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References: <20211013181658.1020262-1-samitolvanen@google.com> <20211019095947.89257-1-alobakin@pm.me>
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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 19 Oct 2021 08:40:55 -0700
Message-ID: <CABCJKufjGL9Zq6CDjDOdimu46ZM7Wj0rFNUD-AQrL=Hb81FZfQ@mail.gmail.com>
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI
To:     Alexander Lobakin <alobakin@pm.me>
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>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, llvm@lists.linux.dev
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On Tue, Oct 19, 2021 at 3:06 AM Alexander Lobakin <alobakin@pm.me> wrote:
>
> From: Sami Tolvanen <samitolvanen@google.com>
> Date: Wed, 13 Oct 2021 11:16:43 -0700
>
> > 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
> >
> > Note that v5 is based on tip/master. The first two patches contain
> > objtool support for CFI, the remaining patches change function
> > declarations to use opaque types, fix type mismatch issues that
> > confuse the compiler, and disable CFI where it can't be used.
> >
> > You can also pull this series from
> >
> >   https://github.com/samitolvanen/linux.git x86-cfi-v5
>
> Hi,
>
> I found [0] while was testing Peter's retpoline series, wanted to
> ask / double check if that is because I'm using ClangCFI for x86
> on unsupported Clang 12. It is fixed in 13 I suppose?
>
> [0] https://lore.kernel.org/all/20211019094038.80569-1-alobakin@pm.me

No, it works exactly the same in later compiler versions. I also
replied to that thread, but this looks like another instance where
using an opaque type instead of a function declaration fixes the
issue, and probably makes sense as the thunks are not directly
callable from C code.

Sami