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Date:   Tue, 24 Aug 2021 10:30:55 -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 Tue, Aug 24, 2021 at 10:26 AM Tom Stellard <tstellar@...hat.com> wrote:
>
> On 8/23/21 10:20 AM, Sami Tolvanen wrote:
> > 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
> >
>
> That looks like something we could backport, I filed a bug to track
> the backport: https://bugs.llvm.org/show_bug.cgi?id=51588.

Great, thanks!

> Do you have any concerns about backporting it or do you think it's pretty
> safe?

No concerns, it should be safe to backport.

Sami

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