Re: KMSAN: uninit-value in can_receive

[Dmitry Vyukov]

On Tue, Nov 19, 2019 at 8:36 AM Oliver Hartkopp <socketcan@xxxxxxxxxxxx> wrote:
> 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.

+Alex, please check re KMSAN false positive.
Oliver, Marc, where this skbcnt should have been initialized in this case?