ARM: 9407/1: Add support for STACKLEAK gcc plugin

Add the STACKLEAK gcc plugin to arm32 by adding the helper used by
stackleak common code: on_thread_stack(). It initialize the stack with the
poison value before returning from system calls which improves the kernel
security. Additionally, this disables the plugin in EFI stub code and
decompress code, which are out of scope for the protection.

Before the test on Qemu versatilepb board:
	# echo STACKLEAK_ERASING  > /sys/kernel/debug/provoke-crash/DIRECT
	lkdtm: Performing direct entry STACKLEAK_ERASING
	lkdtm: XFAIL: stackleak is not supported on this arch (HAVE_ARCH_STACKLEAK=n)

After:
	# echo STACKLEAK_ERASING  > /sys/kernel/debug/provoke-crash/DIRECT
	lkdtm: Performing direct entry STACKLEAK_ERASING
	lkdtm: stackleak stack usage:
	  high offset: 80 bytes
	  current:     280 bytes
	  lowest:      696 bytes
	  tracked:     696 bytes
	  untracked:   192 bytes
	  poisoned:    7220 bytes
	  low offset:  4 bytes
	lkdtm: OK: the rest of the thread stack is properly erased
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/Kconfig

@@ -86,6 +86,7 @@ config ARM
 	select HAVE_ARCH_PFN_VALID
 	select HAVE_ARCH_SECCOMP
 	select HAVE_ARCH_SECCOMP_FILTER if AEABI && !OABI_COMPAT
+	select HAVE_ARCH_STACKLEAK
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARM_LPAE

arch/arm/boot/compressed/Makefile

@@ -9,6 +9,7 @@ OBJS		=
 
 HEAD	= head.o
 OBJS	+= misc.o decompress.o
+CFLAGS_decompress.o += $(DISABLE_STACKLEAK_PLUGIN)
 ifeq ($(CONFIG_DEBUG_UNCOMPRESS),y)
 OBJS	+= debug.o
 AFLAGS_head.o += -DDEBUG

arch/arm/include/asm/stacktrace.h

@@ -26,6 +26,13 @@ struct stackframe {
 #endif
 };
 
+static inline bool on_thread_stack(void)
+{
+	unsigned long delta = current_stack_pointer ^ (unsigned long)current->stack;
+
+	return delta < THREAD_SIZE;
+}
+
 static __always_inline
 void arm_get_current_stackframe(struct pt_regs *regs, struct stackframe *frame)
 {

arch/arm/kernel/entry-common.S

@@ -119,6 +119,9 @@ no_work_pending:
 
 	ct_user_enter save = 0
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	bl	stackleak_erase_on_task_stack
+#endif
 	restore_user_regs fast = 0, offset = 0
 ENDPROC(ret_to_user_from_irq)
 ENDPROC(ret_to_user)

drivers/firmware/efi/libstub/Makefile

@@ -27,7 +27,8 @@ cflags-$(CONFIG_ARM64)		+= -fpie $(DISABLE_STACKLEAK_PLUGIN) \
 cflags-$(CONFIG_ARM)		+= -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \
 				   -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
 				   -DEFI_HAVE_STRCMP -fno-builtin -fpic \
-				   $(call cc-option,-mno-single-pic-base)
+				   $(call cc-option,-mno-single-pic-base) \
+				   $(DISABLE_STACKLEAK_PLUGIN)
 cflags-$(CONFIG_RISCV)		+= -fpic -DNO_ALTERNATIVE -mno-relax
 cflags-$(CONFIG_LOONGARCH)	+= -fpie