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Date:   Thu, 20 Oct 2022 13:04:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Kees Cook <keescook@chromium.org>, x86@kernel.org,
        Joao Moreira <joao@overdrivepizza.com>,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
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On Wed, Oct 19, 2022 at 08:22:17AM -0700, Sami Tolvanen wrote:
> On Wed, Oct 19, 2022 at 5:03 AM Peter Zijlstra <peterz@infradead.org> 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.

Indeed, updated and pushed out. queue/x86/fineibt should have it
momentarily.