arm64: expose user PAC bit positions via ptrace

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 (and replacing them with the value
of bit 55), userspace can acquire the PAC-less versions.

This new regset is exposed when the kernel is built with (user) pointer
authentication support, and the address authentication feature is
enabled. Otherwise, the regset is hidden.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
[will: Fix to use vabits_user instead of VA_BITS and rename macro]
Signed-off-by: Will Deacon <will.deacon@arm.com>

6 files changed, 57 insertions, 2 deletions

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 6747a3eddeb1..bd749033bc88 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -207,6 +207,9 @@ static inline unsigned long kaslr_offset(void)
         return kimage_vaddr - KIMAGE_VADDR;
 }
 
+/* the actual size of a user virtual address */
+extern u64                      vabits_user;
+
 /*
  * Allow all memory at the discovery stage. We will clip it later.
  */
diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index 91c4185dda5b..2a22c03c1540 100644
--- a/arch/arm64/include/asm/pointer_auth.h
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -2,9 +2,11 @@
 #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_PTR_AUTH
@@ -61,6 +63,12 @@ static inline void ptrauth_keys_switch(struct ptrauth_keys *keys)
                 __ptrauth_key_install(APGA, keys->apga);
 }
 
+/*
+ * The EL0 pointer bits used by a pointer authentication code.
+ * This is dependent on TBI0 being enabled, or bits 63:56 would also apply.
+ */
+#define ptrauth_user_pac_mask() GENMASK(54, vabits_user)
+
 #define ptrauth_thread_init_user(tsk)                                   \
 do {                                                                    \
         struct task_struct *__ptiu_tsk = (tsk);                         \
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index bbecc6fe3e5b..f4b8e09aff56 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -53,8 +53,6 @@
  */
 
 #define DEFAULT_MAP_WINDOW_64   (UL(1) << VA_BITS)
-
-extern u64 vabits_user;
 #define TASK_SIZE_64            (UL(1) << vabits_user)
 
 #ifdef CONFIG_COMPAT
diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
index a36227fdb084..c2f249bcd829 100644
--- a/arch/arm64/include/uapi/asm/ptrace.h
+++ b/arch/arm64/include/uapi/asm/ptrace.h
@@ -229,6 +229,13 @@ struct user_sve_header {
                   SVE_PT_SVE_OFFSET + SVE_PT_SVE_SIZE(vq, flags)        \
                 : SVE_PT_FPSIMD_OFFSET + SVE_PT_FPSIMD_SIZE(vq, flags))
 
+/* pointer authentication masks (NT_ARM_PAC_MASK) */
+
+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 1710a2d01669..9dce33b0e260 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -46,6 +46,7 @@
 #include <asm/debug-monitors.h>
 #include <asm/fpsimd.h>
 #include <asm/pgtable.h>
+#include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
 #include <asm/syscall.h>
 #include <asm/traps.h>
@@ -956,6 +957,30 @@ out:
 
 #endif /* CONFIG_ARM64_SVE */
 
+#ifdef CONFIG_ARM64_PTR_AUTH
+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_user_pac_mask();
+        struct user_pac_mask uregs = {
+                .data_mask = mask,
+                .insn_mask = mask,
+        };
+
+        if (!system_supports_address_auth())
+                return -EINVAL;
+
+        return user_regset_copyout(&pos, &count, &kbuf, &ubuf, &uregs, 0, -1);
+}
+#endif /* CONFIG_ARM64_PTR_AUTH */
+
 enum aarch64_regset {
         REGSET_GPR,
         REGSET_FPR,
@@ -968,6 +993,9 @@ enum aarch64_regset {
 #ifdef CONFIG_ARM64_SVE
         REGSET_SVE,
 #endif
+#ifdef CONFIG_ARM64_PTR_AUTH
+        REGSET_PAC_MASK,
+#endif
 };
 
 static const struct user_regset aarch64_regsets[] = {
@@ -1037,6 +1065,16 @@ static const struct user_regset aarch64_regsets[] = {
                 .get_size = sve_get_size,
         },
 #endif
+#ifdef CONFIG_ARM64_PTR_AUTH
+        [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 c5358e0ae7c5..3f23273d690c 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -420,6 +420,7 @@ typedef struct elf64_shdr {
 #define NT_ARM_HW_WATCH 0x403           /* ARM hardware watchpoint registers */
 #define NT_ARM_SYSTEM_CALL      0x404   /* ARM system call number */
 #define NT_ARM_SVE      0x405           /* ARM Scalable Vector Extension registers */
+#define NT_ARM_PAC_MASK         0x406   /* ARM pointer authentication code masks */
 #define NT_ARC_V2       0x600           /* ARCv2 accumulator/extra registers */
 #define NT_VMCOREDD     0x700           /* Vmcore Device Dump Note */
 #define NT_MIPS_DSP     0x800           /* MIPS DSP ASE registers */