Re: [PATCH] Introduce the pkill_on_warn boot parameter

From: Jann Horn <jannh@google.com>
To: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Petr Mladek <pmladek@suse.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Alexander Popov <alex.popov@linux.com>,
	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>,
	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>,
	Thomas Garnier <thgarnie@google.com>,
	Will Deacon <will.deacon@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Laura Abbott <labbott@redhat.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, 27 Jul 2022 18:30:59 +0200	[thread overview]
Message-ID: <CAG48ez2Sh-kngNVeCF9-X550PQMaNnQaEvS+EAiWaDjWnmoHOg@mail.gmail.com> (raw)
In-Reply-To: <7c567acd-1cc1-a480-ca5a-d50a9c5a69ef@ispras.ru>

On Wed, Jul 27, 2022 at 6:17 PM Alexey Khoroshilov
<khoroshilov@ispras.ru> wrote:
> On 01.10.2021 22:59, Linus Torvalds wrote:
> Coming back to the discussion of WARN_ON()/pr_warn("WARNING:") semantics.
>
> We see a number of cases where WARNING is used to inform userspace that
> it is doing something wrong, e.g.
> https://elixir.bootlin.com/linux/v5.19-rc8/source/net/can/j1939/socket.c#L181
> https://elixir.bootlin.com/linux/v5.19-rc8/source/drivers/video/fbdev/core/fbmem.c#L1023
>
> It is definitely useful, but it does not make sense in case of fuzzing
> when the userspace should do wrong things and check if kernel behaves
> correctly.
>
> As a result we have warnings with two different intentions:
> - warn that something wrong happens in kernel, but we are able to continue;
> - warn userspace that it is doing something wrong.
>
> During fuzzing we would like to report the former and to ignore the
> latter. Are any ideas how these intentions can be recognized automatically?

https://elixir.bootlin.com/linux/v5.19-rc8/source/include/asm-generic/bug.h#L74
says:

 * WARN(), WARN_ON(), WARN_ON_ONCE, and so on can be used to report
 * significant kernel issues that need prompt attention if they should ever
 * appear at runtime.
 *
 * Do not use these macros when checking for invalid external inputs
 * (e.g. invalid system call arguments, or invalid data coming from
 * network/devices), and on transient conditions like ENOMEM or EAGAIN.
 * These macros should be used for recoverable kernel issues only.
 * For invalid external inputs, transient conditions, etc use
 * pr_err[_once/_ratelimited]() followed by dump_stack(), if necessary.
 * Do not include "BUG"/"WARNING" in format strings manually to make these
 * conditions distinguishable from kernel issues.

So if you see drivers intentionally using WARN() or printing
"WARNING:" on codepaths that are reachable with bogus inputs from
userspace, those codepaths should be fixed to log warnings in a
different format.