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Pointer authentication in AArch64 Linux
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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.

Recent versions of GCC can compile code with APIAKey-based return
address protection when passed the -msign-return-address option. This
uses instructions in the HINT space (unless -march=armv8.3-a or higher
is also passed), and such code can run on systems without the pointer
authentication extension.

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.