On Tue, Mar 31, 2020 at 08:44:59PM +0100, Mark Brown wrote:
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.
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
Acked-by: Mark Rutland mark.rutland@arm.com
Mark.
arch/arm64/Kconfig | 4 ++++ arch/arm64/Makefile | 7 ++++++- 2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index d3efdc095a17..1e46746e8392 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -1537,6 +1537,10 @@ config ARM64_PTR_AUTH This feature works with FUNCTION_GRAPH_TRACER option only if DYNAMIC_FTRACE_WITH_REGS is enabled. +config CC_HAS_BRANCH_PROT_NONE
- # GCC 9 or later, clang 8 or later
- def_bool $(cc-option,-mbranch-protection=none)
config CC_HAS_BRANCH_PROT_PAC_RET # GCC 9 or later, clang 8 or later def_bool $(cc-option,-mbranch-protection=pac-ret+leaf) diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile index f15f92ba53e6..370fca6663c8 100644 --- a/arch/arm64/Makefile +++ b/arch/arm64/Makefile @@ -65,6 +65,10 @@ stack_protector_prepare: prepare0 include/generated/asm-offsets.h)) endif +# Ensure that if the compiler supports branch protection we default it +# off, this will be overridden if we are using branch protection. +branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_NONE) := -mbranch-protection=none
ifeq ($(CONFIG_ARM64_PTR_AUTH),y) branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf @@ -73,9 +77,10 @@ branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pa # we pass it only to the assembler. This option is utilized only in case of non # integrated assemblers. branch-prot-flags-$(CONFIG_AS_HAS_PAC) += -Wa,-march=armv8.3-a -KBUILD_CFLAGS += $(branch-prot-flags-y) endif +KBUILD_CFLAGS += $(branch-prot-flags-y)
ifeq ($(CONFIG_CPU_BIG_ENDIAN), y) KBUILD_CPPFLAGS += -mbig-endian CHECKFLAGS += -D__AARCH64EB__ -- 2.20.1