linux-hardening - Re: [PATCH v5 00/15] x86: Add support for Clang CFI

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Date:   Tue, 19 Oct 2021 08:40:55 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Alexander Lobakin <alobakin@...me>
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>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Tue, Oct 19, 2021 at 3:06 AM Alexander Lobakin <alobakin@...me> wrote:
>
> From: Sami Tolvanen <samitolvanen@...gle.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