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From:   Jann Horn <jannh@google.com>
Date:   Wed, 27 Jul 2022 18:30:59 +0200
Message-ID: <CAG48ez2Sh-kngNVeCF9-X550PQMaNnQaEvS+EAiWaDjWnmoHOg@mail.gmail.com>
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
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>,
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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.