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Subject[PATCHv2 07/12] arm64: expose user PAC bit positions via ptrace
Date
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.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Yao Qi <yao.qi@arm.com>
---
arch/arm64/include/asm/pointer_auth.h | 8 ++++++++
arch/arm64/include/uapi/asm/ptrace.h | 7 +++++++
arch/arm64/kernel/ptrace.c | 38 +++++++++++++++++++++++++++++++++++
include/uapi/linux/elf.h | 1 +
4 files changed, 54 insertions(+)

diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index 964da0c3dc48..b08ebec4b806 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 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_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 98c4ce55d9c3..4994d718771a 100644
--- a/arch/arm64/include/uapi/asm/ptrace.h
+++ b/arch/arm64/include/uapi/asm/ptrace.h
@@ -228,6 +228,13 @@ 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 7c44658b316d..6901dce44c8d 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -44,6 +44,7 @@
#include <asm/cpufeature.h>
#include <asm/debug-monitors.h>
#include <asm/pgtable.h>
+#include <asm/pointer_auth.h>
#include <asm/stacktrace.h>
#include <asm/syscall.h>
#include <asm/traps.h>
@@ -951,6 +952,30 @@ static int sve_set(struct task_struct *target,

#endif /* CONFIG_ARM64_SVE */

+#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 /* CONFIG_ARM64_POINTER_AUTHENTICATION */
+
enum aarch64_regset {
REGSET_GPR,
REGSET_FPR,
@@ -963,6 +988,9 @@ enum aarch64_regset {
#ifdef CONFIG_ARM64_SVE
REGSET_SVE,
#endif
+#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
+ REGSET_PAC_MASK,
+#endif
};

static const struct user_regset aarch64_regsets[] = {
@@ -1032,6 +1060,16 @@ static const struct user_regset aarch64_regsets[] = {
.get_size = sve_get_size,
},
#endif
+#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 bb6836986200..4ca58005e04a 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -419,6 +419,7 @@ 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_METAG_CBUF 0x500 /* Metag catch buffer registers */
#define NT_METAG_RPIPE 0x501 /* Metag read pipeline state */
#define NT_METAG_TLS 0x502 /* Metag TLS pointer */
--
2.11.0
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