Add support for GCC stack protection

Introduce new build option ENABLE_STACK_PROTECTOR. It enables
compilation of all BL images with one of the GCC -fstack-protector-*
options.

A new platform function plat_get_stack_protector_canary() is introduced.
It returns a value that is used to initialize the canary for stack
corruption detection. Returning a random value will prevent an attacker
from predicting the value and greatly increase the effectiveness of the
protection.

A message is printed at the ERROR level when a stack corruption is
detected.

To be effective, the global data must be stored at an address
lower than the base of the stacks. Failure to do so would allow an
attacker to overwrite the canary as part of an attack which would void
the protection.

FVP implementation of plat_get_stack_protector_canary is weak as
there is no real source of entropy on the FVP. It therefore relies on a
timer's value, which could be predictable.

Change-Id: Icaaee96392733b721fa7c86a81d03660d3c1bc06
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>

Makefile

@@ -246,6 +246,12 @@ endif
         # over the sources.
 endif
 
+################################################################################
+# Include libraries' Makefile that are used in all BL
+################################################################################
+
+include lib/stack_protector/stack_protector.mk
+
 
 ################################################################################
 # Include the platform specific Makefile after the SPD Makefile (the platform

bl1/bl1.ld.S

@@ -111,14 +111,20 @@ SECTIONS
     ASSERT(__CPU_OPS_END__ > __CPU_OPS_START__,
            "cpu_ops not defined for this platform.")
 
+    . = BL1_RW_BASE;
+    ASSERT(BL1_RW_BASE == ALIGN(4096),
+           "BL1_RW_BASE address is not aligned on a page boundary.")
+
     /*
      * The .data section gets copied from ROM to RAM at runtime.
-     * Its LMA must be 16-byte aligned.
+     * Its LMA should be 16-byte aligned to allow efficient copying of 16-bytes
+     * aligned regions in it.
      * Its VMA must be page-aligned as it marks the first read/write page.
+     *
+     * It must be placed at a lower address than the stacks if the stack
+     * protector is enabled. Alternatively, the .data.stack_protector_canary
+     * section can be placed independently of the main .data section.
      */
-    . = BL1_RW_BASE;
-    ASSERT(. == ALIGN(4096),
-           "BL1_RW_BASE address is not aligned on a page boundary.")
     .data . : ALIGN(16) {
         __DATA_RAM_START__ = .;
         *(.data*)

bl2/aarch32/bl2_entrypoint.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -121,6 +121,15 @@ func bl2_entrypoint
 	 */
 	bl	plat_set_my_stack
 
+	/* ---------------------------------------------
+	 * Initialize the stack protector canary before
+	 * any C code is called.
+	 * ---------------------------------------------
+	 */
+#if STACK_PROTECTOR_ENABLED
+	bl	update_stack_protector_canary
+#endif
+
 	/* ---------------------------------------------
 	 * Perform early platform setup & platform
 	 * specific early arch. setup e.g. mmu setup

bl2/aarch64/bl2_entrypoint.S

@@ -112,6 +112,15 @@ func bl2_entrypoint
 	 */
 	bl	plat_set_my_stack
 
+	/* ---------------------------------------------
+	 * Initialize the stack protector canary before
+	 * any C code is called.
+	 * ---------------------------------------------
+	 */
+#if STACK_PROTECTOR_ENABLED
+	bl	update_stack_protector_canary
+#endif
+
 	/* ---------------------------------------------
 	 * Perform early platform setup & platform
 	 * specific early arch. setup e.g. mmu setup

bl2/bl2.ld.S

@@ -99,6 +99,11 @@ SECTIONS
      */
     __RW_START__ = . ;
 
+    /*
+     * .data must be placed at a lower address than the stacks if the stack
+     * protector is enabled. Alternatively, the .data.stack_protector_canary
+     * section can be placed independently of the main .data section.
+     */
     .data . : {
         __DATA_START__ = .;
         *(.data*)

bl2u/aarch64/bl2u_entrypoint.S

@@ -106,6 +106,15 @@ func bl2u_entrypoint
 	 */
 	bl	plat_set_my_stack
 
+	/* ---------------------------------------------
+	 * Initialize the stack protector canary before
+	 * any C code is called.
+	 * ---------------------------------------------
+	 */
+#if STACK_PROTECTOR_ENABLED
+	bl	update_stack_protector_canary
+#endif
+
 	/* ---------------------------------------------
 	 * Perform early platform setup & platform
 	 * specific early arch. setup e.g. mmu setup

bl2u/bl2u.ld.S

@@ -86,6 +86,11 @@ SECTIONS
      */
     __RW_START__ = . ;
 
+    /*
+     * .data must be placed at a lower address than the stacks if the stack
+     * protector is enabled. Alternatively, the .data.stack_protector_canary
+     * section can be placed independently of the main .data section.
+     */
     .data . : {
         __DATA_START__ = .;
         *(.data*)

bl31/bl31.ld.S

@@ -140,7 +140,12 @@ SECTIONS
      */
     __RW_START__ = . ;
 
-    .data . : {
+    /*
+     * .data must be placed at a lower address than the stacks if the stack
+     * protector is enabled. Alternatively, the .data.stack_protector_canary
+     * section can be placed independently of the main .data section.
+     */
+   .data . : {
         __DATA_START__ = .;
         *(.data*)
         __DATA_END__ = .;

bl32/tsp/aarch64/tsp_entrypoint.S

@@ -138,6 +138,15 @@ func tsp_entrypoint
 	 */
 	bl	plat_set_my_stack
 
+	/* ---------------------------------------------
+	 * Initialize the stack protector canary before
+	 * any C code is called.
+	 * ---------------------------------------------
+	 */
+#if STACK_PROTECTOR_ENABLED
+	bl	update_stack_protector_canary
+#endif
+
 	/* ---------------------------------------------
 	 * Perform early platform setup & platform
 	 * specific early arch. setup e.g. mmu setup

docs/porting-guide.md

@@ -920,6 +920,20 @@ kept aside to pass trusted firmware related information that next BL image
 needs. This function is currently invoked in BL2 to pass this information to
 the next BL image, when LOAD_IMAGE_V2 is enabled.
 
+### Function : plat_get_stack_protector_canary()
+    Argument : void
+    Return   : u_register_t
+
+This function returns a random value that is used to initialize the canary used
+when the stack protector is enabled with ENABLE_STACK_PROTECTOR. A predictable
+value will weaken the protection as the attacker could easily write the right
+value as part of the attack most of the time. Therefore, it should return a
+true random number.
+
+Note: For the protection to be effective, the global data need to be placed at
+a lower address than the stack bases. Failure to do so would allow an attacker
+to overwrite the canary as part of the stack buffer overflow attack.
+
 ### Function : plat_flush_next_bl_params()
 
     Argument : void

docs/user-guide.md

@@ -301,6 +301,14 @@ performed.
     Currently, only PSCI is instrumented. Enabling this option enables
     the `ENABLE_PMF` build option as well. Default is 0.
 
+*   `ENABLE_STACK_PROTECTOR`: String option to enable the stack protection
+    checks in GCC. Allowed values are "all", "strong" and "0" (default).
+    "strong" is the recommended stack protection level if this feature is
+    desired. 0 disables the stack protection. For all values other than 0, the
+    `plat_get_stack_protector_canary()` platform hook needs to be implemented.
+    The value is passed as the last component of the option
+    `-fstack-protector-$ENABLE_STACK_PROTECTOR`.
+
 *   `ERROR_DEPRECATED`: This option decides whether to treat the usage of
     deprecated platform APIs, helper functions or drivers within Trusted
     Firmware as error. It can take the value 1 (flag the use of deprecated

include/common/aarch32/el3_common_macros.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -278,6 +278,12 @@
 	 * ---------------------------------------------------------------------
 	 */
 	bl	plat_set_my_stack
+
+#if STACK_PROTECTOR_ENABLED
+	.if \_init_c_runtime
+	bl	update_stack_protector_canary
+	.endif /* _init_c_runtime */
+#endif
 	.endm
 
 #endif /* __EL3_COMMON_MACROS_S__ */

include/common/aarch64/el3_common_macros.S

@@ -283,6 +283,12 @@
 	 * ---------------------------------------------------------------------
 	 */
 	bl	plat_set_my_stack
+
+#if STACK_PROTECTOR_ENABLED
+	.if \_init_c_runtime
+	bl	update_stack_protector_canary
+	.endif /* _init_c_runtime */
+#endif
 	.endm
 
 #endif /* __EL3_COMMON_MACROS_S__ */

include/common/debug.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -84,6 +84,9 @@
 void __dead2 do_panic(void);
 #define panic()	do_panic()
 
+/* Function called when stack protection check code detects a corrupted stack */
+void __dead2 __stack_chk_fail(void);
+
 void tf_printf(const char *fmt, ...) __printflike(1, 2);
 
 #endif /* __ASSEMBLY__ */

include/plat/common/platform.h

@@ -72,6 +72,16 @@ uintptr_t plat_get_ns_image_entrypoint(void);
 unsigned int plat_my_core_pos(void);
 int plat_core_pos_by_mpidr(u_register_t mpidr);
 
+#if STACK_PROTECTOR_ENABLED
+/*
+ * Return a new value to be used for the stack protection's canary.
+ *
+ * Ideally, this value is a random number that is impossible to predict by an
+ * attacker.
+ */
+u_register_t plat_get_stack_protector_canary(void);
+#endif /* STACK_PROTECTOR_ENABLED */
+
 /*******************************************************************************
  * Mandatory interrupt management functions
  ******************************************************************************/
@@ -326,7 +336,7 @@ int platform_setup_pm(const plat_pm_ops_t **);
 
 unsigned int plat_get_aff_count(unsigned int, unsigned long);
 unsigned int plat_get_aff_state(unsigned int, unsigned long);
-#else
+#else /* __ENABLE_PLAT_COMPAT__ */
 /*
  * The below function enable Trusted Firmware components like SPDs which
  * haven't migrated to the new platform API to compile on platforms which
@@ -335,4 +345,6 @@ unsigned int plat_get_aff_state(unsigned int, unsigned long);
 unsigned int platform_get_core_pos(unsigned long mpidr) __deprecated;
 
 #endif /* __ENABLE_PLAT_COMPAT__ */
+
 #endif /* __PLATFORM_H__ */
+

lib/stack_protector/aarch32/asm_stack_protector.S

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ *
+ * Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * Neither the name of ARM nor the names of its contributors may be used
+ * to endorse or promote products derived from this software without specific
+ * prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <arch.h>
+#include <asm_macros.S>
+#include <assert_macros.S>
+
+	.globl	update_stack_protector_canary
+
+/* -----------------------------------------------------------------------
+ * void update_stack_protector_canary(void)
+ *
+ * Change the value of the canary used for stack smashing attacks protection.
+ * Note: This must be called when it is safe to call C code, but this cannot be
+ * called by C code. Doing this will make the check fail when the calling
+ * function returns.
+ * -----------------------------------------------------------------------
+ */
+
+func update_stack_protector_canary
+	/* Use r4 as it is callee-saved */
+	mov	r4, lr
+	bl	plat_get_stack_protector_canary
+
+	/* Update the canary with the returned value */
+	ldr	r1,  =__stack_chk_guard
+	str	r0, [r1]
+	bx	r4
+endfunc update_stack_protector_canary
+
+

lib/stack_protector/aarch64/asm_stack_protector.S

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ *
+ * Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * Neither the name of ARM nor the names of its contributors may be used
+ * to endorse or promote products derived from this software without specific
+ * prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <arch.h>
+#include <asm_macros.S>
+#include <assert_macros.S>
+
+	.globl	update_stack_protector_canary
+
+/* -----------------------------------------------------------------------
+ * void update_stack_protector_canary(void)
+ *
+ * Change the value of the canary used for stack smashing attacks protection.
+ * Note: This must be called when it is safe to call C code, but this cannot be
+ * called by C code. Doing this will make the check fail when the calling
+ * function returns.
+ * -----------------------------------------------------------------------
+ */
+
+func update_stack_protector_canary
+	/* Use x19 as it is callee-saved */
+	mov	x19, x30
+	bl	plat_get_stack_protector_canary
+
+	/* Update the canary with the returned value */
+	adrp	x1,  __stack_chk_guard
+	str	x0, [x1, #:lo12:__stack_chk_guard]
+	ret	x19
+endfunc update_stack_protector_canary
+
+

lib/stack_protector/stack_protector.c

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ *
+ * Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * Neither the name of ARM nor the names of its contributors may be used
+ * to endorse or promote products derived from this software without specific
+ * prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+#include <debug.h>
+#include <platform.h>
+#include <stdint.h>
+
+/*
+ * Canary value used by the compiler runtime checks to detect stack corruption.
+ *
+ * Force the canary to be in .data to allow predictable memory layout relatively
+ * to the stacks.
+ */
+u_register_t  __attribute__((section(".data.stack_protector_canary")))
+	__stack_chk_guard = (u_register_t) 3288484550995823360ULL;
+
+/*
+ * Function called when the stack's canary check fails, which means the stack
+ * was corrupted. It must not return.
+ */
+void __dead2 __stack_chk_fail(void)
+{
+#if DEBUG
+	ERROR("Stack corruption detected\n");
+#endif
+	panic();
+}
+

lib/stack_protector/stack_protector.mk

@@ -0,0 +1,43 @@
+#
+# Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+# Redistributions of source code must retain the above copyright notice, this
+# list of conditions and the following disclaimer.
+#
+# Redistributions in binary form must reproduce the above copyright notice,
+# this list of conditions and the following disclaimer in the documentation
+# and/or other materials provided with the distribution.
+#
+# Neither the name of ARM nor the names of its contributors may be used
+# to endorse or promote products derived from this software without specific
+# prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+#
+
+# Boolean macro to be used in C code
+STACK_PROTECTOR_ENABLED := 0
+
+ifneq (${ENABLE_STACK_PROTECTOR},0)
+STACK_PROTECTOR_ENABLED := 1
+BL_COMMON_SOURCES	+=	lib/stack_protector/stack_protector.c			\
+				lib/stack_protector/${ARCH}/asm_stack_protector.S
+
+TF_CFLAGS		+=	-fstack-protector-${ENABLE_STACK_PROTECTOR}
+endif
+
+$(eval $(call add_define,STACK_PROTECTOR_ENABLED))
+

make_helpers/defaults.mk

@@ -90,6 +90,9 @@ ENABLE_PSCI_STAT		:= 0
 # Flag to enable runtime instrumentation using PMF
 ENABLE_RUNTIME_INSTRUMENTATION	:= 0
 
+# Flag to enable stack corruption protection
+ENABLE_STACK_PROTECTOR		:= 0
+
 # Build flag to treat usage of deprecated platform and framework APIs as error.
 ERROR_DEPRECATED		:= 0
 

plat/arm/board/fvp/fvp_stack_protector.c

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ *
+ * Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * Neither the name of ARM nor the names of its contributors may be used
+ * to endorse or promote products derived from this software without specific
+ * prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <arch_helpers.h>
+#include <platform.h>
+#include <stdint.h>
+
+#define RANDOM_CANARY_VALUE ((u_register_t) 3288484550995823360ULL)
+
+u_register_t plat_get_stack_protector_canary(void)
+{
+	/*
+	 * Ideally, a random number should be returned instead of the
+	 * combination of a timer's value and a compile-time constant. As the
+	 * FVP does not have any random number generator, this is better than
+	 * nothing but not necessarily really secure.
+	 */
+	return RANDOM_CANARY_VALUE ^ read_cntpct_el0();
+}
+

plat/arm/board/fvp/platform.mk

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2013-2016, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are met:
@@ -157,5 +157,9 @@ BL31_SOURCES		+=	plat/arm/board/fvp/fvp_bl31_setup.c		\
 # Disable the PSCI platform compatibility layer
 ENABLE_PLAT_COMPAT	:= 	0
 
+ifneq (${ENABLE_STACK_PROTECTOR},0)
+PLAT_BL_COMMON_SOURCES	+=	plat/arm/board/fvp/fvp_stack_protector.c
+endif
+
 include plat/arm/board/common/board_common.mk
 include plat/arm/common/arm_common.mk

plat/mediatek/mt6795/bl31.ld.S

@@ -95,6 +95,11 @@ SECTIONS
      */
     __RW_START__ = . ;
 
+    /*
+     * .data must be placed at a lower address than the stacks if the stack
+     * protector is enabled. Alternatively, the .data.stack_protector_canary
+     * section can be placed independently of the main .data section.
+     */
     .data . : {
         __DATA_START__ = .;
         *(.data*)