arm64/ptrace: Expose GCS via ptrace and core files

Provide a new register type NT_ARM_GCS reporting the current GCS mode
and pointer for EL0.  Due to the interactions with allocation and
deallocation of Guarded Control Stacks we do not permit any changes to
the GCS mode via ptrace, only GCSPR_EL0 may be changed.

Reviewed-by: Thiago Jung Bauermann <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Signed-off-by: Mark Brown <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: broonie

arch/arm64/include/uapi/asm/ptrace.h

@@ -324,6 +324,14 @@ struct user_za_header {
 #define ZA_PT_SIZE(vq)						\
 	(ZA_PT_ZA_OFFSET + ZA_PT_ZA_SIZE(vq))
 
+/* GCS state (NT_ARM_GCS) */
+
+struct user_gcs {
+	__u64 features_enabled;
+	__u64 features_locked;
+	__u64 gcspr_el0;
+};
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _UAPI__ASM_PTRACE_H */

arch/arm64/kernel/ptrace.c

@@ -34,6 +34,7 @@
 #include <asm/cpufeature.h>
 #include <asm/debug-monitors.h>
 #include <asm/fpsimd.h>
+#include <asm/gcs.h>
 #include <asm/mte.h>
 #include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
@@ -1473,6 +1474,52 @@ static int poe_set(struct task_struct *target, const struct
 }
 #endif
 
+#ifdef CONFIG_ARM64_GCS
+static int gcs_get(struct task_struct *target,
+		   const struct user_regset *regset,
+		   struct membuf to)
+{
+	struct user_gcs user_gcs;
+
+	if (!system_supports_gcs())
+		return -EINVAL;
+
+	if (target == current)
+		gcs_preserve_current_state();
+
+	user_gcs.features_enabled = target->thread.gcs_el0_mode;
+	user_gcs.features_locked = target->thread.gcs_el0_locked;
+	user_gcs.gcspr_el0 = target->thread.gcspr_el0;
+
+	return membuf_write(&to, &user_gcs, sizeof(user_gcs));
+}
+
+static int gcs_set(struct task_struct *target, const struct
+		   user_regset *regset, unsigned int pos,
+		   unsigned int count, const void *kbuf, const
+		   void __user *ubuf)
+{
+	int ret;
+	struct user_gcs user_gcs;
+
+	if (!system_supports_gcs())
+		return -EINVAL;
+
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &user_gcs, 0, -1);
+	if (ret)
+		return ret;
+
+	if (user_gcs.features_enabled & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK)
+		return -EINVAL;
+
+	target->thread.gcs_el0_mode = user_gcs.features_enabled;
+	target->thread.gcs_el0_locked = user_gcs.features_locked;
+	target->thread.gcspr_el0 = user_gcs.gcspr_el0;
+
+	return 0;
+}
+#endif
+
 enum aarch64_regset {
 	REGSET_GPR,
 	REGSET_FPR,
@@ -1503,7 +1550,10 @@ enum aarch64_regset {
 	REGSET_TAGGED_ADDR_CTRL,
 #endif
 #ifdef CONFIG_ARM64_POE
-	REGSET_POE
+	REGSET_POE,
+#endif
+#ifdef CONFIG_ARM64_GCS
+	REGSET_GCS,
 #endif
 };
 
@@ -1674,6 +1724,16 @@ static const struct user_regset aarch64_regsets[] = {
 		.set = poe_set,
 	},
 #endif
+#ifdef CONFIG_ARM64_GCS
+	[REGSET_GCS] = {
+		.core_note_type = NT_ARM_GCS,
+		.n = sizeof(struct user_gcs) / sizeof(u64),
+		.size = sizeof(u64),
+		.align = sizeof(u64),
+		.regset_get = gcs_get,
+		.set = gcs_set,
+	},
+#endif
 };
 
 static const struct user_regset_view user_aarch64_view = {

include/uapi/linux/elf.h

@@ -443,6 +443,7 @@ typedef struct elf64_shdr {
 #define NT_ARM_ZT	0x40d		/* ARM SME ZT registers */
 #define NT_ARM_FPMR	0x40e		/* ARM floating point mode register */
 #define NT_ARM_POE	0x40f		/* ARM POE registers */
+#define NT_ARM_GCS	0x410		/* ARM GCS state */
 #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 */