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Date:   Tue, 10 May 2022 20:00:38 -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
Message-ID: <202205101958.2A33DE20@keescook>
References: <20220427173128.2603085-1-mark.rutland@arm.com>
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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 27.04.2022 20:31, Mark Rutland wrote:
> > > In __stackleak_erase() we check that the `erase_low` value derived from
> > > `current->lowest_stack` is above the lowest legitimate stack pointer
> > > value, but this is already enforced by stackleak_track_stack() when
> > > recording the lowest stack value.
> > > 
> > > Remove the redundant check.
> > > 
> > > There should be no functional change as a result of this patch.
> > 
> > Mark, I can't agree here. I think this check is important.
> > The performance profit from dropping it is less than the confidence decrease :)
> > 
> > With this check, if the 'lowest_stack' value is corrupted, stackleak doesn't
> > overwrite some wrong kernel memory, but simply clears the whole thread
> > stack, which is safe behavior.
> 
> If you feel strongly about it, I can restore the check, but I struggle to
> believe that it's worthwhile. The `lowest_stack` value lives in the
> task_struct, and if you have the power to corrupt that you have the power to do
> much more interesting things.
> 
> If we do restore it, I'd like to add a big fat comment explaining the
> rationale (i.e. that it only matter if someone could corrupt
> `current->lowest_stack`, as otherwise that's guarnateed to be within bounds).

Yeah, let's restore it and add the comment. While I do agree it's likely
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. Or some manipulation
is possible via weird gadgets, etc. It's unlikely, but not impossible,
and a bounds-check for that value is cheap compared to the rest of the
work happening. :)

-Kees

-- 
Kees Cook