kselftest/arm64: Always run signals tests with GCS enabled

Since it is not possible to return from the function that enabled GCS
without disabling GCS it is very inconvenient to use the signal handling
tests to cover GCS when GCS is not enabled by the toolchain and runtime,
something that no current distribution does. Since none of the testcases
do anything with stacks that would cause problems with GCS we can sidestep
this issue by unconditionally enabling GCS on startup and exiting with a
call to exit() rather than a return from main().

Reviewed-by: Thiago Jung Bauermann <[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

tools/testing/selftests/arm64/signal/test_signals.c

@@ -7,6 +7,10 @@
  * Each test provides its own tde struct tdescr descriptor to link with
  * this wrapper. Framework provides common helpers.
  */
+
+#include <sys/auxv.h>
+#include <sys/prctl.h>
+
 #include <kselftest.h>
 
 #include "test_signals.h"
@@ -16,12 +20,23 @@ struct tdescr *current = &tde;
 
 int main(int argc, char *argv[])
 {
+	/*
+	 * Ensure GCS is at least enabled throughout the tests if
+	 * supported, otherwise the inability to return from the
+	 * function that enabled GCS makes it very inconvenient to set
+	 * up test cases.  The prctl() may fail if GCS was locked by
+	 * libc setup code.
+	 */
+	if (getauxval(AT_HWCAP) & HWCAP_GCS)
+		gcs_set_state(PR_SHADOW_STACK_ENABLE);
+
 	ksft_print_msg("%s :: %s\n", current->name, current->descr);
 	if (test_setup(current) && test_init(current)) {
 		test_run(current);
 		test_cleanup(current);
 	}
 	test_result(current);
 
-	return current->result;
+	/* Do not return in case GCS was enabled */
+	exit(current->result);
 }

tools/testing/selftests/arm64/signal/test_signals_utils.h

@@ -18,6 +18,35 @@ void test_cleanup(struct tdescr *td);
 int test_run(struct tdescr *td);
 void test_result(struct tdescr *td);
 
+#ifndef __NR_prctl
+#define __NR_prctl 167
+#endif
+
+/*
+ * The prctl takes 1 argument but we need to ensure that the other
+ * values passed in registers to the syscall are zero since the kernel
+ * validates them.
+ */
+#define gcs_set_state(state)					\
+	({								\
+		register long _num  __asm__ ("x8") = __NR_prctl;	\
+		register long _arg1 __asm__ ("x0") =  PR_SET_SHADOW_STACK_STATUS; \
+		register long _arg2 __asm__ ("x1") = (long)(state);	\
+		register long _arg3 __asm__ ("x2") = 0;			\
+		register long _arg4 __asm__ ("x3") = 0;			\
+		register long _arg5 __asm__ ("x4") = 0;			\
+	                                                                      \
+		__asm__  volatile (					\
+			"svc #0\n"					\
+			: "=r"(_arg1)					\
+			: "r"(_arg1), "r"(_arg2),			\
+			  "r"(_arg3), "r"(_arg4),			\
+			  "r"(_arg5), "r"(_num)				\
+			: "memory", "cc"				\
+			);						\
+		_arg1;							\
+	})
+
 static inline bool feats_ok(struct tdescr *td)
 {
 	if (td->feats_incompatible & td->feats_supported)