netdev - Re: KMSAN: uninit-value in can

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Date:   Mon, 18 Nov 2019 21:49:30 +0100
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        syzbot <syzbot+b02ff0707a97e4e79ebb@...kaller.appspotmail.com>,
        davem@...emloft.net, glider@...gle.com, linux-can@...r.kernel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: KMSAN: uninit-value in can_receive



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@...kaller.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.

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?

Regards,
Oliver