Re: KMSAN: uninit-value in can_receive

[Oleksij Rempel]

Hi,

On 19.11.19 08:35, Oliver Hartkopp 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))
> > > > > ÂÂÂÂcan_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)
> > > > > {
> > > > > ÂÂÂÂreturn (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.

It can be other incornation of this bug:
https://github.com/linux-can/linux/issues/1

The echo skd was free, because socket which send this skb was closed before it was received.

Kind regards,
Oleksij Rempel

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