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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.

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