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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>,
        davem@davemloft.net, glider@google.com, linux-can@vger.kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
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 <7f5c4546-0c1a-86ae-581e-0203b5fca446@pengutronix.de>
 <1f7d6ea7-152e-ff18-549c-b196d8b5e3a7@hartkopp.net>
From:   Oleksij Rempel <o.rempel@pengutronix.de>
Message-ID: <d87d0751-1433-386d-48aa-d41686106ecc@pengutronix.de>
Date:   Tue, 19 Nov 2019 10:00:56 +0100
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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@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.

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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