[PATCH v2 2/2] firmware: Initial compiler built-in stack protector support

[Alvin Chang]

Add __stack_chk_fail() and __stack_chk_guard variable which are used by
compiler built-in stack protector.

This patch just try to support stack-protector so the value of the stack
guard variable is simply fixed for now. It could be improved by
deriving from a random number generator, such as Zkr extension or any
platform-specific random number sources.

Introduce three configurations for the stack protector:
1. CONFIG_STACK_PROTECTOR to enable the stack protector feature by
   providing "-fstack-protector" compiler flag
2. CONFIG_STACK_PROTECTOR_STRONG to provide "-fstack-protector-strong"
3. CONFIG_STACK_PROTECTOR_ALL to provide "-fstack-protector-all"

Instead of fixing the compiler flag of stack-protector feature as
"-fstack-protector", we derive it from the introduced Kconfig
configurations. The compiler flag "stack-protector-cflags-y" is defined
as Makefile "immediately expanded variables" with ":=". Thus, the
stronger configuration of the stack protector can overwrite the
preceding one.

Signed-off-by: Alvin Chang <alvinga at andestech.com>
---
 firmware/Kconfig              | 27 +++++++++++++++++++++++++++
 firmware/fw_base.S            | 17 +++++++++++++++++
 firmware/objects.mk           |  9 +++++++++
 firmware/payloads/test_head.S | 17 +++++++++++++++++
 4 files changed, 70 insertions(+)

diff --git a/firmware/Kconfig b/firmware/Kconfig
index d6e0506..c1fee19 100644
--- a/firmware/Kconfig
+++ b/firmware/Kconfig
@@ -1 +1,28 @@
 # SPDX-License-Identifier: BSD-2-Clause
+
+menu "Stack Protector Support"
+
+config STACK_PROTECTOR
+	bool "Stack Protector buffer overflow detection"
+	default n
+	help
+	  This option turns on the "stack-protector" compiler feature.
+
+config STACK_PROTECTOR_STRONG
+	bool "Strong Stack Protector"
+	depends on STACK_PROTECTOR
+	default n
+	help
+	  Turn on the "stack-protector" with "-fstack-protector-strong" option.
+	  Like -fstack-protector but includes additional functions to be
+	  protected.
+
+config STACK_PROTECTOR_ALL
+	bool "Almighty Stack Protector"
+	depends on STACK_PROTECTOR
+	default n
+	help
+	  Turn on the "stack-protector" with "-fstack-protector-all" option.
+	  Like -fstack-protector except that all functions are protected.
+
+endmenu
diff --git a/firmware/fw_base.S b/firmware/fw_base.S
index 2498797..9dc73d0 100644
--- a/firmware/fw_base.S
+++ b/firmware/fw_base.S
@@ -736,6 +736,23 @@ _reset_regs:
 
 	ret
 
+	/* This will be called when the stack corruption is detected */
+	.section .text
+	.align 3
+	.globl __stack_chk_fail
+	.type __stack_chk_fail, %function
+__stack_chk_fail:
+	wfi
+	j	__stack_chk_fail
+
+	/* Initial value of the stack guard variable */
+	.section .data
+	.align 3
+	.globl __stack_chk_guard
+	.type __stack_chk_guard, %object
+__stack_chk_guard:
+	RISCV_PTR	0x95B5FF5A
+
 #ifdef FW_FDT_PATH
 	.section .rodata
 	.align 4
diff --git a/firmware/objects.mk b/firmware/objects.mk
index a90485d..bfec467 100644
--- a/firmware/objects.mk
+++ b/firmware/objects.mk
@@ -66,3 +66,12 @@ endif
 ifdef FW_OPTIONS
 firmware-genflags-y += -DFW_OPTIONS=$(FW_OPTIONS)
 endif
+
+ifeq ($(CONFIG_STACK_PROTECTOR),y)
+stack-protector-cflags-$(CONFIG_STACK_PROTECTOR) := -fstack-protector
+stack-protector-cflags-$(CONFIG_STACK_PROTECTOR_STRONG) := -fstack-protector-strong
+stack-protector-cflags-$(CONFIG_STACK_PROTECTOR_ALL) := -fstack-protector-all
+else
+stack-protector-cflags-y := -fno-stack-protector
+endif
+firmware-cflags-y += $(stack-protector-cflags-y)
diff --git a/firmware/payloads/test_head.S b/firmware/payloads/test_head.S
index 7d25e07..b5fd46b 100644
--- a/firmware/payloads/test_head.S
+++ b/firmware/payloads/test_head.S
@@ -97,3 +97,20 @@ _boot_a0:
 	RISCV_PTR	0
 _boot_a1:
 	RISCV_PTR	0
+
+	/* This will be called when the stack corruption is detected */
+	.section .text
+	.align 3
+	.globl __stack_chk_fail
+	.type __stack_chk_fail, %function
+__stack_chk_fail:
+	wfi
+	j	__stack_chk_fail
+
+	/* Initial value of the stack guard variable */
+	.section .data
+	.align 3
+	.globl __stack_chk_guard
+	.type __stack_chk_guard, %object
+__stack_chk_guard:
+	RISCV_PTR	0x95B5FF5A
-- 
2.43.0