Message-ID: <77b79f0c-48f2-16dd-1d00-22f3a1b1f5a6@linux.com>
Date:   Sat, 13 Nov 2021 21:14:39 +0300
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Reply-To: alex.popov@linux.com
Subject: Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter
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To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Paul McKenney <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Joerg Roedel <jroedel@suse.de>,
        Maciej Rozycki <macro@orcam.me.uk>,
        Muchun Song <songmuchun@bytedance.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Petr Mladek <pmladek@suse.com>,
        Kees Cook <keescook@chromium.org>,
        Luis Chamberlain <mcgrof@kernel.org>, Wei Liu <wl@xen.org>,
        John Ogness <john.ogness@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Jann Horn <jannh@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Laura Abbott <labbott@kernel.org>,
        David S Miller <davem@davemloft.net>,
        Borislav Petkov <bp@alien8.de>, Arnd Bergmann <arnd@arndb.de>,
        Andrew Scull <ascull@google.com>,
        Marc Zyngier <maz@kernel.org>, Jessica Yu <jeyu@kernel.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Wang Qing <wangqing@vivo.com>, Mel Gorman <mgorman@suse.de>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Andrew Klychkov <andrew.a.klychkov@gmail.com>,
        Mathieu Chouquet-Stringer <me@mathieu.digital>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Stephen Kitt <steve@sk2.org>, Stephen Boyd <sboyd@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Mike Rapoport <rppt@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-hardening@vger.kernel.org,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>, notify@kernel.org,
        main@lists.elisa.tech, safety-architecture@lists.elisa.tech,
        devel@lists.elisa.tech, Shuah Khan <shuah@kernel.org>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>
References: <20211027233215.306111-1-alex.popov@linux.com>
 <ac989387-3359-f8da-23f9-f5f6deca4db8@linux.com>
 <CAHk-=wgRmjkP3+32XPULMLTkv24AkA=nNLa7xxvSg-F0G1sJ9g@mail.gmail.com>
From:   Alexander Popov <alex.popov@linux.com>
In-Reply-To: <CAHk-=wgRmjkP3+32XPULMLTkv24AkA=nNLa7xxvSg-F0G1sJ9g@mail.gmail.com>
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On 13.11.2021 00:26, Linus Torvalds wrote:
> On Fri, Nov 12, 2021 at 10:52 AM Alexander Popov <alex.popov@linux.com> 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/