2023-06-13 08:10:52 +08:00
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// SPDX-License-Identifier: GPL-2.0
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/*
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* shstk.c - Intel shadow stack support
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*
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* Copyright (c) 2021, Intel Corporation.
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* Yu-cheng Yu <yu-cheng.yu@intel.com>
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*/
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#include <linux/sched.h>
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#include <linux/bitops.h>
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2023-06-13 08:10:54 +08:00
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#include <linux/types.h>
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#include <linux/mm.h>
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#include <linux/mman.h>
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#include <linux/slab.h>
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#include <linux/uaccess.h>
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#include <linux/sched/signal.h>
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#include <linux/compat.h>
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#include <linux/sizes.h>
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#include <linux/user.h>
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x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
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#include <linux/syscalls.h>
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2023-06-13 08:10:54 +08:00
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#include <asm/msr.h>
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#include <asm/fpu/xstate.h>
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#include <asm/fpu/types.h>
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#include <asm/shstk.h>
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#include <asm/special_insns.h>
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#include <asm/fpu/api.h>
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2023-06-13 08:10:52 +08:00
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#include <asm/prctl.h>
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2023-06-13 08:10:56 +08:00
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#define SS_FRAME_SIZE 8
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2023-06-13 08:10:54 +08:00
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static bool features_enabled(unsigned long features)
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{
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return current->thread.features & features;
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}
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static void features_set(unsigned long features)
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{
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current->thread.features |= features;
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}
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static void features_clr(unsigned long features)
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{
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current->thread.features &= ~features;
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}
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2023-06-13 08:10:56 +08:00
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/*
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* Create a restore token on the shadow stack. A token is always 8-byte
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* and aligned to 8.
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*/
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static int create_rstor_token(unsigned long ssp, unsigned long *token_addr)
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{
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unsigned long addr;
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/* Token must be aligned */
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if (!IS_ALIGNED(ssp, 8))
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return -EINVAL;
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addr = ssp - SS_FRAME_SIZE;
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/*
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* SSP is aligned, so reserved bits and mode bit are a zero, just mark
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* the token 64-bit.
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*/
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ssp |= BIT(0);
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if (write_user_shstk_64((u64 __user *)addr, (u64)ssp))
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return -EFAULT;
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if (token_addr)
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*token_addr = addr;
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return 0;
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}
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2023-07-07 07:32:48 +08:00
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/*
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* VM_SHADOW_STACK will have a guard page. This helps userspace protect
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* itself from attacks. The reasoning is as follows:
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*
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* The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The
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* INCSSP instruction can increment the shadow stack pointer. It is the
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* shadow stack analog of an instruction like:
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*
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* addq $0x80, %rsp
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*
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* However, there is one important difference between an ADD on %rsp
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* and INCSSP. In addition to modifying SSP, INCSSP also reads from the
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* memory of the first and last elements that were "popped". It can be
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* thought of as acting like this:
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*
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* READ_ONCE(ssp); // read+discard top element on stack
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* ssp += nr_to_pop * 8; // move the shadow stack
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* READ_ONCE(ssp-8); // read+discard last popped stack element
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*
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* The maximum distance INCSSP can move the SSP is 2040 bytes, before
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* it would read the memory. Therefore a single page gap will be enough
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* to prevent any operation from shifting the SSP to an adjacent stack,
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* since it would have to land in the gap at least once, causing a
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* fault.
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*/
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x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
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static unsigned long alloc_shstk(unsigned long addr, unsigned long size,
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unsigned long token_offset, bool set_res_tok)
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2023-06-13 08:10:54 +08:00
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{
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int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_ABOVE4G;
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struct mm_struct *mm = current->mm;
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x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
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unsigned long mapped_addr, unused;
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2023-06-13 08:10:54 +08:00
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|
x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
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if (addr)
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flags |= MAP_FIXED_NOREPLACE;
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2023-06-13 08:10:54 +08:00
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x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
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mmap_write_lock(mm);
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mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags,
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VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
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2023-06-13 08:10:54 +08:00
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mmap_write_unlock(mm);
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x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
|
|
|
if (!set_res_tok || IS_ERR_VALUE(mapped_addr))
|
|
|
|
|
goto out;
|
|
|
|
|
|
|
|
|
|
if (create_rstor_token(mapped_addr + token_offset, NULL)) {
|
|
|
|
|
vm_munmap(mapped_addr, size);
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
out:
|
|
|
|
|
return mapped_addr;
|
2023-06-13 08:10:54 +08:00
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static unsigned long adjust_shstk_size(unsigned long size)
|
|
|
|
|
{
|
|
|
|
|
if (size)
|
|
|
|
|
return PAGE_ALIGN(size);
|
|
|
|
|
|
|
|
|
|
return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G));
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static void unmap_shadow_stack(u64 base, u64 size)
|
|
|
|
|
{
|
2023-07-07 07:38:58 +08:00
|
|
|
int r;
|
|
|
|
|
|
|
|
|
|
r = vm_munmap(base, size);
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* mmap_write_lock_killable() failed with -EINTR. This means
|
|
|
|
|
* the process is about to die and have it's MM cleaned up.
|
|
|
|
|
* This task shouldn't ever make it back to userspace. In this
|
|
|
|
|
* case it is ok to leak a shadow stack, so just exit out.
|
|
|
|
|
*/
|
|
|
|
|
if (r == -EINTR)
|
|
|
|
|
return;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* For all other types of vm_munmap() failure, either the
|
|
|
|
|
* system is out of memory or there is bug.
|
|
|
|
|
*/
|
|
|
|
|
WARN_ON_ONCE(r);
|
2023-06-13 08:10:54 +08:00
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static int shstk_setup(void)
|
|
|
|
|
{
|
|
|
|
|
struct thread_shstk *shstk = ¤t->thread.shstk;
|
|
|
|
|
unsigned long addr, size;
|
|
|
|
|
|
|
|
|
|
/* Already enabled */
|
|
|
|
|
if (features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
/* Also not supported for 32 bit and x32 */
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || in_32bit_syscall())
|
|
|
|
|
return -EOPNOTSUPP;
|
|
|
|
|
|
|
|
|
|
size = adjust_shstk_size(0);
|
x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
|
|
|
addr = alloc_shstk(0, size, 0, false);
|
2023-06-13 08:10:54 +08:00
|
|
|
if (IS_ERR_VALUE(addr))
|
|
|
|
|
return PTR_ERR((void *)addr);
|
|
|
|
|
|
|
|
|
|
fpregs_lock_and_load();
|
|
|
|
|
wrmsrl(MSR_IA32_PL3_SSP, addr + size);
|
|
|
|
|
wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);
|
|
|
|
|
fpregs_unlock();
|
|
|
|
|
|
|
|
|
|
shstk->base = addr;
|
|
|
|
|
shstk->size = size;
|
|
|
|
|
features_set(ARCH_SHSTK_SHSTK);
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:10:52 +08:00
|
|
|
void reset_thread_features(void)
|
|
|
|
|
{
|
2023-06-13 08:10:54 +08:00
|
|
|
memset(¤t->thread.shstk, 0, sizeof(struct thread_shstk));
|
2023-06-13 08:10:52 +08:00
|
|
|
current->thread.features = 0;
|
|
|
|
|
current->thread.features_locked = 0;
|
|
|
|
|
}
|
|
|
|
|
|
x86/shstk: Handle thread shadow stack
When a process is duplicated, but the child shares the address space with
the parent, there is potential for the threads sharing a single stack to
cause conflicts for each other. In the normal non-CET case this is handled
in two ways.
With regular CLONE_VM a new stack is provided by userspace such that the
parent and child have different stacks.
For vfork, the parent is suspended until the child exits. So as long as
the child doesn't return from the vfork()/CLONE_VFORK calling function and
sticks to a limited set of operations, the parent and child can share the
same stack.
For shadow stack, these scenarios present similar sharing problems. For the
CLONE_VM case, the child and the parent must have separate shadow stacks.
Instead of changing clone to take a shadow stack, have the kernel just
allocate one and switch to it.
Use stack_size passed from clone3() syscall for thread shadow stack size. A
compat-mode thread shadow stack size is further reduced to 1/4. This
allows more threads to run in a 32-bit address space. The clone() does not
pass stack_size, which was added to clone3(). In that case, use
RLIMIT_STACK size and cap to 4 GB.
For shadow stack enabled vfork(), the parent and child can share the same
shadow stack, like they can share a normal stack. Since the parent is
suspended until the child terminates, the child will not interfere with
the parent while executing as long as it doesn't return from the vfork()
and overwrite up the shadow stack. The child can safely overwrite down
the shadow stack, as the parent can just overwrite this later. So CET does
not add any additional limitations for vfork().
Free the shadow stack on thread exit by doing it in mm_release(). Skip
this when exiting a vfork() child since the stack is shared in the
parent.
During this operation, the shadow stack pointer of the new thread needs
to be updated to point to the newly allocated shadow stack. Since the
ability to do this is confined to the FPU subsystem, change
fpu_clone() to take the new shadow stack pointer, and update it
internally inside the FPU subsystem. This part was suggested by Thomas
Gleixner.
Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-30-rick.p.edgecombe%40intel.com
2023-06-13 08:10:55 +08:00
|
|
|
unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
|
|
|
|
|
unsigned long stack_size)
|
|
|
|
|
{
|
|
|
|
|
struct thread_shstk *shstk = &tsk->thread.shstk;
|
|
|
|
|
unsigned long addr, size;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* If shadow stack is not enabled on the new thread, skip any
|
|
|
|
|
* switch to a new shadow stack.
|
|
|
|
|
*/
|
|
|
|
|
if (!features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* For CLONE_VM, except vfork, the child needs a separate shadow
|
|
|
|
|
* stack.
|
|
|
|
|
*/
|
|
|
|
|
if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
size = adjust_shstk_size(stack_size);
|
x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
|
|
|
addr = alloc_shstk(0, size, 0, false);
|
x86/shstk: Handle thread shadow stack
When a process is duplicated, but the child shares the address space with
the parent, there is potential for the threads sharing a single stack to
cause conflicts for each other. In the normal non-CET case this is handled
in two ways.
With regular CLONE_VM a new stack is provided by userspace such that the
parent and child have different stacks.
For vfork, the parent is suspended until the child exits. So as long as
the child doesn't return from the vfork()/CLONE_VFORK calling function and
sticks to a limited set of operations, the parent and child can share the
same stack.
For shadow stack, these scenarios present similar sharing problems. For the
CLONE_VM case, the child and the parent must have separate shadow stacks.
Instead of changing clone to take a shadow stack, have the kernel just
allocate one and switch to it.
Use stack_size passed from clone3() syscall for thread shadow stack size. A
compat-mode thread shadow stack size is further reduced to 1/4. This
allows more threads to run in a 32-bit address space. The clone() does not
pass stack_size, which was added to clone3(). In that case, use
RLIMIT_STACK size and cap to 4 GB.
For shadow stack enabled vfork(), the parent and child can share the same
shadow stack, like they can share a normal stack. Since the parent is
suspended until the child terminates, the child will not interfere with
the parent while executing as long as it doesn't return from the vfork()
and overwrite up the shadow stack. The child can safely overwrite down
the shadow stack, as the parent can just overwrite this later. So CET does
not add any additional limitations for vfork().
Free the shadow stack on thread exit by doing it in mm_release(). Skip
this when exiting a vfork() child since the stack is shared in the
parent.
During this operation, the shadow stack pointer of the new thread needs
to be updated to point to the newly allocated shadow stack. Since the
ability to do this is confined to the FPU subsystem, change
fpu_clone() to take the new shadow stack pointer, and update it
internally inside the FPU subsystem. This part was suggested by Thomas
Gleixner.
Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-30-rick.p.edgecombe%40intel.com
2023-06-13 08:10:55 +08:00
|
|
|
if (IS_ERR_VALUE(addr))
|
|
|
|
|
return addr;
|
|
|
|
|
|
|
|
|
|
shstk->base = addr;
|
|
|
|
|
shstk->size = size;
|
|
|
|
|
|
|
|
|
|
return addr + size;
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:10:56 +08:00
|
|
|
static unsigned long get_user_shstk_addr(void)
|
|
|
|
|
{
|
|
|
|
|
unsigned long long ssp;
|
|
|
|
|
|
|
|
|
|
fpregs_lock_and_load();
|
|
|
|
|
|
|
|
|
|
rdmsrl(MSR_IA32_PL3_SSP, ssp);
|
|
|
|
|
|
|
|
|
|
fpregs_unlock();
|
|
|
|
|
|
|
|
|
|
return ssp;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
#define SHSTK_DATA_BIT BIT(63)
|
|
|
|
|
|
|
|
|
|
static int put_shstk_data(u64 __user *addr, u64 data)
|
|
|
|
|
{
|
|
|
|
|
if (WARN_ON_ONCE(data & SHSTK_DATA_BIT))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Mark the high bit so that the sigframe can't be processed as a
|
|
|
|
|
* return address.
|
|
|
|
|
*/
|
|
|
|
|
if (write_user_shstk_64(addr, data | SHSTK_DATA_BIT))
|
|
|
|
|
return -EFAULT;
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static int get_shstk_data(unsigned long *data, unsigned long __user *addr)
|
|
|
|
|
{
|
|
|
|
|
unsigned long ldata;
|
|
|
|
|
|
|
|
|
|
if (unlikely(get_user(ldata, addr)))
|
|
|
|
|
return -EFAULT;
|
|
|
|
|
|
|
|
|
|
if (!(ldata & SHSTK_DATA_BIT))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
*data = ldata & ~SHSTK_DATA_BIT;
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:10:57 +08:00
|
|
|
static int shstk_push_sigframe(unsigned long *ssp)
|
|
|
|
|
{
|
|
|
|
|
unsigned long target_ssp = *ssp;
|
|
|
|
|
|
|
|
|
|
/* Token must be aligned */
|
|
|
|
|
if (!IS_ALIGNED(target_ssp, 8))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
*ssp -= SS_FRAME_SIZE;
|
2023-08-25 09:45:54 +08:00
|
|
|
if (put_shstk_data((void __user *)*ssp, target_ssp))
|
2023-06-13 08:10:57 +08:00
|
|
|
return -EFAULT;
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static int shstk_pop_sigframe(unsigned long *ssp)
|
|
|
|
|
{
|
x86/shstk: Check that signal frame is shadow stack mem
The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). This behavior helps
userspace protect itself. However, using the INCSSP instruction userspace
can adjust the SSP to 8 bytes beyond the end of a shadow stack. INCSSP
performs shadow stack reads to make sure it doesn’t increment off of the
shadow stack, but on the end position it actually reads 8 bytes below the
new SSP.
For the shadow stack HW operations, this situation (INCSSP off the end
of a shadow stack by 8 bytes) would be fine. If the a RET is executed, the
push to the shadow stack would fail to write to the shadow stack. If a
CALL is executed, the SSP will be incremented back onto the stack and the
return address will be written successfully to the very end. That is
expected behavior around shadow stack underflow.
However, the kernel doesn’t have a way to read shadow stack memory using
shadow stack accesses. WRUSS can write to shadow stack memory with a
shadow stack access which ensures the access is to shadow stack memory.
But unfortunately for this case, there is no equivalent instruction for
shadow stack reads. So when reading the shadow stack signal frames, the
kernel currently assumes the SSP is pointing to the shadow stack and uses
a normal read.
The SSP pointing to shadow stack memory will be true in most cases, but as
described above, in can be untrue by 8 bytes. So lookup the VMA of the
shadow stack sigframe being read to verify it is shadow stack.
Since the SSP can only be beyond the shadow stack by 8 bytes, and
shadow stack memory is page aligned, this check only needs to be done
when this type of relative position to a page boundary is encountered.
So skip the extra work otherwise.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230613001108.3040476-34-rick.p.edgecombe%40intel.com
2023-06-13 08:10:59 +08:00
|
|
|
struct vm_area_struct *vma;
|
2023-06-13 08:10:57 +08:00
|
|
|
unsigned long token_addr;
|
x86/shstk: Check that signal frame is shadow stack mem
The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). This behavior helps
userspace protect itself. However, using the INCSSP instruction userspace
can adjust the SSP to 8 bytes beyond the end of a shadow stack. INCSSP
performs shadow stack reads to make sure it doesn’t increment off of the
shadow stack, but on the end position it actually reads 8 bytes below the
new SSP.
For the shadow stack HW operations, this situation (INCSSP off the end
of a shadow stack by 8 bytes) would be fine. If the a RET is executed, the
push to the shadow stack would fail to write to the shadow stack. If a
CALL is executed, the SSP will be incremented back onto the stack and the
return address will be written successfully to the very end. That is
expected behavior around shadow stack underflow.
However, the kernel doesn’t have a way to read shadow stack memory using
shadow stack accesses. WRUSS can write to shadow stack memory with a
shadow stack access which ensures the access is to shadow stack memory.
But unfortunately for this case, there is no equivalent instruction for
shadow stack reads. So when reading the shadow stack signal frames, the
kernel currently assumes the SSP is pointing to the shadow stack and uses
a normal read.
The SSP pointing to shadow stack memory will be true in most cases, but as
described above, in can be untrue by 8 bytes. So lookup the VMA of the
shadow stack sigframe being read to verify it is shadow stack.
Since the SSP can only be beyond the shadow stack by 8 bytes, and
shadow stack memory is page aligned, this check only needs to be done
when this type of relative position to a page boundary is encountered.
So skip the extra work otherwise.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230613001108.3040476-34-rick.p.edgecombe%40intel.com
2023-06-13 08:10:59 +08:00
|
|
|
bool need_to_check_vma;
|
|
|
|
|
int err = 1;
|
2023-06-13 08:10:57 +08:00
|
|
|
|
x86/shstk: Check that signal frame is shadow stack mem
The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). This behavior helps
userspace protect itself. However, using the INCSSP instruction userspace
can adjust the SSP to 8 bytes beyond the end of a shadow stack. INCSSP
performs shadow stack reads to make sure it doesn’t increment off of the
shadow stack, but on the end position it actually reads 8 bytes below the
new SSP.
For the shadow stack HW operations, this situation (INCSSP off the end
of a shadow stack by 8 bytes) would be fine. If the a RET is executed, the
push to the shadow stack would fail to write to the shadow stack. If a
CALL is executed, the SSP will be incremented back onto the stack and the
return address will be written successfully to the very end. That is
expected behavior around shadow stack underflow.
However, the kernel doesn’t have a way to read shadow stack memory using
shadow stack accesses. WRUSS can write to shadow stack memory with a
shadow stack access which ensures the access is to shadow stack memory.
But unfortunately for this case, there is no equivalent instruction for
shadow stack reads. So when reading the shadow stack signal frames, the
kernel currently assumes the SSP is pointing to the shadow stack and uses
a normal read.
The SSP pointing to shadow stack memory will be true in most cases, but as
described above, in can be untrue by 8 bytes. So lookup the VMA of the
shadow stack sigframe being read to verify it is shadow stack.
Since the SSP can only be beyond the shadow stack by 8 bytes, and
shadow stack memory is page aligned, this check only needs to be done
when this type of relative position to a page boundary is encountered.
So skip the extra work otherwise.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230613001108.3040476-34-rick.p.edgecombe%40intel.com
2023-06-13 08:10:59 +08:00
|
|
|
/*
|
|
|
|
|
* It is possible for the SSP to be off the end of a shadow stack by 4
|
|
|
|
|
* or 8 bytes. If the shadow stack is at the start of a page or 4 bytes
|
|
|
|
|
* before it, it might be this case, so check that the address being
|
|
|
|
|
* read is actually shadow stack.
|
|
|
|
|
*/
|
2023-06-13 08:10:58 +08:00
|
|
|
if (!IS_ALIGNED(*ssp, 8))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
x86/shstk: Check that signal frame is shadow stack mem
The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). This behavior helps
userspace protect itself. However, using the INCSSP instruction userspace
can adjust the SSP to 8 bytes beyond the end of a shadow stack. INCSSP
performs shadow stack reads to make sure it doesn’t increment off of the
shadow stack, but on the end position it actually reads 8 bytes below the
new SSP.
For the shadow stack HW operations, this situation (INCSSP off the end
of a shadow stack by 8 bytes) would be fine. If the a RET is executed, the
push to the shadow stack would fail to write to the shadow stack. If a
CALL is executed, the SSP will be incremented back onto the stack and the
return address will be written successfully to the very end. That is
expected behavior around shadow stack underflow.
However, the kernel doesn’t have a way to read shadow stack memory using
shadow stack accesses. WRUSS can write to shadow stack memory with a
shadow stack access which ensures the access is to shadow stack memory.
But unfortunately for this case, there is no equivalent instruction for
shadow stack reads. So when reading the shadow stack signal frames, the
kernel currently assumes the SSP is pointing to the shadow stack and uses
a normal read.
The SSP pointing to shadow stack memory will be true in most cases, but as
described above, in can be untrue by 8 bytes. So lookup the VMA of the
shadow stack sigframe being read to verify it is shadow stack.
Since the SSP can only be beyond the shadow stack by 8 bytes, and
shadow stack memory is page aligned, this check only needs to be done
when this type of relative position to a page boundary is encountered.
So skip the extra work otherwise.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230613001108.3040476-34-rick.p.edgecombe%40intel.com
2023-06-13 08:10:59 +08:00
|
|
|
need_to_check_vma = PAGE_ALIGN(*ssp) == *ssp;
|
|
|
|
|
|
|
|
|
|
if (need_to_check_vma)
|
|
|
|
|
mmap_read_lock_killable(current->mm);
|
|
|
|
|
|
2023-06-13 08:10:57 +08:00
|
|
|
err = get_shstk_data(&token_addr, (unsigned long __user *)*ssp);
|
|
|
|
|
if (unlikely(err))
|
x86/shstk: Check that signal frame is shadow stack mem
The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). This behavior helps
userspace protect itself. However, using the INCSSP instruction userspace
can adjust the SSP to 8 bytes beyond the end of a shadow stack. INCSSP
performs shadow stack reads to make sure it doesn’t increment off of the
shadow stack, but on the end position it actually reads 8 bytes below the
new SSP.
For the shadow stack HW operations, this situation (INCSSP off the end
of a shadow stack by 8 bytes) would be fine. If the a RET is executed, the
push to the shadow stack would fail to write to the shadow stack. If a
CALL is executed, the SSP will be incremented back onto the stack and the
return address will be written successfully to the very end. That is
expected behavior around shadow stack underflow.
However, the kernel doesn’t have a way to read shadow stack memory using
shadow stack accesses. WRUSS can write to shadow stack memory with a
shadow stack access which ensures the access is to shadow stack memory.
But unfortunately for this case, there is no equivalent instruction for
shadow stack reads. So when reading the shadow stack signal frames, the
kernel currently assumes the SSP is pointing to the shadow stack and uses
a normal read.
The SSP pointing to shadow stack memory will be true in most cases, but as
described above, in can be untrue by 8 bytes. So lookup the VMA of the
shadow stack sigframe being read to verify it is shadow stack.
Since the SSP can only be beyond the shadow stack by 8 bytes, and
shadow stack memory is page aligned, this check only needs to be done
when this type of relative position to a page boundary is encountered.
So skip the extra work otherwise.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230613001108.3040476-34-rick.p.edgecombe%40intel.com
2023-06-13 08:10:59 +08:00
|
|
|
goto out_err;
|
|
|
|
|
|
|
|
|
|
if (need_to_check_vma) {
|
|
|
|
|
vma = find_vma(current->mm, *ssp);
|
|
|
|
|
if (!vma || !(vma->vm_flags & VM_SHADOW_STACK)) {
|
|
|
|
|
err = -EFAULT;
|
|
|
|
|
goto out_err;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
mmap_read_unlock(current->mm);
|
|
|
|
|
}
|
2023-06-13 08:10:57 +08:00
|
|
|
|
|
|
|
|
/* Restore SSP aligned? */
|
|
|
|
|
if (unlikely(!IS_ALIGNED(token_addr, 8)))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
/* SSP in userspace? */
|
|
|
|
|
if (unlikely(token_addr >= TASK_SIZE_MAX))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
*ssp = token_addr;
|
|
|
|
|
|
|
|
|
|
return 0;
|
x86/shstk: Check that signal frame is shadow stack mem
The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). This behavior helps
userspace protect itself. However, using the INCSSP instruction userspace
can adjust the SSP to 8 bytes beyond the end of a shadow stack. INCSSP
performs shadow stack reads to make sure it doesn’t increment off of the
shadow stack, but on the end position it actually reads 8 bytes below the
new SSP.
For the shadow stack HW operations, this situation (INCSSP off the end
of a shadow stack by 8 bytes) would be fine. If the a RET is executed, the
push to the shadow stack would fail to write to the shadow stack. If a
CALL is executed, the SSP will be incremented back onto the stack and the
return address will be written successfully to the very end. That is
expected behavior around shadow stack underflow.
However, the kernel doesn’t have a way to read shadow stack memory using
shadow stack accesses. WRUSS can write to shadow stack memory with a
shadow stack access which ensures the access is to shadow stack memory.
But unfortunately for this case, there is no equivalent instruction for
shadow stack reads. So when reading the shadow stack signal frames, the
kernel currently assumes the SSP is pointing to the shadow stack and uses
a normal read.
The SSP pointing to shadow stack memory will be true in most cases, but as
described above, in can be untrue by 8 bytes. So lookup the VMA of the
shadow stack sigframe being read to verify it is shadow stack.
Since the SSP can only be beyond the shadow stack by 8 bytes, and
shadow stack memory is page aligned, this check only needs to be done
when this type of relative position to a page boundary is encountered.
So skip the extra work otherwise.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230613001108.3040476-34-rick.p.edgecombe%40intel.com
2023-06-13 08:10:59 +08:00
|
|
|
out_err:
|
|
|
|
|
if (need_to_check_vma)
|
|
|
|
|
mmap_read_unlock(current->mm);
|
|
|
|
|
return err;
|
2023-06-13 08:10:57 +08:00
|
|
|
}
|
|
|
|
|
|
|
|
|
|
int setup_signal_shadow_stack(struct ksignal *ksig)
|
|
|
|
|
{
|
|
|
|
|
void __user *restorer = ksig->ka.sa.sa_restorer;
|
|
|
|
|
unsigned long ssp;
|
|
|
|
|
int err;
|
|
|
|
|
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) ||
|
|
|
|
|
!features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
if (!restorer)
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
ssp = get_user_shstk_addr();
|
|
|
|
|
if (unlikely(!ssp))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
err = shstk_push_sigframe(&ssp);
|
|
|
|
|
if (unlikely(err))
|
|
|
|
|
return err;
|
|
|
|
|
|
|
|
|
|
/* Push restorer address */
|
|
|
|
|
ssp -= SS_FRAME_SIZE;
|
|
|
|
|
err = write_user_shstk_64((u64 __user *)ssp, (u64)restorer);
|
|
|
|
|
if (unlikely(err))
|
|
|
|
|
return -EFAULT;
|
|
|
|
|
|
|
|
|
|
fpregs_lock_and_load();
|
|
|
|
|
wrmsrl(MSR_IA32_PL3_SSP, ssp);
|
|
|
|
|
fpregs_unlock();
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
int restore_signal_shadow_stack(void)
|
|
|
|
|
{
|
|
|
|
|
unsigned long ssp;
|
|
|
|
|
int err;
|
|
|
|
|
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) ||
|
|
|
|
|
!features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
ssp = get_user_shstk_addr();
|
|
|
|
|
if (unlikely(!ssp))
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
err = shstk_pop_sigframe(&ssp);
|
|
|
|
|
if (unlikely(err))
|
|
|
|
|
return err;
|
|
|
|
|
|
|
|
|
|
fpregs_lock_and_load();
|
|
|
|
|
wrmsrl(MSR_IA32_PL3_SSP, ssp);
|
|
|
|
|
fpregs_unlock();
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:10:54 +08:00
|
|
|
void shstk_free(struct task_struct *tsk)
|
|
|
|
|
{
|
|
|
|
|
struct thread_shstk *shstk = &tsk->thread.shstk;
|
|
|
|
|
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) ||
|
|
|
|
|
!features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return;
|
|
|
|
|
|
x86/shstk: Handle thread shadow stack
When a process is duplicated, but the child shares the address space with
the parent, there is potential for the threads sharing a single stack to
cause conflicts for each other. In the normal non-CET case this is handled
in two ways.
With regular CLONE_VM a new stack is provided by userspace such that the
parent and child have different stacks.
For vfork, the parent is suspended until the child exits. So as long as
the child doesn't return from the vfork()/CLONE_VFORK calling function and
sticks to a limited set of operations, the parent and child can share the
same stack.
For shadow stack, these scenarios present similar sharing problems. For the
CLONE_VM case, the child and the parent must have separate shadow stacks.
Instead of changing clone to take a shadow stack, have the kernel just
allocate one and switch to it.
Use stack_size passed from clone3() syscall for thread shadow stack size. A
compat-mode thread shadow stack size is further reduced to 1/4. This
allows more threads to run in a 32-bit address space. The clone() does not
pass stack_size, which was added to clone3(). In that case, use
RLIMIT_STACK size and cap to 4 GB.
For shadow stack enabled vfork(), the parent and child can share the same
shadow stack, like they can share a normal stack. Since the parent is
suspended until the child terminates, the child will not interfere with
the parent while executing as long as it doesn't return from the vfork()
and overwrite up the shadow stack. The child can safely overwrite down
the shadow stack, as the parent can just overwrite this later. So CET does
not add any additional limitations for vfork().
Free the shadow stack on thread exit by doing it in mm_release(). Skip
this when exiting a vfork() child since the stack is shared in the
parent.
During this operation, the shadow stack pointer of the new thread needs
to be updated to point to the newly allocated shadow stack. Since the
ability to do this is confined to the FPU subsystem, change
fpu_clone() to take the new shadow stack pointer, and update it
internally inside the FPU subsystem. This part was suggested by Thomas
Gleixner.
Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-30-rick.p.edgecombe%40intel.com
2023-06-13 08:10:55 +08:00
|
|
|
/*
|
|
|
|
|
* When fork() with CLONE_VM fails, the child (tsk) already has a
|
|
|
|
|
* shadow stack allocated, and exit_thread() calls this function to
|
|
|
|
|
* free it. In this case the parent (current) and the child share
|
|
|
|
|
* the same mm struct.
|
|
|
|
|
*/
|
|
|
|
|
if (!tsk->mm || tsk->mm != current->mm)
|
2023-06-13 08:10:54 +08:00
|
|
|
return;
|
|
|
|
|
|
|
|
|
|
unmap_shadow_stack(shstk->base, shstk->size);
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:11:01 +08:00
|
|
|
static int wrss_control(bool enable)
|
|
|
|
|
{
|
|
|
|
|
u64 msrval;
|
|
|
|
|
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
|
|
|
|
|
return -EOPNOTSUPP;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Only enable WRSS if shadow stack is enabled. If shadow stack is not
|
|
|
|
|
* enabled, WRSS will already be disabled, so don't bother clearing it
|
|
|
|
|
* when disabling.
|
|
|
|
|
*/
|
|
|
|
|
if (!features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return -EPERM;
|
|
|
|
|
|
|
|
|
|
/* Already enabled/disabled? */
|
|
|
|
|
if (features_enabled(ARCH_SHSTK_WRSS) == enable)
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
fpregs_lock_and_load();
|
|
|
|
|
rdmsrl(MSR_IA32_U_CET, msrval);
|
|
|
|
|
|
|
|
|
|
if (enable) {
|
|
|
|
|
features_set(ARCH_SHSTK_WRSS);
|
|
|
|
|
msrval |= CET_WRSS_EN;
|
|
|
|
|
} else {
|
|
|
|
|
features_clr(ARCH_SHSTK_WRSS);
|
|
|
|
|
if (!(msrval & CET_WRSS_EN))
|
|
|
|
|
goto unlock;
|
|
|
|
|
|
|
|
|
|
msrval &= ~CET_WRSS_EN;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
wrmsrl(MSR_IA32_U_CET, msrval);
|
|
|
|
|
|
|
|
|
|
unlock:
|
|
|
|
|
fpregs_unlock();
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:10:54 +08:00
|
|
|
static int shstk_disable(void)
|
|
|
|
|
{
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
|
|
|
|
|
return -EOPNOTSUPP;
|
|
|
|
|
|
|
|
|
|
/* Already disabled? */
|
|
|
|
|
if (!features_enabled(ARCH_SHSTK_SHSTK))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
fpregs_lock_and_load();
|
|
|
|
|
/* Disable WRSS too when disabling shadow stack */
|
|
|
|
|
wrmsrl(MSR_IA32_U_CET, 0);
|
|
|
|
|
wrmsrl(MSR_IA32_PL3_SSP, 0);
|
|
|
|
|
fpregs_unlock();
|
|
|
|
|
|
|
|
|
|
shstk_free(current);
|
2023-06-13 08:11:01 +08:00
|
|
|
features_clr(ARCH_SHSTK_SHSTK | ARCH_SHSTK_WRSS);
|
2023-06-13 08:10:54 +08:00
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
x86/shstk: Introduce map_shadow_stack syscall
When operating with shadow stacks enabled, the kernel will automatically
allocate shadow stacks for new threads, however in some cases userspace
will need additional shadow stacks. The main example of this is the
ucontext family of functions, which require userspace allocating and
pivoting to userspace managed stacks.
Unlike most other user memory permissions, shadow stacks need to be
provisioned with special data in order to be useful. They need to be setup
with a restore token so that userspace can pivot to them via the RSTORSSP
instruction. But, the security design of shadow stacks is that they
should not be written to except in limited circumstances. This presents a
problem for userspace, as to how userspace can provision this special
data, without allowing for the shadow stack to be generally writable.
Previously, a new PROT_SHADOW_STACK was attempted, which could be
mprotect()ed from RW permissions after the data was provisioned. This was
found to not be secure enough, as other threads could write to the
shadow stack during the writable window.
The kernel can use a special instruction, WRUSS, to write directly to
userspace shadow stacks. So the solution can be that memory can be mapped
as shadow stack permissions from the beginning (never generally writable
in userspace), and the kernel itself can write the restore token.
First, a new madvise() flag was explored, which could operate on the
PROT_SHADOW_STACK memory. This had a couple of downsides:
1. Extra checks were needed in mprotect() to prevent writable memory from
ever becoming PROT_SHADOW_STACK.
2. Extra checks/vma state were needed in the new madvise() to prevent
restore tokens being written into the middle of pre-used shadow stacks.
It is ideal to prevent restore tokens being added at arbitrary
locations, so the check was to make sure the shadow stack had never been
written to.
3. It stood out from the rest of the madvise flags, as more of direct
action than a hint at future desired behavior.
So rather than repurpose two existing syscalls (mmap, madvise) that don't
quite fit, just implement a new map_shadow_stack syscall to allow
userspace to map and setup new shadow stacks in one step. While ucontext
is the primary motivator, userspace may have other unforeseen reasons to
setup its own shadow stacks using the WRSS instruction. Towards this
provide a flag so that stacks can be optionally setup securely for the
common case of ucontext without enabling WRSS. Or potentially have the
kernel set up the shadow stack in some new way.
The following example demonstrates how to create a new shadow stack with
map_shadow_stack:
void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-35-rick.p.edgecombe%40intel.com
2023-06-13 08:11:00 +08:00
|
|
|
SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
|
|
|
|
|
{
|
|
|
|
|
bool set_tok = flags & SHADOW_STACK_SET_TOKEN;
|
|
|
|
|
unsigned long aligned_size;
|
|
|
|
|
|
|
|
|
|
if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
|
|
|
|
|
return -EOPNOTSUPP;
|
|
|
|
|
|
|
|
|
|
if (flags & ~SHADOW_STACK_SET_TOKEN)
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
/* If there isn't space for a token */
|
|
|
|
|
if (set_tok && size < 8)
|
|
|
|
|
return -ENOSPC;
|
|
|
|
|
|
|
|
|
|
if (addr && addr < SZ_4G)
|
|
|
|
|
return -ERANGE;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* An overflow would result in attempting to write the restore token
|
|
|
|
|
* to the wrong location. Not catastrophic, but just return the right
|
|
|
|
|
* error code and block it.
|
|
|
|
|
*/
|
|
|
|
|
aligned_size = PAGE_ALIGN(size);
|
|
|
|
|
if (aligned_size < size)
|
|
|
|
|
return -EOVERFLOW;
|
|
|
|
|
|
|
|
|
|
return alloc_shstk(addr, aligned_size, size, set_tok);
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:11:08 +08:00
|
|
|
long shstk_prctl(struct task_struct *task, int option, unsigned long arg2)
|
2023-06-13 08:10:52 +08:00
|
|
|
{
|
2023-06-13 08:11:08 +08:00
|
|
|
unsigned long features = arg2;
|
|
|
|
|
|
|
|
|
|
if (option == ARCH_SHSTK_STATUS) {
|
|
|
|
|
return put_user(task->thread.features, (unsigned long __user *)arg2);
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:10:52 +08:00
|
|
|
if (option == ARCH_SHSTK_LOCK) {
|
|
|
|
|
task->thread.features_locked |= features;
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2023-06-13 08:11:07 +08:00
|
|
|
/* Only allow via ptrace */
|
|
|
|
|
if (task != current) {
|
|
|
|
|
if (option == ARCH_SHSTK_UNLOCK && IS_ENABLED(CONFIG_CHECKPOINT_RESTORE)) {
|
|
|
|
|
task->thread.features_locked &= ~features;
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
2023-06-13 08:10:52 +08:00
|
|
|
return -EINVAL;
|
2023-06-13 08:11:07 +08:00
|
|
|
}
|
2023-06-13 08:10:52 +08:00
|
|
|
|
|
|
|
|
/* Do not allow to change locked features */
|
|
|
|
|
if (features & task->thread.features_locked)
|
|
|
|
|
return -EPERM;
|
|
|
|
|
|
|
|
|
|
/* Only support enabling/disabling one feature at a time. */
|
|
|
|
|
if (hweight_long(features) > 1)
|
|
|
|
|
return -EINVAL;
|
|
|
|
|
|
|
|
|
|
if (option == ARCH_SHSTK_DISABLE) {
|
2023-06-13 08:11:03 +08:00
|
|
|
if (features & ARCH_SHSTK_WRSS)
|
|
|
|
|
return wrss_control(false);
|
|
|
|
|
if (features & ARCH_SHSTK_SHSTK)
|
|
|
|
|
return shstk_disable();
|
2023-06-13 08:10:52 +08:00
|
|
|
return -EINVAL;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
/* Handle ARCH_SHSTK_ENABLE */
|
2023-06-13 08:11:03 +08:00
|
|
|
if (features & ARCH_SHSTK_SHSTK)
|
|
|
|
|
return shstk_setup();
|
|
|
|
|
if (features & ARCH_SHSTK_WRSS)
|
|
|
|
|
return wrss_control(true);
|
2023-06-13 08:10:52 +08:00
|
|
|
return -EINVAL;
|
|
|
|
|
}
|
