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

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Date:   Wed, 27 Oct 2021 14:48:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        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,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Wed, Oct 27, 2021 at 02:22:27PM +0200, Ard Biesheuvel wrote:
> On Wed, 27 Oct 2021 at 14:05, Mark Rutland <mark.rutland@....com> wrote:

> > > Should not this jump-table thingy get converted to an actual function
> > > address somewhere around arch_static_call_transform() ? This also seems
> > > relevant for arm64 (which already has CLANG_CFI supported) given:
> > >
> > >   https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org
> >
> > Ugh, yeah, we'll need to do the function_nocfi() dance somewhere...
> >
> 
> Sadly, that only works on symbol names, so we cannot use it to strip
> CFI-ness from void *func arguments passed into the static call API,
> unfortunately.

Right, and while mostly static_call_update() is used, whcih is a macro
and could possibly be used to wrap this, we very much rely on
__static_call_update() also working without that wrapper and then we're
up a creek without no paddles.