mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-16 17:23:55 +08:00
ce39d473d1
Remove the paragraph which has nothing to do with the kernel and add PAC description related to kernel. Suggested-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com> Link: https://lore.kernel.org/r/20211201034014.20048-1-Kuan-Ying.Lee@mediatek.com Signed-off-by: Will Deacon <will@kernel.org>
143 lines
5.9 KiB
ReStructuredText
143 lines
5.9 KiB
ReStructuredText
=======================================
|
|
Pointer authentication in AArch64 Linux
|
|
=======================================
|
|
|
|
Author: Mark Rutland <mark.rutland@arm.com>
|
|
|
|
Date: 2017-07-19
|
|
|
|
This document briefly describes the provision of pointer authentication
|
|
functionality in AArch64 Linux.
|
|
|
|
|
|
Architecture overview
|
|
---------------------
|
|
|
|
The ARMv8.3 Pointer Authentication extension adds primitives that can be
|
|
used to mitigate certain classes of attack where an attacker can corrupt
|
|
the contents of some memory (e.g. the stack).
|
|
|
|
The extension uses a Pointer Authentication Code (PAC) to determine
|
|
whether pointers have been modified unexpectedly. A PAC is derived from
|
|
a pointer, another value (such as the stack pointer), and a secret key
|
|
held in system registers.
|
|
|
|
The extension adds instructions to insert a valid PAC into a pointer,
|
|
and to verify/remove the PAC from a pointer. The PAC occupies a number
|
|
of high-order bits of the pointer, which varies dependent on the
|
|
configured virtual address size and whether pointer tagging is in use.
|
|
|
|
A subset of these instructions have been allocated from the HINT
|
|
encoding space. In the absence of the extension (or when disabled),
|
|
these instructions behave as NOPs. Applications and libraries using
|
|
these instructions operate correctly regardless of the presence of the
|
|
extension.
|
|
|
|
The extension provides five separate keys to generate PACs - two for
|
|
instruction addresses (APIAKey, APIBKey), two for data addresses
|
|
(APDAKey, APDBKey), and one for generic authentication (APGAKey).
|
|
|
|
|
|
Basic support
|
|
-------------
|
|
|
|
When CONFIG_ARM64_PTR_AUTH is selected, and relevant HW support is
|
|
present, the kernel will assign random key values to each process at
|
|
exec*() time. The keys are shared by all threads within the process, and
|
|
are preserved across fork().
|
|
|
|
Presence of address authentication functionality is advertised via
|
|
HWCAP_PACA, and generic authentication functionality via HWCAP_PACG.
|
|
|
|
The number of bits that the PAC occupies in a pointer is 55 minus the
|
|
virtual address size configured by the kernel. For example, with a
|
|
virtual address size of 48, the PAC is 7 bits wide.
|
|
|
|
When ARM64_PTR_AUTH_KERNEL is selected, the kernel will be compiled
|
|
with HINT space pointer authentication instructions protecting
|
|
function returns. Kernels built with this option will work on hardware
|
|
with or without pointer authentication support.
|
|
|
|
In addition to exec(), keys can also be reinitialized to random values
|
|
using the PR_PAC_RESET_KEYS prctl. A bitmask of PR_PAC_APIAKEY,
|
|
PR_PAC_APIBKEY, PR_PAC_APDAKEY, PR_PAC_APDBKEY and PR_PAC_APGAKEY
|
|
specifies which keys are to be reinitialized; specifying 0 means "all
|
|
keys".
|
|
|
|
|
|
Debugging
|
|
---------
|
|
|
|
When CONFIG_ARM64_PTR_AUTH is selected, and HW support for address
|
|
authentication is present, the kernel will expose the position of TTBR0
|
|
PAC bits in the NT_ARM_PAC_MASK regset (struct user_pac_mask), which
|
|
userspace can acquire via PTRACE_GETREGSET.
|
|
|
|
The regset is exposed only when HWCAP_PACA is set. Separate masks are
|
|
exposed for data pointers and instruction pointers, as the set of PAC
|
|
bits can vary between the two. Note that the masks apply to TTBR0
|
|
addresses, and are not valid to apply to TTBR1 addresses (e.g. kernel
|
|
pointers).
|
|
|
|
Additionally, when CONFIG_CHECKPOINT_RESTORE is also set, the kernel
|
|
will expose the NT_ARM_PACA_KEYS and NT_ARM_PACG_KEYS regsets (struct
|
|
user_pac_address_keys and struct user_pac_generic_keys). These can be
|
|
used to get and set the keys for a thread.
|
|
|
|
|
|
Virtualization
|
|
--------------
|
|
|
|
Pointer authentication is enabled in KVM guest when each virtual cpu is
|
|
initialised by passing flags KVM_ARM_VCPU_PTRAUTH_[ADDRESS/GENERIC] and
|
|
requesting these two separate cpu features to be enabled. The current KVM
|
|
guest implementation works by enabling both features together, so both
|
|
these userspace flags are checked before enabling pointer authentication.
|
|
The separate userspace flag will allow to have no userspace ABI changes
|
|
if support is added in the future to allow these two features to be
|
|
enabled independently of one another.
|
|
|
|
As Arm Architecture specifies that Pointer Authentication feature is
|
|
implemented along with the VHE feature so KVM arm64 ptrauth code relies
|
|
on VHE mode to be present.
|
|
|
|
Additionally, when these vcpu feature flags are not set then KVM will
|
|
filter out the Pointer Authentication system key registers from
|
|
KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID
|
|
register. Any attempt to use the Pointer Authentication instructions will
|
|
result in an UNDEFINED exception being injected into the guest.
|
|
|
|
|
|
Enabling and disabling keys
|
|
---------------------------
|
|
|
|
The prctl PR_PAC_SET_ENABLED_KEYS allows the user program to control which
|
|
PAC keys are enabled in a particular task. It takes two arguments, the
|
|
first being a bitmask of PR_PAC_APIAKEY, PR_PAC_APIBKEY, PR_PAC_APDAKEY
|
|
and PR_PAC_APDBKEY specifying which keys shall be affected by this prctl,
|
|
and the second being a bitmask of the same bits specifying whether the key
|
|
should be enabled or disabled. For example::
|
|
|
|
prctl(PR_PAC_SET_ENABLED_KEYS,
|
|
PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY | PR_PAC_APDBKEY,
|
|
PR_PAC_APIBKEY, 0, 0);
|
|
|
|
disables all keys except the IB key.
|
|
|
|
The main reason why this is useful is to enable a userspace ABI that uses PAC
|
|
instructions to sign and authenticate function pointers and other pointers
|
|
exposed outside of the function, while still allowing binaries conforming to
|
|
the ABI to interoperate with legacy binaries that do not sign or authenticate
|
|
pointers.
|
|
|
|
The idea is that a dynamic loader or early startup code would issue this
|
|
prctl very early after establishing that a process may load legacy binaries,
|
|
but before executing any PAC instructions.
|
|
|
|
For compatibility with previous kernel versions, processes start up with IA,
|
|
IB, DA and DB enabled, and are reset to this state on exec(). Processes created
|
|
via fork() and clone() inherit the key enabled state from the calling process.
|
|
|
|
It is recommended to avoid disabling the IA key, as this has higher performance
|
|
overhead than disabling any of the other keys.
|