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diff --git a/Documentation/arm64/pointer-authentication.rst b/Documentation/arm64/pointer-authentication.rst new file mode 100644 index 000000000000..30b2ab06526b --- /dev/null +++ b/Documentation/arm64/pointer-authentication.rst @@ -0,0 +1,109 @@ +======================================= +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. + +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 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. |