Re: [RFC PATCH 00/21] KCFI support

Sami Tolvanen
2022-05-02T15:24:41Z

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.
> >
> > 🎉 :)
> >
> > > 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?

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