Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210823171318.2801096-1-samitolvanen@google.com>
 <1706ee8e-c21c-f867-c0be-24814a92b853@redhat.com> <CABCJKufrpx9arM-hfX_bR-efO+13VBMFNBTe4ff036VEZi1LZQ@mail.gmail.com>
 <9349a92d-f2a7-9ee4-64db-98d30eadc505@redhat.com>
In-Reply-To: <9349a92d-f2a7-9ee4-64db-98d30eadc505@redhat.com>
From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 24 Aug 2021 10:30:55 -0700
Message-ID: <CABCJKudiTVXOdzxzY1OxF4MMtwNt4XZ_-JJ70dM9H8AEHJmLNw@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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Aug 24, 2021 at 10:26 AM Tom Stellard <tstellar@redhat.com> wrote:
>
> On 8/23/21 10:20 AM, Sami Tolvanen wrote:
> > 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
> >
>
> 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