Re: KMSAN: uninit-value in can_receive

[Oliver Hartkopp]

Hi Eric,

On 19/11/2019 17.53, Eric Dumazet wrote:
> On 11/18/19 11:35 PM, Oliver Hartkopp wrote:
> >
>
> > 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.
>
> And what entity sets the can_skb_prv(skb)->skbcnt to zero exactly ?
>
> > > > 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.
>
> I do not believe it is a false positive.
>
> It seems CAN relies on some properties of low level drivers using alloc_can_skb() or similar function.
>
> Why not simply fix this like that ?
>
> diff --git a/net/can/af_can.c b/net/can/af_can.c
> index 128d37a4c2e0ba5d8db69fcceec8cbd6a79380df..3e71a78d82af84caaacd0ef512b5e894efbf4852 100644
> --- a/net/can/af_can.c
> +++ b/net/can/af_can.c
> @@ -647,8 +647,9 @@ static void can_receive(struct sk_buff *skb, struct net_device *dev)
>          pkg_stats->rx_frames_delta++;
>   
>          /* create non-zero unique skb identifier together with *skb */
> -       while (!(can_skb_prv(skb)->skbcnt))
> +       do {
>                  can_skb_prv(skb)->skbcnt = atomic_inc_return(&skbcounter);
> +       } while (!(can_skb_prv(skb)->skbcnt));
>   
>          rcu_read_lock();
>   

Please check commit d3b58c47d330d ("can: replace timestamp as unique skb 
attribute").

can_skb_prv(skb)->skbcnt is set to 0 at skb creation time when sending 
CAN frames from local host or receiving CAN frames from a real CAN 
interface.

When a CAN skb is received by the net layer the *first* time it gets a 
unique value which we need for a per-cpu filter mechanism in raw_rcv().

So where's the problem to check for (!(can_skb_prv(skb)->skbcnt)) in a 
while statement? I can't see a chance for an uninitialized value there.

Regards,
Oliver