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;
Date:   Wed, 19 Oct 2022 14:03:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     x86@kernel.org, Sami Tolvanen <samitolvanen@google.com>,
        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
Message-ID: <Y0/npwZ3UO9+ZSS4@hirez.programming.kicks-ass.net>
References: <Y06rtoE9BsERG9uv@hirez.programming.kicks-ass.net>
 <202210181020.79AF7F7@keescook>
 <Y08FhjK3fKsfRAaw@hirez.programming.kicks-ass.net>
 <202210182200.50680AE@keescook>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <202210182200.50680AE@keescook>
Precedence: bulk

On Tue, Oct 18, 2022 at 10:05:26PM -0700, Kees Cook wrote:

> > +static int cfi_rand_preamble(s32 *start, s32 *end)
> > +{
> > +	s32 *s;
> > +
> > +	for (s = start; s < end; s++) {
> > +		void *addr = (void *)s + *s;
> > +		u32 hash;
> > +
> > +		hash = decode_preamble_hash(addr);
> > +		if (WARN(!hash, "no CFI hash found at: %pS %px %*ph\n",
> > +			 addr, addr, 5, addr))
> > +			return -EINVAL;
> > +
> > +		hash ^= cfi_seed;
> > +		text_poke_early(addr + 1, &hash, 4);
> > +	}
> > +
> > +	return 0;
> > +}
> 
> 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 :-)