From: Mark Brown broonie@kernel.org
commit b8fdef311a0bd9223f10754f94fdcf1a594a3457 upstream.
Compilers with branch protection support can be configured to enable it by default, it is likely that distributions will do this as part of deploying branch protection system wide. As well as the slight overhead from having some extra NOPs for unused branch protection features this can cause more serious problems when the kernel is providing pointer authentication to userspace but not built for pointer authentication itself. In that case our switching of keys for userspace can affect the kernel unexpectedly, causing pointer authentication instructions in the kernel to corrupt addresses.
To ensure that we get consistent and reliable behaviour always explicitly initialise the branch protection mode, ensuring that the kernel is built the same way regardless of the compiler defaults.
[This is a reworked version of b8fdef311a0bd9223f1075 ("arm64: Always force a branch protection mode when the compiler has one") for backport. Kernels prior to 74afda4016a7 ("arm64: compile the kernel with ptrauth return address signing") don't have any Makefile machinery for forcing on pointer auth but still have issues if the compiler defaults it on so need this reworked version. -- broonie]
Fixes: 7503197562567 (arm64: add basic pointer authentication support) Reported-by: Szabolcs Nagy szabolcs.nagy@arm.com Signed-off-by: Mark Brown broonie@kernel.org Cc: stable@vger.kernel.org [catalin.marinas@arm.com: remove Kconfig option in favour of Makefile check] Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/arm64/Makefile | 4 ++++ 1 file changed, 4 insertions(+)
--- a/arch/arm64/Makefile +++ b/arch/arm64/Makefile @@ -72,6 +72,10 @@ stack_protector_prepare: prepare0 include/generated/asm-offsets.h)) endif
+# Ensure that if the compiler supports branch protection we default it +# off. +KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none) + ifeq ($(CONFIG_CPU_BIG_ENDIAN), y) KBUILD_CPPFLAGS += -mbig-endian CHECKFLAGS += -D__AARCH64EB__