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

Hello Ziyang Xuan,

On 21.01.22 02:50, Ziyang Xuan (William) wrote:

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On 20.01.22 12:28, Ziyang Xuan (William) wrote:

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On 20.01.22 07:24, Ziyang Xuan (William) wrote:

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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.

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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/ (local)