plat/qemu: Use RNDR in stack protector

When getting a stack protector canary value, check
if cpu supports FEAT_RNG and use that. Fallback to
old method of using a (hardcoded value ^ timer).
Signed-off-by: Tomas Pilar <tomas@nuviainc.com>
Change-Id: I8181acf8e31661d4cc82bc3a4078f8751909e725

plat/qemu/common/qemu_stack_protector.c

 /*
- * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2021, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -7,17 +7,25 @@
 #include <stdint.h>
 
 #include <arch_helpers.h>
+#include <arch_features.h>
 #include <plat/common/platform.h>
 
 #define RANDOM_CANARY_VALUE ((u_register_t) 3288484550995823360ULL)
 
 u_register_t plat_get_stack_protector_canary(void)
 {
+#if ENABLE_FEAT_RNG
+	/* Use the RNDR instruction if the CPU supports it */
+	if (is_armv8_5_rng_present()) {
+		return read_rndr();
+	}
+#endif
+
 	/*
-	 * Ideally, a random number should be returned instead of the
+	 * Ideally, a random number should be returned above. If a random
+	 * number generator is not supported, return instead a
 	 * combination of a timer's value and a compile-time constant.
-	 * As the virt platform does not have any random number generator,
-	 * this is better than nothing but not necessarily really secure.
+	 * This is better than nothing but not necessarily really secure.
 	 */
 	return RANDOM_CANARY_VALUE ^ read_cntpct_el0();
 }