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Reply-To: alex.popov@linux.com
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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        Lu Baolu <baolu.lu@linux.intel.com>,
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        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
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 <0e847d7f-7bf0-cdd4-ba6e-a742ce877a38@linux.com> <87zgrnqmlc.fsf@disp2133>
From:   Alexander Popov <alex.popov@linux.com>
Message-ID: <c48643fa-213d-7391-9f5d-c75efe709c3f@linux.com>
Date:   Wed, 6 Oct 2021 17:56:55 +0300
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On 05.10.2021 22:48, Eric W. Biederman wrote:
> Alexander Popov <alex.popov@linux.com> writes:
> 
>> 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?
> 
> I won't comment on the sanity of the feature but I will say that calling
> it oops_on_warn (rather than pkill_on_warn), and using the usual oops
> facilities rather than rolling oops by hand sounds like a better
> implementation.
> 
> Especially as calling do_group_exit(SIGKILL) from a random location is
> not a clean way to kill a process.  Strictly speaking it is not even
> killing the process.
> 
> Partly this is just me seeing the introduction of a
> do_group_exit(SIGKILL) call and not likely the maintenance that will be
> needed.  I am still sorting out the problems with other randomly placed
> calls to do_group_exit(SIGKILL) and interactions with ptrace and
> PTRACE_EVENT_EXIT in particular.
> 
> Which is a long winded way of saying if I can predictably trigger a
> warning that calls do_group_exit(SIGKILL), on some architectures I can
> use ptrace and  can convert that warning into a way to manipulate the
> kernel stack to have the contents of my choice.
> 
> If anyone goes forward with this please use the existing oops
> infrastructure so the ptrace interactions and anything else that comes
> up only needs to be fixed once.

Eric, thanks a lot.

I will learn the oops infrastructure deeper.
I will do more experiments and come with version 2.

Currently, I think I will save the pkill_on_warn option name because I want to
avoid kernel crashes.

Thanks to everyone who gave feedback on this patch!

Best regards,
Alexander