arm64/bti: Disable in kernel BTI when cross section thunks are broken

commit c0a454b9044fdc99486853aa424e5b3be2107078 upstream.

GCC does not insert a `bti c` instruction at the beginning of a function
when it believes that all callers reach the function through a direct
branch[1]. Unfortunately the logic it uses to determine this is not
sufficiently robust, for example not taking account of functions being
placed in different sections which may be loaded separately, so we may
still see thunks being generated to these functions. If that happens,
the first instruction in the callee function will result in a Branch
Target Exception due to the missing landing pad.

While this has currently only been observed in the case of modules
having their main code loaded sufficiently far from their init section
to require thunks it could potentially happen for other cases so the
safest thing is to disable BTI for the kernel when building with an
affected toolchain.

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671

Reported-by: D Scott Phillips <scott@os.amperecomputing.com>
[Bits of the commit message are lifted from his report & workaround]
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220905142255.591990-1-broonie@kernel.org
Cc: <stable@vger.kernel.org> # v5.10+
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 21f33d8d6668529b507a63d731db62f47aaa9ca2..cc1e7bb49d38b6f53b7ce376a2a9eae1504c2af2 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1884,6 +1884,8 @@ config ARM64_BTI_KERNEL
        depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
        # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
        depends on !CC_IS_GCC || GCC_VERSION >= 100100
+       # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
+       depends on !CC_IS_GCC
        # https://github.com/llvm/llvm-project/commit/a88c722e687e6780dcd6a58718350dc76fcc4cc9
        depends on !CC_IS_CLANG || CLANG_VERSION >= 120000
        depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)