Pointer authentication in AArch64 Linux ======================================= Author: Mark Rutland 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 not currently supported in KVM guests. KVM will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of the feature will result in an UNDEFINED exception being injected into the guest.