Re: KMSAN: uninit-value in can_receive

[Oliver Hartkopp]

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.

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?

Regards,
Oliver