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Date:   Tue, 01 Nov 2022 14:50:22 -0700
From:   Joao Moreira <joao@overdrivepizza.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        Sami Tolvanen <samitolvanen@google.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 2022-11-01 14:39, Kees Cook wrote:
> On Mon, Oct 31, 2022 at 12:13:50PM -0700, Joao Moreira wrote:
>> On 2022-10-18 22:19, Kees Cook wrote:
>> > On Tue, Oct 18, 2022 at 09:48:42PM -0700, Joao Moreira wrote:
>> > > > > Is it useful to get the compiler to emit 0xcc with
>> > > > > -fpatchable-function-entry under any circumstance? I can probably
>> > > > > change
>> > > > > that quickly if needed/useful.
>> > > >
>> > > > Having it emit 0xcc for the bytes in front of the symbol might be
>> > > > interesting. It would mean a few kernel changes, but nothing too hard.
>> 
>> Should I push for this within clang? I have the patch semi-ready 
>> (below) and
>> would have some cycles this week for polishing it.
> 
> Sure! While the NOP vs CC issue isn't very interesting when IBT is
> available, it's nice for non-IBT to make attackers have to target
> addresses precisely.
> 
> If it's really invasive or hard to maintain in Clang (or objtool),
> then I'd say leave it as-is.

The Clang implementation is actually quite simple and, IIRC, I heard in 
the past someone mentioning that trapping instructions actually provide 
benefits for holding undesired straight-line speculation. Maybe someone 
can comment on that, or even if that is really relevant.

Meanwhile I'll work on pushing it then.