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Subject: Re: KMSAN: uninit-value in can_receive
To:     Oliver Hartkopp <socketcan@hartkopp.net>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        syzbot <syzbot+b02ff0707a97e4e79ebb@syzkaller.appspotmail.com>,
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        syzkaller-bugs@googlegroups.com
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From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <9e06266a-67f3-7352-7b87-2b9144c7c9a9@gmail.com>
Date:   Tue, 19 Nov 2019 08:53:33 -0800
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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();