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+=======================================
+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.