Re: KMSAN: uninit-value in can_receive

[Alexander Potapenko <glider@google.com>]

On Tue, Nov 19, 2019 at 11:09 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, Nov 19, 2019 at 8:36 AM Oliver Hartkopp <socketcan@hartkopp.net> 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@syzkaller.appspotmail.com
> > >>>>>
> > >>>>> =====================================================
> > >>>>> 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.
Unfortunately syzbot didn't give a repro for this bug. I've tried
replaying the log, but it didn't work (or maybe the bug is fixed
already).
> Oliver, Marc, where this skbcnt should have been initialized in this case?



-- 
Alexander Potapenko
Software Engineer

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