linux-hardening - Re: [PATCH v2 00/14] x86: Add support for Clang CFI

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Date:   Mon, 23 Aug 2021 10:20:33 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Tom Stellard <tstellar@...hat.com>
Cc:     X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI

On Mon, Aug 23, 2021 at 10:16 AM Tom Stellard <tstellar@...hat.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