From: Mark Brown broonie@kernel.org
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 --- arch/arm64/Kconfig | 2 ++ 1 file changed, 2 insertions(+)
--- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -1626,6 +1626,8 @@ 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/a88c722e687e6780dcd6a58718350dc7... depends on !CC_IS_CLANG || CLANG_VERSION >= 120000 depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)