[PATCH v5 3/6] selftests/x86: add shadow stack lock test

[Bill Roberts <bill.roberts@arm.com>]

The shadow stack selftests do not currently verify that locking
a feature prevents subsequent changes to its enabled state.

Add a test that locks the WRSS feature while enabled and verifies
that attempting to disable it fails with -EPERM.

Despite WRSS being locked, this allows the existing code in main()
to disable shadow stack before returning to prevent a SIGSEGV from
a shadow stack underflow. This is why shadow stack enable is not
locked.

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
---
 .../testing/selftests/x86/test_shadow_stack.c | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/tools/testing/selftests/x86/test_shadow_stack.c b/tools/testing/selftests/x86/test_shadow_stack.c
index b52c5420c137..e047f1ceae0a 100644
--- a/tools/testing/selftests/x86/test_shadow_stack.c
+++ b/tools/testing/selftests/x86/test_shadow_stack.c
@@ -1060,9 +1060,28 @@ int test_ptrace(void)
 	return 1;
 }
 
+static int test_locking(void)
+{
+
+	if (ARCH_PRCTL(ARCH_SHSTK_LOCK, ARCH_SHSTK_WRSS)) {
+		printf("[FAIL]\tCould not lock Shadow stack write\n");
+		return 1;
+	}
+
+	if (ARCH_PRCTL(ARCH_SHSTK_DISABLE, ARCH_SHSTK_WRSS) != -EPERM) {
+		printf("[FAIL]\tCould change Shadow stack write after lock\n");
+		return 1;
+	}
+
+	printf("[OK]\tShadow stack locking\n");
+
+	return 0;
+}
+
 int main(int argc, char *argv[])
 {
 	int ret = 0;
+	unsigned long status = 0;
 
 	if (ARCH_PRCTL(ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK)) {
 		printf("[SKIP]\tCould not enable Shadow stack\n");
@@ -1079,12 +1098,31 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
+	/* Note: test_shadow_stack_lock() needs write enabled */
 	if (ARCH_PRCTL(ARCH_SHSTK_ENABLE, ARCH_SHSTK_WRSS)) {
 		printf("[SKIP]\tCould not enable WRSS\n");
 		ret = 1;
 		goto out;
 	}
 
+	if (ARCH_PRCTL(ARCH_SHSTK_STATUS, &status)) {
+		printf("[FAIL]\tCould not get Shadow stack status\n");
+		ret = 1;
+		goto out;
+	}
+
+	if (status != (ARCH_SHSTK_WRSS|ARCH_SHSTK_SHSTK)) {
+		printf("[FAIL]\tStatus not as expected, got 0x%lx status, wanted: 0x%llx\n",
+			status, (ARCH_SHSTK_WRSS|ARCH_SHSTK_SHSTK));
+		ret = 1;
+		goto out;
+	}
+
+	if (test_locking()) {
+		ret = 1;
+		goto out;
+	}
+
 	/* Should have succeeded if here, but this is a test, so double check. */
 	if (!get_ssp()) {
 		printf("[FAIL]\tShadow stack disabled\n");
-- 
2.55.0