linux-hardening - [PATCH] kstack_erase: Disable kstack_erase for all of arm compressed boot code

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Message-Id: <20250726212945.work.975-kees@kernel.org>
Date: Sat, 26 Jul 2025 14:29:45 -0700
From: Kees Cook <kees@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
	Linux Kernel Functional Testing <lkft@...aro.org>,
	Russell King <linux@...linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Ard Biesheuvel <ardb@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] kstack_erase: Disable kstack_erase for all of arm compressed boot code

When building with CONFIG_KSTACK_ERASE=y and CONFIG_ARM_ATAG_DTB_COMPAT=y,
the compressed boot environment encounters an undefined symbol error:

    ld.lld: error: undefined symbol: __sanitizer_cov_stack_depth
    >>> referenced by atags_to_fdt.c:135

This occurs because the compiler instruments the atags_to_fdt() function
with sanitizer coverage calls, but the minimal compressed boot environment
lacks access to sanitizer runtime support.

The compressed boot environment already disables stack protector with
-fno-stack-protector. Similarly disable sanitizer coverage by adding
$(DISABLE_KSTACK_ERASE) to the general compiler flags (and remove it
from the one place it was noticed before), which contains the appropriate
flags to prevent sanitizer instrumentation.

This follows the same pattern used in other early boot contexts where
sanitizer runtime support is unavailable.

Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
Closes: https://lore.kernel.org/all/CA+G9fYtBk8qnpWvoaFwymCx5s5i-5KXtPGpmf=_+UKJddCOnLA@mail.gmail.com
Reported-by: Nathan Chancellor <nathan@...nel.org>
Closes: https://lore.kernel.org/all/20250726004313.GA3650901@ax162
Suggested-by: Nathan Chancellor <nathan@...nel.org>
Signed-off-by: Kees Cook <kees@...nel.org>
---
 arch/arm/boot/compressed/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index f9075edfd773..a159120d1e42 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -9,7 +9,6 @@ OBJS		=
 
 HEAD	= head.o
 OBJS	+= misc.o decompress.o
-CFLAGS_decompress.o += $(DISABLE_KSTACK_ERASE)
 ifeq ($(CONFIG_DEBUG_UNCOMPRESS),y)
 OBJS	+= debug.o
 AFLAGS_head.o += -DDEBUG
@@ -96,6 +95,7 @@ KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
 
 ccflags-y := -fpic $(call cc-option,-mno-single-pic-base,) -fno-builtin \
 	     -I$(srctree)/scripts/dtc/libfdt -fno-stack-protector \
+	     $(DISABLE_KSTACK_ERASE) \
 	     -I$(obj)
 ccflags-remove-$(CONFIG_FUNCTION_TRACER) += -pg
 asflags-y := -DZIMAGE
-- 
2.34.1