linux-hardening - Re: [RFC PATCH 00/21] KCFI support

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Date:   Sat, 30 Apr 2022 11:02:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 00/21] KCFI support

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.
> 
> 🎉 :)
> 
> > 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 dependency
> with regard to the design of __builtin_kcfi_call_unchecked (D124211
> above), as the implementation has gone around a few times in review within
> 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?

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.