[PATCH -stable] efi/libstub: arm64: Don't use the platform register x18

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

commit 4c86b5f3fa78347fa9cbc9e8ec06615b2f80e397 upstream

The UEFI specification designates x18, the platform register, as "do not
use" on AArch64, so that UEFI code is compatible both with an OS that
assigns a fixed role to x18, and with one that uses it as a temporary
register.

The arm64 EFI stub and the EFI zboot decompressor only avoid x18 if the
kernel is built with shadow call stacks, as that is the only case where
-ffixed-x18 is added to KBUILD_CFLAGS, which the stub inherits.
Otherwise, the compiler is free to use x18 as a temporary register, and
it does so.

So pass -ffixed-x18 unconditionally when building the stub for arm64,
so that the image can be run by loaders that rely on x18 being left
alone, such as a kernel that uses it for its shadow call stack.

Cc: <stable@vger.kernel.org> # 6.1.y and older
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 748781c25787..b85d3cae997e 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -19,7 +19,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ \
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
-				   -fpie $(DISABLE_STACKLEAK_PLUGIN) \
+				   -fpie $(DISABLE_STACKLEAK_PLUGIN) -ffixed-x18 \
 				   $(call cc-option,-mbranch-protection=none)
 cflags-$(CONFIG_ARM)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic \
-- 
2.56.0.455.g6c5a503b38-goog