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Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Petr Mladek <pmladek@suse.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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From:   Alexander Popov <alex.popov@linux.com>
Message-ID: <0e847d7f-7bf0-cdd4-ba6e-a742ce877a38@linux.com>
Date:   Sun, 3 Oct 2021 00:05:56 +0300
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On 02.10.2021 19:52, Linus Torvalds wrote:
> On Sat, Oct 2, 2021 at 4:41 AM Alexander Popov <alex.popov@linux.com> wrote:
>>
>> And what do you think about the proposed pkill_on_warn?
> 
> Honestly, I don't see the point.
> 
> If you can reliably trigger the WARN_ON some way, you can probably
> cause more problems by fooling some other process to trigger it.
> 
> And if it's unintentional, then what does the signal help?
> 
> So rather than a "rationale" that makes little sense, I'd like to hear
> of an actual _use_ case. That's different. That's somebody actually
> _using_ that pkill to good effect for some particular load.

I was thinking about a use case for you and got an insight.

Bugs usually don't come alone. Killing the process that got WARN_ON() prevents
possible bad effects **after** the warning. For example, in my exploit for
CVE-2019-18683, the kernel warning happens **before** the memory corruption
(use-after-free in the V4L2 subsystem).
https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html

So pkill_on_warn allows the kernel to stop the process when the first signs of
wrong behavior are detected. In other words, proceeding with the code execution
from the wrong state can bring more disasters later.

> That said, I don't much care in the end. But it sounds like a
> pointless option to just introduce yet another behavior to something
> that should never happen anyway, and where the actual
> honest-to-goodness reason for WARN_ON() existing is already being
> fulfilled (ie syzbot has been very effective at flushing things like
> that out).

Yes, we slowly get rid of kernel warnings.
However, the syzbot dashboard still shows a lot of them.
Even my small syzkaller setup finds plenty of new warnings.
I believe fixing all of them will take some time.
And during that time, pkill_on_warn may be a better reaction to WARN_ON() than
ignoring and proceeding with the execution.

Is that reasonable?

Best regards,
Alexander