arm64/signal: Expose GCS state in signal frames

Add a context for the GCS state and include it in the signal context when
running on a system that supports GCS. We reuse the same flags that the
prctl() uses to specify which GCS features are enabled and also provide the
current GCS pointer.

We do not support enabling GCS via signal return, there is a conflict
between specifying GCSPR_EL0 and allocation of a new GCS and this is not
an ancticipated use case.  We also enforce GCS configuration locking on
signal return.

Reviewed-by: Catalin Marinas <[email protected]>
Reviewed-by: Thiago Jung Bauermann <[email protected]>
Acked-by: Yury Khrustalev <[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/sigcontext.h

@@ -183,6 +183,15 @@ struct zt_context {
 	__u16 __reserved[3];
 };
 
+#define GCS_MAGIC	0x47435300
+
+struct gcs_context {
+	struct _aarch64_ctx head;
+	__u64 gcspr;
+	__u64 features_enabled;
+	__u64 reserved;
+};
+
 #endif /* !__ASSEMBLY__ */
 
 #include <asm/sve_context.h>

arch/arm64/kernel/signal.c

@@ -66,6 +66,7 @@ struct rt_sigframe_user_layout {
 
 	unsigned long fpsimd_offset;
 	unsigned long esr_offset;
+	unsigned long gcs_offset;
 	unsigned long sve_offset;
 	unsigned long tpidr2_offset;
 	unsigned long za_offset;
@@ -198,6 +199,8 @@ struct user_ctxs {
 	u32 fpmr_size;
 	struct poe_context __user *poe;
 	u32 poe_size;
+	struct gcs_context __user *gcs;
+	u32 gcs_size;
 };
 
 static int preserve_fpsimd_context(struct fpsimd_context __user *ctx)
@@ -643,6 +646,82 @@ extern int restore_zt_context(struct user_ctxs *user);
 
 #endif /* ! CONFIG_ARM64_SME */
 
+#ifdef CONFIG_ARM64_GCS
+
+static int preserve_gcs_context(struct gcs_context __user *ctx)
+{
+	int err = 0;
+	u64 gcspr = read_sysreg_s(SYS_GCSPR_EL0);
+
+	/*
+	 * If GCS is enabled we will add a cap token to the frame,
+	 * include it in the GCSPR_EL0 we report to support stack
+	 * switching via sigreturn if GCS is enabled.  We do not allow
+	 * enabling via sigreturn so the token is only relevant for
+	 * threads with GCS enabled.
+	 */
+	if (task_gcs_el0_enabled(current))
+		gcspr -= 8;
+
+	__put_user_error(GCS_MAGIC, &ctx->head.magic, err);
+	__put_user_error(sizeof(*ctx), &ctx->head.size, err);
+	__put_user_error(gcspr, &ctx->gcspr, err);
+	__put_user_error(0, &ctx->reserved, err);
+	__put_user_error(current->thread.gcs_el0_mode,
+			 &ctx->features_enabled, err);
+
+	return err;
+}
+
+static int restore_gcs_context(struct user_ctxs *user)
+{
+	u64 gcspr, enabled;
+	int err = 0;
+
+	if (user->gcs_size != sizeof(*user->gcs))
+		return -EINVAL;
+
+	__get_user_error(gcspr, &user->gcs->gcspr, err);
+	__get_user_error(enabled, &user->gcs->features_enabled, err);
+	if (err)
+		return err;
+
+	/* Don't allow unknown modes */
+	if (enabled & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK)
+		return -EINVAL;
+
+	err = gcs_check_locked(current, enabled);
+	if (err != 0)
+		return err;
+
+	/* Don't allow enabling */
+	if (!task_gcs_el0_enabled(current) &&
+	    (enabled & PR_SHADOW_STACK_ENABLE))
+		return -EINVAL;
+
+	/* If we are disabling disable everything */
+	if (!(enabled & PR_SHADOW_STACK_ENABLE))
+		enabled = 0;
+
+	current->thread.gcs_el0_mode = enabled;
+
+	/*
+	 * We let userspace set GCSPR_EL0 to anything here, we will
+	 * validate later in gcs_restore_signal().
+	 */
+	write_sysreg_s(gcspr, SYS_GCSPR_EL0);
+
+	return 0;
+}
+
+#else /* ! CONFIG_ARM64_GCS */
+
+/* Turn any non-optimised out attempts to use these into a link error: */
+extern int preserve_gcs_context(void __user *ctx);
+extern int restore_gcs_context(struct user_ctxs *user);
+
+#endif /* ! CONFIG_ARM64_GCS */
+
 static int parse_user_sigframe(struct user_ctxs *user,
 			       struct rt_sigframe __user *sf)
 {
@@ -661,6 +740,7 @@ static int parse_user_sigframe(struct user_ctxs *user,
 	user->zt = NULL;
 	user->fpmr = NULL;
 	user->poe = NULL;
+	user->gcs = NULL;
 
 	if (!IS_ALIGNED((unsigned long)base, 16))
 		goto invalid;
@@ -777,6 +857,17 @@ static int parse_user_sigframe(struct user_ctxs *user,
 			user->fpmr_size = size;
 			break;
 
+		case GCS_MAGIC:
+			if (!system_supports_gcs())
+				goto invalid;
+
+			if (user->gcs)
+				goto invalid;
+
+			user->gcs = (struct gcs_context __user *)head;
+			user->gcs_size = size;
+			break;
+
 		case EXTRA_MAGIC:
 			if (have_extra_context)
 				goto invalid;
@@ -896,6 +987,9 @@ static int restore_sigframe(struct pt_regs *regs,
 			err = restore_fpsimd_context(&user);
 	}
 
+	if (err == 0 && system_supports_gcs() && user.gcs)
+		err = restore_gcs_context(&user);
+
 	if (err == 0 && system_supports_tpidr2() && user.tpidr2)
 		err = restore_tpidr2_context(&user);
 
@@ -1029,6 +1123,15 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user,
 			return err;
 	}
 
+#ifdef CONFIG_ARM64_GCS
+	if (system_supports_gcs() && (add_all || current->thread.gcspr_el0)) {
+		err = sigframe_alloc(user, &user->gcs_offset,
+				     sizeof(struct gcs_context));
+		if (err)
+			return err;
+	}
+#endif
+
 	if (system_supports_sve() || system_supports_sme()) {
 		unsigned int vq = 0;
 
@@ -1136,6 +1239,12 @@ static int setup_sigframe(struct rt_sigframe_user_layout *user,
 		__put_user_error(current->thread.fault_code, &esr_ctx->esr, err);
 	}
 
+	if (system_supports_gcs() && err == 0 && user->gcs_offset) {
+		struct gcs_context __user *gcs_ctx =
+			apply_user_offset(user, user->gcs_offset);
+		err |= preserve_gcs_context(gcs_ctx);
+	}
+
 	/* Scalable Vector Extension state (including streaming), if present */
 	if ((system_supports_sve() || system_supports_sme()) &&
 	    err == 0 && user->sve_offset) {