linux-kernel - Re: [PATCH] x86/ibt: Implement FineIBT

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Message-ID: <CABCJKudc4uS87oxdXyXOyuoT1XqwR4ARrS2N9htGbz1y016AcQ@mail.gmail.com>
Date:   Wed, 19 Oct 2022 08:22:17 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, x86@...nel.org,
        Joao Moreira <joao@...rdrivepizza.com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT

On Wed, Oct 19, 2022 at 5:03 AM Peter Zijlstra <peterz@...radead.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.

Sami