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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 23 Aug 2021 10:20:33 -0700
Message-ID: <CABCJKufrpx9arM-hfX_bR-efO+13VBMFNBTe4ff036VEZi1LZQ@mail.gmail.com>
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI
To:     Tom Stellard <tstellar@redhat.com>
Cc:     X86 ML <x86@kernel.org>, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        linux-hardening@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Mon, Aug 23, 2021 at 10:16 AM Tom Stellard <tstellar@redhat.com> wrote:
>
> On 8/23/21 10:13 AM, 'Sami Tolvanen' via Clang Built Linux wrote:
> > This series adds support for Clang's Control-Flow Integrity (CFI)
> > checking to x86_64. With CFI, the compiler injects a runtime
> > check before each indirect function call to ensure the target is
> > a valid function with the correct static type. This restricts
> > possible call targets and makes it more difficult for an attacker
> > to exploit bugs that allow the modification of stored function
> > pointers. For more details, see:
> >
> >    https://clang.llvm.org/docs/ControlFlowIntegrity.html
> >
> > Version 2 depends on Clang >=14, where we fixed the issue with
> > referencing static functions from inline assembly. Based on the
> > feedback for v1, this version also changes the declaration of
> > functions that are not callable from C to use an opaque type,
> > which stops the compiler from replacing references to them. This
> > avoids the need to sprinkle function_nocfi() macros in the kernel
> > code.
>
> How invasive are the changes in clang 14 necessary to make CFI work?
> Would it be possible to backport them to LLVM 13?

I'm not sure what the LLVM backport policy is, but this specific fix
was quite simple:

https://reviews.llvm.org/rG7ce1c4da7726

Sami