Re: WARNING in can_rcv

[Eric Biggers]

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@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > 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.

Eric