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Date:   Wed, 11 May 2022 07:44:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Mark Rutland <mark.rutland@arm.com>
CC:     Alexander Popov <alex.popov@linux.com>,
        linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org,
        catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
        luto@kernel.org, will@kernel.org
Subject: Re: [PATCH v2 03/13] stackleak: remove redundant check
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On May 11, 2022 1:02:45 AM PDT, Mark Rutland <mark=2Erutland@arm=2Ecom> wr=
ote:
>On Tue, May 10, 2022 at 08:00:38PM -0700, Kees Cook wrote:
>> On Tue, May 10, 2022 at 12:46:48PM +0100, Mark Rutland wrote:
>> > On Sun, May 08, 2022 at 09:17:01PM +0300, Alexander Popov wrote:
>> > > On 27=2E04=2E2022 20:31, Mark Rutland wrote:
>> > > > In __stackleak_erase() we check that the `erase_low` value derive=
d from
>> > > > `current->lowest_stack` is above the lowest legitimate stack poin=
ter
>> > > > value, but this is already enforced by stackleak_track_stack() wh=
en
>> > > > recording the lowest stack value=2E
>> > > >=20
>> > > > Remove the redundant check=2E
>> > > >=20
>> > > > There should be no functional change as a result of this patch=2E
>> > >=20
>> > > Mark, I can't agree here=2E I think this check is important=2E
>> > > The performance profit from dropping it is less than the confidence=
 decrease :)
>> > >=20
>> > > With this check, if the 'lowest_stack' value is corrupted, stacklea=
k doesn't
>> > > overwrite some wrong kernel memory, but simply clears the whole thr=
ead
>> > > stack, which is safe behavior=2E
>> >=20
>> > If you feel strongly about it, I can restore the check, but I struggl=
e to
>> > believe that it's worthwhile=2E The `lowest_stack` value lives in the
>> > task_struct, and if you have the power to corrupt that you have the p=
ower to do
>> > much more interesting things=2E
>> >=20
>> > If we do restore it, I'd like to add a big fat comment explaining the
>> > rationale (i=2Ee=2E that it only matter if someone could corrupt
>> > `current->lowest_stack`, as otherwise that's guarnateed to be within =
bounds)=2E
>>=20
>> Yeah, let's restore it and add the comment=2E While I do agree it's lik=
ely
>> that such an corruption would likely mean an attacker had significant
>> control over kernel memory already, it is not uncommon that an attack
>> only has a limited index from a given address, etc=2E Or some manipulat=
ion
>> is possible via weird gadgets, etc=2E It's unlikely, but not impossible=
,
>> and a bounds-check for that value is cheap compared to the rest of the
>> work happening=2E :)
>
>Fair enough; I can go spin a patch restoring this=2E I'm somewhat unhappy=
 with
>silently fixing that up, though -- IMO it'd be better to BUG() or similar=
 in
>that case=2E

I share your desires, and this was exactly what Alexander originally propo=
sed, but Linus rejected it violently=2E :(
https://lore=2Ekernel=2Eorg/lkml/CA+55aFy6jNLsywVYdGp83AMrXBo_P-pkjkphPGrO=
=3D82SPKCpLQ@mail=2Egmail=2Ecom/


--=20
Kees Cook