From: Ard Biesheuvel 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: # 6.1.y and older Signed-off-by: Ard Biesheuvel --- 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