rpi3: Add support for the stack protector

It uses the hardware RNG in a similar way as Juno (it gets 128 bits of
entropy and does xor on them).

It is disabled by default.

Change-Id: I8b3adb61f5a5623716e0e8b6799404c68dd94c60
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

docs/plat/rpi3.rst

@@ -212,6 +212,9 @@ instructions in `Setup SD card`_.
 
 The following build options are supported:
 
+- ``ENABLE_STACK_PROTECTOR``: Disabled by default. It uses the hardware RNG of
+  the board.
+
 - ``PRELOADED_BL33_BASE``: Specially useful because the file ``kernel8.img`` can
   be loaded anywhere by modifying the file ``config.txt``. It doesn't have to
   contain a kernel, it could have any arbitrary payload.

plat/rpi3/platform.mk

@@ -90,6 +90,9 @@ WORKAROUND_CVE_2017_5715	:= 0
 # Disable the PSCI platform compatibility layer by default
 ENABLE_PLAT_COMPAT		:= 0
 
+# Disable stack protector by default
+ENABLE_STACK_PROTECTOR	 	:= 0
+
 # Reset to BL31 isn't supported
 RESET_TO_BL31			:= 0
 
@@ -146,6 +149,11 @@ ifeq (${ARCH},aarch32)
   $(error Error: AArch32 not supported on rpi3)
 endif
 
+ifneq ($(ENABLE_STACK_PROTECTOR), 0)
+PLAT_BL_COMMON_SOURCES	+=	plat/rpi3/rpi3_rng.c			\
+				plat/rpi3/rpi3_stack_protector.c
+endif
+
 ifeq (${SPD},opteed)
 BL2_SOURCES	+=							\
 		lib/optee/optee_utils.c

plat/rpi3/rpi3_stack_protector.c

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <sys/types.h>
+#include <utils.h>
+
+#include "rpi3_private.h"
+
+/* Get 128 bits of entropy and fuse the values together to form the canary. */
+#define TRNG_NBYTES	16U
+
+u_register_t plat_get_stack_protector_canary(void)
+{
+	size_t i;
+	u_register_t buf[TRNG_NBYTES / sizeof(u_register_t)];
+	u_register_t ret = 0U;
+
+	rpi3_rng_read(buf, sizeof(buf));
+
+	for (i = 0U; i < ARRAY_SIZE(buf); i++)
+		ret ^= buf[i];
+
+	return ret;
+}