1======================================= 2Pointer authentication in AArch64 Linux 3======================================= 4 5Author: Mark Rutland <mark.rutland@arm.com> 6 7Date: 2017-07-19 8 9This document briefly describes the provision of pointer authentication 10functionality in AArch64 Linux. 11 12 13Architecture overview 14--------------------- 15 16The ARMv8.3 Pointer Authentication extension adds primitives that can be 17used to mitigate certain classes of attack where an attacker can corrupt 18the contents of some memory (e.g. the stack). 19 20The extension uses a Pointer Authentication Code (PAC) to determine 21whether pointers have been modified unexpectedly. A PAC is derived from 22a pointer, another value (such as the stack pointer), and a secret key 23held in system registers. 24 25The extension adds instructions to insert a valid PAC into a pointer, 26and to verify/remove the PAC from a pointer. The PAC occupies a number 27of high-order bits of the pointer, which varies dependent on the 28configured virtual address size and whether pointer tagging is in use. 29 30A subset of these instructions have been allocated from the HINT 31encoding space. In the absence of the extension (or when disabled), 32these instructions behave as NOPs. Applications and libraries using 33these instructions operate correctly regardless of the presence of the 34extension. 35 36The extension provides five separate keys to generate PACs - two for 37instruction addresses (APIAKey, APIBKey), two for data addresses 38(APDAKey, APDBKey), and one for generic authentication (APGAKey). 39 40 41Basic support 42------------- 43 44When CONFIG_ARM64_PTR_AUTH is selected, and relevant HW support is 45present, the kernel will assign random key values to each process at 46exec*() time. The keys are shared by all threads within the process, and 47are preserved across fork(). 48 49Presence of address authentication functionality is advertised via 50HWCAP_PACA, and generic authentication functionality via HWCAP_PACG. 51 52The number of bits that the PAC occupies in a pointer is 55 minus the 53virtual address size configured by the kernel. For example, with a 54virtual address size of 48, the PAC is 7 bits wide. 55 56Recent versions of GCC can compile code with APIAKey-based return 57address protection when passed the -msign-return-address option. This 58uses instructions in the HINT space (unless -march=armv8.3-a or higher 59is also passed), and such code can run on systems without the pointer 60authentication extension. 61 62In addition to exec(), keys can also be reinitialized to random values 63using the PR_PAC_RESET_KEYS prctl. A bitmask of PR_PAC_APIAKEY, 64PR_PAC_APIBKEY, PR_PAC_APDAKEY, PR_PAC_APDBKEY and PR_PAC_APGAKEY 65specifies which keys are to be reinitialized; specifying 0 means "all 66keys". 67 68 69Debugging 70--------- 71 72When CONFIG_ARM64_PTR_AUTH is selected, and HW support for address 73authentication is present, the kernel will expose the position of TTBR0 74PAC bits in the NT_ARM_PAC_MASK regset (struct user_pac_mask), which 75userspace can acquire via PTRACE_GETREGSET. 76 77The regset is exposed only when HWCAP_PACA is set. Separate masks are 78exposed for data pointers and instruction pointers, as the set of PAC 79bits can vary between the two. Note that the masks apply to TTBR0 80addresses, and are not valid to apply to TTBR1 addresses (e.g. kernel 81pointers). 82 83Additionally, when CONFIG_CHECKPOINT_RESTORE is also set, the kernel 84will expose the NT_ARM_PACA_KEYS and NT_ARM_PACG_KEYS regsets (struct 85user_pac_address_keys and struct user_pac_generic_keys). These can be 86used to get and set the keys for a thread. 87 88 89Virtualization 90-------------- 91 92Pointer authentication is enabled in KVM guest when each virtual cpu is 93initialised by passing flags KVM_ARM_VCPU_PTRAUTH_[ADDRESS/GENERIC] and 94requesting these two separate cpu features to be enabled. The current KVM 95guest implementation works by enabling both features together, so both 96these userspace flags are checked before enabling pointer authentication. 97The separate userspace flag will allow to have no userspace ABI changes 98if support is added in the future to allow these two features to be 99enabled independently of one another. 100 101As Arm Architecture specifies that Pointer Authentication feature is 102implemented along with the VHE feature so KVM arm64 ptrauth code relies 103on VHE mode to be present. 104 105Additionally, when these vcpu feature flags are not set then KVM will 106filter out the Pointer Authentication system key registers from 107KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID 108register. Any attempt to use the Pointer Authentication instructions will 109result in an UNDEFINED exception being injected into the guest. 110