Re: KMSAN: uninit-value in can_receive

[Marc Kleine-Budde]

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))
>>> 	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.

> 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?

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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