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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 2 May 2022 08:22:57 -0700
Message-ID: <CABCJKuckt75qA1op-LpkJnQsJC36m9fstbY3uD=7pET2VyyZSg@mail.gmail.com>
Subject: Re: [RFC PATCH 00/21] KCFI support
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        linux-hardening@vger.kernel.org,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        llvm@lists.linux.dev
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On Sat, Apr 30, 2022 at 2:02 AM Peter Zijlstra <peterz@infradead.org> wrote=
:
>
> On Fri, Apr 29, 2022 at 03:53:12PM -0700, Kees Cook wrote:
> > On Fri, Apr 29, 2022 at 01:36:23PM -0700, Sami Tolvanen wrote:
> > > KCFI is a proposed forward-edge control-flow integrity scheme for
> > > Clang, which is more suitable for kernel use than the existing CFI
> > > scheme used by CONFIG_CFI_CLANG. KCFI doesn't require LTO, doesn't
> > > alter function references to point to a jump table, and won't break
> > > function address equality.
> >
> > =F0=9F=8E=89 :)
> >
> > > The latest LLVM patches are here:
> > >
> > >   https://reviews.llvm.org/D119296
> > >   https://reviews.llvm.org/D124211
> > >
> > > [...]
> > > To test this series, you'll need to compile your own Clang toolchain
> > > with the patches linked above. You can also find the complete source
> > > tree here:
> > >
> > >   https://github.com/samitolvanen/llvm-project/commits/kcfi-rfc
> >
> > And note that this RFC is seeking to break a bit of a circular dependen=
cy
> > with regard to the design of __builtin_kcfi_call_unchecked (D124211
> > above), as the implementation has gone around a few times in review wit=
hin
> > LLVM, and we want to make sure that kernel folks are okay with what was
> > settled on. If there are no objections on the kernel side, then we can
> > land the KCFI patches, as this is basically the only remaining blocker.
>
> So aside from the static_call usage, was there any other?

Not at the moment, and it looks like we can get rid of that too.

> Anyway, I think I hate that __builtin, I'd *much* rather see a variable
> attribute or qualifier for this, such that one can mark a function
> pointer as not doing CFI.
>
> I simply doesn't make sense to have a builtin that operates on an
> expression. The whole thing is about indirect calls, IOW function
> pointers.

I also thought an attribute would be more convenient, but the compiler
folks prefer a built-in:

https://reviews.llvm.org/D122673

Sami