Re: WARNING in can_rcv

[Oliver Hartkopp <socketcan@hartkopp.net>]

On 01/17/2018 08:12 AM, Eric Biggers wrote:
> On Wed, Jan 17, 2018 at 07:39:24AM +0100, Oliver Hartkopp wrote:
>>
>>
>> On 01/16/2018 07:11 PM, Dmitry Vyukov wrote:
>>> On Tue, Jan 16, 2018 at 7:07 PM, Marc Kleine-Budde <mkl@pengutronix.de> wrote:
>>>> On 01/16/2018 06:58 PM, syzbot wrote:
>>>>> Hello,
>>>>>
>>>>> syzkaller hit the following crash on
>>>>> a8750ddca918032d6349adbf9a4b6555e7db20da
>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
>>>>> compiler: gcc (GCC) 7.1.1 20170620
>>>>> .config is attached
>>>>> Raw console output is attached.
>>>>> C reproducer is attached
>>>>> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
>>>>> for information about syzkaller reproducers
>>>>>
>>>>>
>>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>>> Reported-by: syzbot+4386709c0c1284dca827@syzkaller.appspotmail.com
>>>>> It will help syzbot understand when the bug is fixed. See footer for
>>>>> details.
>>>>> If you forward the report, please keep this part and the footer.
>>>>>
>>>>> device eql entered promiscuous mode
>>>>> ------------[ cut here ]------------
>>>>> PF_CAN: dropped non conform CAN skbuf: dev type 65534, len 42, datalen 0
>>>>> WARNING: CPU: 0 PID: 3650 at net/can/af_can.c:729 can_rcv+0x1c5/0x200
>>>>> net/can/af_can.c:724
>>>>> Kernel panic - not syncing: panic_on_warn set ...
>>>>
>>>> Invalid packages generate a warning (WARN_ONCE()), and you have
>>>> panic_on_warn active. Should we better silently drop these CAN packages?
>>>
>>> Hi,
>>>
>>> pr_warn_once() will be more appropriate. It prints a single line.
>>>
>>
>> The idea behind this WARN() is to detect really bad things that might have
>> happen on network driver level:
>>
>> The CAN subsystem registers with dev_add_pack() for ETH_P_CAN and
>> ETH_P_CANFD only. These ETH_P_ types are only allowed to be created by CAN
>> network devices (like vcan, vxcan, and real CAN drivers).
>>
>> I don't have any strong opinion on using WARN() or pr_warn_once().
>> Is this detected violation worth using WARN(), as something already must
>> have gone really wrong to trigger this issue?
>>
> 
> WARN() indicates a kernel bug.  If it's instead "userspace did something
> stupid", or "someone sent some unexpected network packet", it needs to be
> pr_warn_once(), pr_warn_ratelimited(), or removed entirely.

Ok. Thanks for the explanation!
It is "some bogus network driver sent something unexpected" - but that 
does not harm the entire system.

pr_warn_once() seems the right way to go then.

Thanks,
Oliver