Re: [PATCH net] can: isotp: isotp_rcv_cf(): fix so->rx race problem

[Oliver Hartkopp <socketcan@hartkopp.net>]

Hello Ziyang Xuan,

On 21.01.22 02:50, Ziyang Xuan (William) wrote:
>>
>> On 20.01.22 12:28, Ziyang Xuan (William) wrote:
>>>>
>>>> On 20.01.22 07:24, Ziyang Xuan (William) wrote:
>>>>
>>>>> I have reproduced the syz problem with Marc's commit, the commit can not fix the panic problem.
>>>>> So I tried to find the root cause for panic and gave my solution.
>>>>>
>>>>> Marc's commit just fix the condition that packet size bigger than INT_MAX which trigger
>>>>> tpcon::{idx,len} integer overflow, but the packet size is 4096 in the syz problem.
>>>>>
>>>>> so->rx.len is 0 after the following logic in isotp_rcv_ff():
>>>>>
>>>>> /* get the FF_DL */
>>>>> so->rx.len = (cf->data[ae] & 0x0F) << 8;
>>>>> so->rx.len += cf->data[ae + 1];
>>>>>
>>>>> so->rx.len is 4096 after the following logic in isotp_rcv_ff():
>>>>>
>>>>> /* FF_DL = 0 => get real length from next 4 bytes */
>>>>> so->rx.len = cf->data[ae + 2] << 24;
>>>>> so->rx.len += cf->data[ae + 3] << 16;
>>>>> so->rx.len += cf->data[ae + 4] << 8;
>>>>> so->rx.len += cf->data[ae + 5];
>>>>>
>>>>
>>>> In these cases the values 0 could be the minimum value in so->rx.len - but e.g. the value 0 can not show up in isotp_rcv_cf() as this function requires so->rx.state to be ISOTP_WAIT_DATA.
>>>
>>> Consider the scenario that isotp_rcv_cf() and isotp_rcv_cf() are concurrent for the same isotp_sock as following sequence:
>>
>> o_O
>>
>> Sorry but the receive path is not designed to handle concurrent receptions that would run isotp_rcv_cf() and isotp_rcv_ff() simultaneously.
>>
>>> isotp_rcv_cf()
>>> if (so->rx.state != ISOTP_WAIT_DATA) [false]
>>>                          isotp_rcv_ff()
>>>                          so->rx.state = ISOTP_IDLE
>>>                          /* get the FF_DL */ [so->rx.len == 0]
>>> alloc_skb() [so->rx.len == 0]
>>>                          /* FF_DL = 0 => get real length from next 4 bytes */ [so->rx.len == 4096]
>>> skb_put(nskb, so->rx.len) [so->rx.len == 4096]
>>> skb_over_panic()
>>>
>>
>> Even though this case is not possible with a real CAN bus due to the CAN frame transmission times we could introduce some locking (or dropping of concurrent CAN frames) in isotp_rcv() - but this code runs in net softirq context ...
>>

As discussed off-list I added a spin_lock() in isotp_rcv() as 
https://www.kernel.org/doc/htmldocs/kernel-locking/lock-softirqs.html 
suggests.

Please give this patch[1] a try in your test setup.

Many thanks,
Oliver

[1]: 
https://lore.kernel.org/linux-can/20220127192429.336335-1-socketcan@hartkopp.net/T/