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Date:   Wed, 19 Oct 2022 12:35:25 -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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>> >
>> If we look back at how well ASLR did over the years I think we can't 
>> really
>> rely that randomizing the hashes will solve anything. So what you are
>> suggesting is that we flip a "viable defence against SpectreBHB" for a
>> randomization-based scheme, when what we really should be doing is 
>> getting
>> constant blinding enabled by default.
> 
> I don't think any of these things are mutually exclusive. The
> randomization means an additional step (and possibly additional 
> primitive)
> is needed for an attack chain. Since we get this from a one-time cost
> on our end, that seems like reasonable value.
> 
I think I misunderstood your original comment/suggestion, so my bad for 
the noise.

And yeah, I agree that randomization is relevant from the perspective of 
security in depth. With this said, FWIIW, all suggestions sound good to 
me.

>> 
>> Assuming we got 16 bytes padding to play with on each function 
>> prologue, you
>> can randomize between 0-11 in which offset you emit the ENDBR 
>> instruction.
>> Caller/Callee would look like (hopefully I did not mess-up offset):
>> 
>> <caller>:
>> and 0xf3, r11b
>> call *r11
>> 
>> <callee>:
>> nop
>> nop
>> nop
>> endbr // <- this position is randomized/patched during boot time.
>> nop
>> nop
>> ...
>> 
>> And of course, you get more entropy as you increase the padding nop 
>> area.
> 
> Oh, I kind of like this -- it'd need to be per matching hash. This 
> would
> require roughly 3 bits of entropy exposure of the .text area. For X^R,
> that becomes annoying for an attacker, though likely once close enough,
> multiple attempts could find it, assume panic_on_oops/warn wasn't set.
> 
> Anyway, this sounds like an interesting idea to keep in our back
> pocket...

Agreed. It is hard to implement this because the space overhead would be 
too big for meaningful entropy. Yet, again, could be a trick in a swiss 
army knife for future problems.

Tks,
Joao