Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter

[Alexander Popov]

On 13.11.2021 00:26, Linus Torvalds wrote:
> On Fri, Nov 12, 2021 at 10:52 AM Alexander Popov <alex.popov@xxxxxxxxx> wrote:
> > Hello everyone!
> > Friendly ping for your feedback.
>
> I still haven't heard a compelling _reason_ for this all, and why
> anybody should ever use this or care?

Ok, to sum up:

Killing the process that hit a kernel warning complies with the Fail-Fast 
principle [1]. pkill_on_warn sysctl allows the kernel to stop the process when 
the **first signs** of wrong behavior are detected.

By default, the Linux kernel ignores a warning and proceeds the execution from 
the flawed state. That is opposite to the Fail-Fast principle.
A kernel warning may be followed by memory corruption or other negative effects, 
like in CVE-2019-18683 exploit [2] or many other cases detected by the SyzScope 
project [3]. pkill_on_warn would prevent the system from the errors going after 
a warning in the process context.

At the same time, pkill_on_warn does not kill the entire system like 
panic_on_warn. That is the middle way of handling kernel warnings.
Linus, it's similar to your BUG_ON() policy [4]. The process hitting BUG_ON() is 
killed, and the system proceeds to work. pkill_on_warn just brings a similar 
policy to WARN_ON() handling.

I believe that many Linux distros (which don't hit WARN_ON() here and there) 
will enable pkill_on_warn because it's reasonable from the safety and security 
points of view.

And I'm sure that the ELISA project by the Linux Foundation (Enabling Linux In 
Safety Applications [5]) would support the pkill_on_warn sysctl.
[Adding people from this project to CC]

I hope that I managed to show the rationale.

Best regards,
Alexander


[1]: https://en.wikipedia.org/wiki/Fail-fast
[2]: https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
[3]: https://www.usenix.org/system/files/sec22summer_zou.pdf
[4]: http://lkml.iu.edu/hypermail/linux/kernel/1610.0/01217.html
[5]: https://elisa.tech/