kselftest: arm64: Check access to GCS after mprotect(PROT_NONE)

A GCS mapping should not be accessible after mprotect(PROT_NONE). Add a
kselftest for this scenario.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

Author: ctmarinas

tools/testing/selftests/arm64/signal/testcases/gcs_prot_none_fault.c

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2026 ARM Limited
+ */
+
+#include <errno.h>
+#include <signal.h>
+#include <unistd.h>
+
+#include <sys/mman.h>
+#include <sys/prctl.h>
+
+#include "test_signals_utils.h"
+#include "testcases.h"
+
+static uint64_t *gcs_page;
+static bool post_mprotect;
+
+#ifndef __NR_map_shadow_stack
+#define __NR_map_shadow_stack 453
+#endif
+
+static bool alloc_gcs(struct tdescr *td)
+{
+	long page_size = sysconf(_SC_PAGE_SIZE);
+
+	gcs_page = (void *)syscall(__NR_map_shadow_stack, 0,
+				   page_size, 0);
+	if (gcs_page == MAP_FAILED) {
+		fprintf(stderr, "Failed to map %ld byte GCS: %d\n",
+			page_size, errno);
+		return false;
+	}
+
+	return true;
+}
+
+static int gcs_prot_none_fault_trigger(struct tdescr *td)
+{
+	/* Verify that the page is readable (ie, not completely unmapped) */
+	fprintf(stderr, "Read value 0x%lx\n", gcs_page[0]);
+
+	if (mprotect(gcs_page, sysconf(_SC_PAGE_SIZE), PROT_NONE) != 0) {
+		fprintf(stderr, "mprotect(PROT_NONE) failed: %d\n", errno);
+		return 0;
+	}
+	post_mprotect = true;
+
+	/* This should trigger a fault if PROT_NONE is honoured for the GCS page */
+	fprintf(stderr, "Read value after mprotect(PROT_NONE) 0x%lx\n", gcs_page[0]);
+	return 0;
+}
+
+static int gcs_prot_none_fault_signal(struct tdescr *td, siginfo_t *si,
+				      ucontext_t *uc)
+{
+	ASSERT_GOOD_CONTEXT(uc);
+
+	/* A fault before mprotect(PROT_NONE) is unexpected. */
+	if (!post_mprotect)
+		return 0;
+
+	return 1;
+}
+
+struct tdescr tde = {
+	.name = "GCS PROT_NONE fault",
+	.descr = "Read from GCS after mprotect(PROT_NONE) segfaults",
+	.feats_required = FEAT_GCS,
+	.timeout = 3,
+	.sig_ok = SIGSEGV,
+	.sanity_disabled = true,
+	.init = alloc_gcs,
+	.trigger = gcs_prot_none_fault_trigger,
+	.run = gcs_prot_none_fault_signal,
+};