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Date:   Tue, 24 Aug 2021 10:26:16 -0700
From:   Tom Stellard <tstellar@...hat.com>
To:     Sami Tolvanen <samitolvanen@...gle.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 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.

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

-Tom


> Sami
> 

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