Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <Y06rtoE9BsERG9uv@hirez.programming.kicks-ass.net>
 <202210181020.79AF7F7@keescook> <Y08FhjK3fKsfRAaw@hirez.programming.kicks-ass.net>
 <202210182200.50680AE@keescook> <Y0/npwZ3UO9+ZSS4@hirez.programming.kicks-ass.net>
In-Reply-To: <Y0/npwZ3UO9+ZSS4@hirez.programming.kicks-ass.net>
From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 19 Oct 2022 08:22:17 -0700
Message-ID: <CABCJKudc4uS87oxdXyXOyuoT1XqwR4ARrS2N9htGbz1y016AcQ@mail.gmail.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
To:     Peter Zijlstra <peterz@infradead.org>
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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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.

Sami