Re: WARNING in can_rcv

[Oliver Hartkopp]

On 01/17/2018 08:39 AM, Dmitry Vyukov wrote:
> On Wed, Jan 17, 2018 at 8:12 AM, Eric Biggers <ebiggers3@xxxxxxxxx> 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@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.
>
>
> The packet comes from tun device. We could change tun to filter out
> such packages earlier. However, in the context of "syzkaller support
> for AF_CAN" discussion, it would actually be useful for fuzzer to be
> able emit can packets for testing purposes. 

Yes - definitely! It's a safer process to check the reception side 
instead of maintaining thousands of potential transmitters.

> For example, for tcp it
> can not just emit random packets, it can build complex user<->network
> interactions, for example, open a listening socket, connect to it
> "from outside", accept the connection, and then exchange some data
> over the active connection. It could do the same for can.

Yes.

> Is it possible to allow can packets via tun?

Hm - didn't even think about it.
CAN frames have a fixed data structure (struct can_frame) so the tunnel 
would need to be capable to process SOCK_SEQPACKET (?!?) traffic.

Right now there has been no work to 'tunnel' CAN traffic.

> Then we could leave this
> WARNING in place.

Yes.

> tun/vcan are contained within a net namespace, so
> this should not be a security problem, right?

vcan can be created in or moved into a namespace. vxcan can bridge 
namespaces similar to veth. This is all local traffic then.

What kind of security problem would you have in mind there?

> Or is there a way to do the same with vcan? If yes, then fuzzer could
> use vcan.

Yes. This would be my idea too. Unfortunately I'm very busy @work this 
week - so I would like to dig deeper into your mail some days ago at the 
beginning of next week.

> But then we need some fix for this WARNING: either change it
> to pr_warn or change tun (I don't have strong preference which one).

From the discussions (also with Eric) I think going with pr_warn is the 
right way for now.

Tnx & best regards,
Oliver