Re: KMSAN: uninit-value in can_receive

[Oliver Hartkopp]

On 18/11/2019 22.15, Marc Kleine-Budde wrote:
> On 11/18/19 9:49 PM, Oliver Hartkopp wrote:
> > On 18/11/2019 21.29, Marc Kleine-Budde wrote:
> > > On 11/18/19 9:25 PM, Oliver Hartkopp wrote:
> >
> > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > > Reported-by: syzbot+b02ff0707a97e4e79ebb@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > >
> > > > > =====================================================
> > > > > BUG: KMSAN: uninit-value in can_receive+0x23c/0x5e0 net/can/af_can.c:649
> > > > > CPU: 1 PID: 3490 Comm: syz-executor.2 Not tainted 5.4.0-rc5+ #0
> >
> > > > In line 649 of 5.4.0-rc5+ we can find a while() statement:
> > > >
> > > > while (!(can_skb_prv(skb)->skbcnt))
> > > > 	can_skb_prv(skb)->skbcnt = atomic_inc_return(&skbcounter);
> > > >
> > > > In linux/include/linux/can/skb.h we see:
> > > >
> > > > static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb)
> > > > {
> > > > 	return (struct can_skb_priv *)(skb->head);
> > > > }
> > > >
> > > > IMO accessing can_skb_prv(skb)->skbcnt at this point is a valid
> > > > operation which has no uninitialized value.
> > > >
> > > > Can this probably be a false positive of KMSAN?
> > >
> > > The packet is injected via the packet socket into the kernel. Where does
> > > skb->head point to in this case? When the skb is a proper
> > > kernel-generated skb containing a CAN-2.0 or CAN-FD frame skb->head is
> > > maybe properly initialized?
> >
> > The packet is either received via vcan or vxcan which checks via
> > can_dropped_invalid_skb() if we have a valid ETH_P_CAN type skb.
>
> According to the call stack it's injected into the kernel via a packet
> socket and not via v(x)can.

See ioctl$ifreq https://syzkaller.appspot.com/x/log.txt?x=14563416e00000

23:11:34 executing program 2:
r0 = socket(0x200000000000011, 0x3, 0x0)
ioctl$ifreq_SIOCGIFINDEX_vcan(r0, 0x8933, 
&(0x7f0000000040)={'vxcan1\x00', <r1=>0x0})
bind$packet(r0, &(0x7f0000000300)={0x11, 0xc, r1}, 0x14)
sendmmsg(r0, &(0x7f0000000d00), 0x400004e, 0x0)

We only can receive skbs from (v(x))can devices.
No matter if someone wrote to them via PF_CAN or PF_PACKET.
We check for ETH_P_CAN(FD) type and ARPHRD_CAN dev type at rx time.

> > We additionally might think about introducing a check whether we have a
> > can_skb_reserve() created skbuff.
> >
> > But even if someone forged a skbuff without this reserved space the
> > access to can_skb_prv(skb)->skbcnt would point into some CAN frame
> > content - which is still no access to uninitialized content, right?

So this question remains still valid whether we have a false positive 
from KMSAN here.

Regards,
Oliver