Re: [PATCH] x86/ibt: Implement FineIBT

[Sami Tolvanen]

On Wed, Oct 19, 2022 at 5:03 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Oct 18, 2022 at 10:05:26PM -0700, Kees Cook wrote:
> >
> > The one glitch here is that the resulting hash needs to not contain
> > an endbr...
>
> Oh right,.. duh. How about something like:
>
> static u32 cfi_rehash(u32 hash)
> {
>         hash ^= cfi_hash;
>         while (unlikely(is_endbr(hash))) {
>                 bool lsb = hash & 1;
>                 hash >>= 1;
>                 if (lsb)
>                         hash ^= 0x80200003;
>         }
>         return hash;
> }
>
> Which seems properly over-engineered :-)

Also, -hash can't be endbr with KCFI since we use that in the check itself.

Sami