When pointer authentication is in use, data/instruction pointers have a number of PAC bits inserted into them. The number and position of these bits depends on the configured TCR_ELx.TxSZ and whether tagging is enabled. ARMv8.3 allows tagging to differ for instruction and data pointers.
For userspace debuggers to unwind the stack and/or to follow pointer chains, they need to be able to remove the PAC bits before attempting to use a pointer. This patch adds a new structure with masks describing the location of the PAC bits in userspace instruction and data pointers (i.e. those addressable via TTBR0), which userspace can query via PTRACE_GETREGSET. By clearing these bits from pointers, userspace can acquire the PAC-less versions. This new regset is exposed when the kernel is built with (user) pointer authentication support, and the feature is enabled. Otherwise, it is hidden. Signed-off-by: Mark Rutland <mark.rutl...@arm.com> Cc: Catalin Marinas <catalin.mari...@arm.com> Cc: Jiong Wang <jiong.w...@arm.com> Cc: Will Deacon <will.dea...@arm.com> Cc: Yao Qi <yao...@arm.com> --- arch/arm64/include/asm/pointer_auth.h | 8 +++++++ arch/arm64/include/uapi/asm/ptrace.h | 5 +++++ arch/arm64/kernel/ptrace.c | 39 +++++++++++++++++++++++++++++++++++ include/uapi/linux/elf.h | 1 + 4 files changed, 53 insertions(+) diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h index 964da0c..ae72c7c 100644 --- a/arch/arm64/include/asm/pointer_auth.h +++ b/arch/arm64/include/asm/pointer_auth.h @@ -16,9 +16,11 @@ #ifndef __ASM_POINTER_AUTH_H #define __ASM_POINTER_AUTH_H +#include <linux/bitops.h> #include <linux/random.h> #include <asm/cpufeature.h> +#include <asm/memory.h> #include <asm/sysreg.h> #ifdef CONFIG_ARM64_POINTER_AUTHENTICATION @@ -71,6 +73,12 @@ static inline void ptrauth_keys_dup(struct ptrauth_keys *old, *new = *old; } +/* + * The pointer bits used by a pointer authentication code. + * If we were to use tagged pointers, bits 63:56 would also apply. + */ +#define ptrauth_pac_mask() GENMASK(54, VA_BITS) + #define mm_ctx_ptrauth_init(ctx) \ ptrauth_keys_init(&(ctx)->ptrauth_keys) diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h index d1ff83d..5092fbf 100644 --- a/arch/arm64/include/uapi/asm/ptrace.h +++ b/arch/arm64/include/uapi/asm/ptrace.h @@ -90,6 +90,11 @@ struct user_hwdebug_state { } dbg_regs[16]; }; +struct user_pac_mask { + __u64 data_mask; + __u64 insn_mask; +}; + #endif /* __ASSEMBLY__ */ #endif /* _UAPI__ASM_PTRACE_H */ diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 1b38c01..fae9d50 100644 --- a/arch/arm64/kernel/ptrace.c +++ b/arch/arm64/kernel/ptrace.c @@ -40,8 +40,10 @@ #include <linux/elf.h> #include <asm/compat.h> +#include <asm/cpufeature.h> #include <asm/debug-monitors.h> #include <asm/pgtable.h> +#include <asm/pointer_auth.h> #include <asm/syscall.h> #include <asm/traps.h> #include <asm/system_misc.h> @@ -701,6 +703,30 @@ static int system_call_set(struct task_struct *target, return ret; } +#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION +static int pac_mask_get(struct task_struct *target, + const struct user_regset *regset, + unsigned int pos, unsigned int count, + void *kbuf, void __user *ubuf) +{ + /* + * The PAC bits can differ across data and instruction pointers + * depending on TCR_EL1.TBID*, which we may make use of in future, so + * we expose separate masks. + */ + unsigned long mask = ptrauth_pac_mask(); + struct user_pac_mask uregs = { + .data_mask = mask, + .insn_mask = mask, + }; + + if (!cpus_have_cap(ARM64_HAS_ADDRESS_AUTH)) + return -EINVAL; + + return user_regset_copyout(&pos, &count, &kbuf, &ubuf, &uregs, 0, -1); +} +#endif + enum aarch64_regset { REGSET_GPR, REGSET_FPR, @@ -710,6 +736,9 @@ enum aarch64_regset { REGSET_HW_WATCH, #endif REGSET_SYSTEM_CALL, +#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION + REGSET_PAC_MASK, +#endif }; static const struct user_regset aarch64_regsets[] = { @@ -767,6 +796,16 @@ enum aarch64_regset { .get = system_call_get, .set = system_call_set, }, +#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION + [REGSET_PAC_MASK] = { + .core_note_type = NT_ARM_PAC_MASK, + .n = sizeof(struct user_pac_mask) / sizeof(u64), + .size = sizeof(u64), + .align = sizeof(u64), + .get = pac_mask_get, + /* this cannot be set dynamically */ + }, +#endif }; static const struct user_regset_view user_aarch64_view = { diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h index b5280db..60652f1 100644 --- a/include/uapi/linux/elf.h +++ b/include/uapi/linux/elf.h @@ -416,6 +416,7 @@ #define NT_ARM_HW_BREAK 0x402 /* ARM hardware breakpoint registers */ #define NT_ARM_HW_WATCH 0x403 /* ARM hardware watchpoint registers */ #define NT_ARM_SYSTEM_CALL 0x404 /* ARM system call number */ +#define NT_ARM_PAC_MASK 0x406 /* ARM pointer authentication code masks */ #define NT_METAG_CBUF 0x500 /* Metag catch buffer registers */ #define NT_METAG_RPIPE 0x501 /* Metag read pipeline state */ #define NT_METAG_TLS 0x502 /* Metag TLS pointer */ -- 1.9.1 _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm