arm64: Always force a branch protection mode when the compiler has one

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: 75031975 (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>

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-y += $(call cc-option,-mbranch-protection=none)
+
 ifeq ($(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__