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linux-next/lib/strncpy_from_user.c
Andrey Konovalov 903f433f8f lib: untag user pointers in strn*_user
Patch series "arm64: untag user pointers passed to the kernel", v19.

=== Overview

arm64 has a feature called Top Byte Ignore, which allows to embed pointer
tags into the top byte of each pointer.  Userspace programs (such as
HWASan, a memory debugging tool [1]) might use this feature and pass
tagged user pointers to the kernel through syscalls or other interfaces.

Right now the kernel is already able to handle user faults with tagged
pointers, due to these patches:

1. 81cddd65 ("arm64: traps: fix userspace cache maintenance emulation on a
             tagged pointer")
2. 7dcd9dd8 ("arm64: hw_breakpoint: fix watchpoint matching for tagged
	      pointers")
3. 276e9327 ("arm64: entry: improve data abort handling of tagged
	      pointers")

This patchset extends tagged pointer support to syscall arguments.

As per the proposed ABI change [3], tagged pointers are only allowed to be
passed to syscalls when they point to memory ranges obtained by anonymous
mmap() or sbrk() (see the patchset [3] for more details).

For non-memory syscalls this is done by untaging user pointers when the
kernel performs pointer checking to find out whether the pointer comes
from userspace (most notably in access_ok).  The untagging is done only
when the pointer is being checked, the tag is preserved as the pointer
makes its way through the kernel and stays tagged when the kernel
dereferences the pointer when perfoming user memory accesses.

The mmap and mremap (only new_addr) syscalls do not currently accept
tagged addresses.  Architectures may interpret the tag as a background
colour for the corresponding vma.

Other memory syscalls (mprotect, etc.) don't do user memory accesses but
rather deal with memory ranges, and untagged pointers are better suited to
describe memory ranges internally.  Thus for memory syscalls we untag
pointers completely when they enter the kernel.

=== Other approaches

One of the alternative approaches to untagging that was considered is to
completely strip the pointer tag as the pointer enters the kernel with
some kind of a syscall wrapper, but that won't work with the countless
number of different ioctl calls.  With this approach we would need a
custom wrapper for each ioctl variation, which doesn't seem practical.

An alternative approach to untagging pointers in memory syscalls prologues
is to inspead allow tagged pointers to be passed to find_vma() (and other
vma related functions) and untag them there.  Unfortunately, a lot of
find_vma() callers then compare or subtract the returned vma start and end
fields against the pointer that was being searched.  Thus this approach
would still require changing all find_vma() callers.

=== Testing

The following testing approaches has been taken to find potential issues
with user pointer untagging:

1. Static testing (with sparse [2] and separately with a custom static
   analyzer based on Clang) to track casts of __user pointers to integer
   types to find places where untagging needs to be done.

2. Static testing with grep to find parts of the kernel that call
   find_vma() (and other similar functions) or directly compare against
   vm_start/vm_end fields of vma.

3. Static testing with grep to find parts of the kernel that compare
   user pointers with TASK_SIZE or other similar consts and macros.

4. Dynamic testing: adding BUG_ON(has_tag(addr)) to find_vma() and running
   a modified syzkaller version that passes tagged pointers to the kernel.

Based on the results of the testing the requried patches have been added
to the patchset.

=== Notes

This patchset is meant to be merged together with "arm64 relaxed ABI" [3].

This patchset is a prerequisite for ARM's memory tagging hardware feature
support [4].

This patchset has been merged into the Pixel 2 & 3 kernel trees and is
now being used to enable testing of Pixel phones with HWASan.

Thanks!

[1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html

[2] 5f960cb10f

[3] https://lkml.org/lkml/2019/6/12/745

[4] https://community.arm.com/processors/b/blog/posts/arm-a-profile-architecture-2018-developments-armv85a

This patch (of 11)

This patch is a part of a series that extends kernel ABI to allow to pass
tagged user pointers (with the top byte set to something else other than
0x00) as syscall arguments.

strncpy_from_user and strnlen_user accept user addresses as arguments, and
do not go through the same path as copy_from_user and others, so here we
need to handle the case of tagged user addresses separately.

Untag user pointers passed to these functions.

Note, that this patch only temporarily untags the pointers to perform
validity checks, but then uses them as is to perform user memory accesses.

[andreyknvl@google.com: fix sparc4 build]
 Link: http://lkml.kernel.org/r/CAAeHK+yx4a-P0sDrXTUxMvO2H0CJZUFPffBrg_cU7oJOZyC7ew@mail.gmail.com
Link: http://lkml.kernel.org/r/c5a78bcad3e94d6cda71fcaa60a423231ae71e4c.1563904656.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2019-09-25 17:51:41 -07:00

128 lines
3.2 KiB
C

// SPDX-License-Identifier: GPL-2.0
#include <linux/compiler.h>
#include <linux/export.h>
#include <linux/kasan-checks.h>
#include <linux/thread_info.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/mm.h>
#include <asm/byteorder.h>
#include <asm/word-at-a-time.h>
#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
#define IS_UNALIGNED(src, dst) 0
#else
#define IS_UNALIGNED(src, dst) \
(((long) dst | (long) src) & (sizeof(long) - 1))
#endif
/*
* Do a strncpy, return length of string without final '\0'.
* 'count' is the user-supplied count (return 'count' if we
* hit it), 'max' is the address space maximum (and we return
* -EFAULT if we hit it).
*/
static inline long do_strncpy_from_user(char *dst, const char __user *src,
unsigned long count, unsigned long max)
{
const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
unsigned long res = 0;
/*
* Truncate 'max' to the user-specified limit, so that
* we only have one limit we need to check in the loop
*/
if (max > count)
max = count;
if (IS_UNALIGNED(src, dst))
goto byte_at_a_time;
while (max >= sizeof(unsigned long)) {
unsigned long c, data;
/* Fall back to byte-at-a-time if we get a page fault */
unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
*(unsigned long *)(dst+res) = c;
if (has_zero(c, &data, &constants)) {
data = prep_zero_mask(c, data, &constants);
data = create_zero_mask(data);
return res + find_zero(data);
}
res += sizeof(unsigned long);
max -= sizeof(unsigned long);
}
byte_at_a_time:
while (max) {
char c;
unsafe_get_user(c,src+res, efault);
dst[res] = c;
if (!c)
return res;
res++;
max--;
}
/*
* Uhhuh. We hit 'max'. But was that the user-specified maximum
* too? If so, that's ok - we got as much as the user asked for.
*/
if (res >= count)
return res;
/*
* Nope: we hit the address space limit, and we still had more
* characters the caller would have wanted. That's an EFAULT.
*/
efault:
return -EFAULT;
}
/**
* strncpy_from_user: - Copy a NUL terminated string from userspace.
* @dst: Destination address, in kernel space. This buffer must be at
* least @count bytes long.
* @src: Source address, in user space.
* @count: Maximum number of bytes to copy, including the trailing NUL.
*
* Copies a NUL-terminated string from userspace to kernel space.
*
* On success, returns the length of the string (not including the trailing
* NUL).
*
* If access to userspace fails, returns -EFAULT (some data may have been
* copied).
*
* If @count is smaller than the length of the string, copies @count bytes
* and returns @count.
*/
long strncpy_from_user(char *dst, const char __user *src, long count)
{
unsigned long max_addr, src_addr;
if (unlikely(count <= 0))
return 0;
max_addr = user_addr_max();
src_addr = (unsigned long)untagged_addr(src);
if (likely(src_addr < max_addr)) {
unsigned long max = max_addr - src_addr;
long retval;
kasan_check_write(dst, count);
check_object_size(dst, count, false);
if (user_access_begin(src, max)) {
retval = do_strncpy_from_user(dst, src, count, max);
user_access_end();
return retval;
}
}
return -EFAULT;
}
EXPORT_SYMBOL(strncpy_from_user);