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. With 5.7 you won't be able to configure user and kernel PAC support independently. So, I guess that's something only for prior kernel versions. > 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@xxxxxxx> > Signed-off-by: Mark Brown <broonie@xxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > --- > 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) I don't think we need to bother with a Kconfig entry here. We did it for the other options since CONFIG_ARM64_PTR_AUTH has a dependency on them. > + > 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 And a $(call cc-option,-mbranch-protection=none) here. branch-prot-flags-y is only introduced in 5.7, so backporting may look slightly weirder. > + > ifeq ($(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 += $(branch-prot-flags-y) Or just use an else clause here with: KBUILD_CFLAGS += ($call cc-option,-mbranch-protection=none). On backports, we just drop else/endif since they don't exist. Not a strong preference really, just looking to have backports resemble upstream better. I can fix it up locally, whichever variant we go for (or even this one). -- Catalin
