From: Alexander Popov <alex.popov@linux.com>
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>,
Jonathan Corbet <corbet@lwn.net>,
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>,
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.deacon@arm.com>,
David S Miller <davem@davemloft.net>,
Borislav Petkov <bp@alien8.de>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
linux-hardening@vger.kernel.org,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
notify@kernel.org
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
Date: Wed, 6 Oct 2021 17:56:55 +0300 [thread overview]
Message-ID: <c48643fa-213d-7391-9f5d-c75efe709c3f@linux.com> (raw)
In-Reply-To: <87zgrnqmlc.fsf@disp2133>
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
next prev parent reply other threads:[~2021-10-06 14:57 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-29 18:58 [PATCH] Introduce the pkill_on_warn boot parameter Alexander Popov
2021-09-29 19:01 ` Alexander Popov
2021-09-29 19:49 ` Paul E. McKenney
2021-09-30 9:15 ` Petr Mladek
2021-09-30 15:05 ` Alexander Popov
2021-10-01 12:23 ` Petr Mladek
2021-09-30 16:59 ` Steven Rostedt
2021-10-01 12:09 ` Petr Mladek
2021-09-30 18:28 ` Kees Cook
2021-10-01 19:59 ` Linus Torvalds
2021-10-02 11:41 ` Alexander Popov
2021-10-02 12:13 ` Steven Rostedt
2021-10-02 16:33 ` Alexander Popov
2021-10-02 16:52 ` Linus Torvalds
2021-10-02 21:05 ` Alexander Popov
2021-10-05 19:48 ` Eric W. Biederman
2021-10-06 14:56 ` Alexander Popov [this message]
2021-10-22 17:30 ` Alexander Popov
2022-07-27 16:17 ` Alexey Khoroshilov
2022-07-27 16:30 ` Jann Horn
2022-07-27 16:43 ` Alexey Khoroshilov
2022-07-27 16:42 ` Linus Torvalds
2022-07-27 17:47 ` Alexey Khoroshilov
2021-09-29 23:31 ` Andrew Morton
2021-09-30 18:27 ` Alexander Popov
2021-09-30 18:36 ` Kees Cook
2021-09-29 19:03 ` Dave Hansen
2021-09-29 19:47 ` Peter Zijlstra
2021-09-29 20:06 ` Kees Cook
2021-09-30 13:55 ` Alexander Popov
2021-09-30 18:20 ` Kees Cook
2021-10-02 18:04 ` Al Viro
2021-10-02 18:31 ` Steven Rostedt
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