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    Subject[PATCH 5.19 187/192] arm64/bti: Disable in kernel BTI when cross section thunks are broken
    Date
    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
    @@ -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)

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