arm64: bti: Provide Kconfig for kernel mode BTI

Now that all the code is in place provide a Kconfig option allowing users
to enable BTI for the kernel if their toolchain supports it, defaulting it
on since this has security benefits. This is a separate configuration
option since we currently don't support secondary CPUs that lack BTI if
the boot CPU supports it.

Code generation issues mean that current GCC 9 versions are not able to
produce usable BTI binaries so we disable support for building with GCC
versions prior to 10, once a fix is backported to GCC 9 the dependencies
will be updated.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200506195138.22086-8-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 6f199d8..77d5fa9 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1610,6 +1610,25 @@ config ARM64_BTI
          BTI, such binaries can still run, but you get no additional
          enforcement of branch destinations.
 
+config ARM64_BTI_KERNEL
+       bool "Use Branch Target Identification for kernel"
+       default y
+       depends on ARM64_BTI
+       depends on ARM64_PTR_AUTH
+       depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
+       depends on !CC_IS_GCC || GCC_VERSION >= 100000
+       depends on !(CC_IS_CLANG && GCOV_KERNEL)
+       depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
+       help
+         Build the kernel with Branch Target Identification annotations
+         and enable enforcement of this for kernel code. When this option
+         is enabled and the system supports BTI all kernel code including
+         modular code must have BTI enabled.
+
+config CC_HAS_BRANCH_PROT_PAC_RET_BTI
+       # GCC 9 or later, clang 8 or later
+       def_bool $(cc-option,-mbranch-protection=pac-ret+leaf+bti)
+
 config ARM64_E0PD
        bool "Enable support for E0PD"
        default y