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