7.3-stable review patch. If anyone has any objections, please let me know. ------------------ From: Ard Biesheuvel [ Upstream commit 4c86b5f3fa78347fa9cbc9e8ec06615b2f80e397 ] 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: Signed-off-by: Ard Biesheuvel Signed-off-by: Sasha Levin --- 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 77a2b2d74f3f6..48982d05c441e 100644 --- a/drivers/firmware/efi/libstub/Makefile +++ b/drivers/firmware/efi/libstub/Makefile @@ -21,7 +21,7 @@ cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ $(CC_FLAGS_DIALECT) \ # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly # disable the stackleak plugin -cflags-$(CONFIG_ARM64) += -fpie $(DISABLE_KSTACK_ERASE) \ +cflags-$(CONFIG_ARM64) += -fpie $(DISABLE_KSTACK_ERASE) -ffixed-x18 \ -fno-unwind-tables -fno-asynchronous-unwind-tables cflags-$(CONFIG_ARM) += -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \ -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \ -- 2.53.0