Re: [RFC/RFT] CFI: Add support for gcc CFI in aarch64

Kees Cook
2023-02-08T19:35:42Z

On Sat, Jan 07, 2023 at 07:42:13AM -0800, Dan Li wrote:
> Hi Kees,
>
> On 01/06, Kees Cook wrote:
> > On Mon, Dec 19, 2022 at 05:32:04AM -0800, Dan Li wrote:
> > > Hi Peter,
> > Hi!
> >
> > First of all, thank you thank you for working on this in GCC. This will
> > make a big difference for folks that don't have the option to build with
> > Clang to gain CFI coverage.
> >
> > As for the implementation details, the core issue is really that this
> > type of CFI is specifically designed for the Linux kernel, and it took a
> > rather long time to figure out all the specifics needed (down to the
> > byte counts and instruction layouts). GCC's version will ultimately need
> > to exactly match the Clang output, or Linux is unlikely to support it.
> >
> > We're already on our second CFI -- the original Clang CFI was just too
> > clunky for long-term use in Linux, so unless we're going to improve on
> > the latest Clang KCFI implementation in some way, it's better to stick
> > to exactly byte-for-byte identical results. The KCFI support in Linux
> > depends on the arm64 and x86_64 runtimes for catching the traps, and the
> > post-processing done (on x86_64) with objtool that prepares the kernel
> > for IBT use, and converts to the optional FineIBT CFI mechanism. With
> > all those moving parts, there needs to be a very compelling reason to
> > have GCC KCFI implementation differ from Clang's.
> >
> > Hopefully that context helps a little. I'm excited to try out future
> > versions!
>
> Thanks for the context, it makes sense and helped me a lot. :)
>
> In the next version I'll make the gcc implementation consistent with clang.

Hi!

Just checking in on this, since there are a lot of interested folks. :)
What's the status on the next version (and has anyone been found to
tackle the x86 backend part)? Is there anything we can help with?

Thanks!

-Kees

--
Kees Cook