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Message-ID: <64695483-ff75-4872-db81-ca55763f95cf@hartkopp.net>
Date: Thu, 20 Jan 2022 15:46:13 +0100
From: Oliver Hartkopp <socketcan@...tkopp.net>
To: "Ziyang Xuan (William)" <william.xuanziyang@...wei.com>,
mkl@...gutronix.de
Cc: davem@...emloft.net, kuba@...nel.org, linux-can@...r.kernel.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] can: isotp: isotp_rcv_cf(): fix so->rx race problem
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 ...
Regards,
Oliver
>>
>> And when so->rx.len is 0 in isotp_rcv_ff() this check
>>
>> if (so->rx.len + ae + off + ff_pci_sz < so->rx.ll_dl)
>> return 1;
>>
>> will return from isotp_rcv_ff() before ISOTP_WAIT_DATA is set at the end. So after that above check we are still in ISOTP_IDLE state.
>>
>> Or did I miss something here?
>>
>>> so->rx.len is 0 before alloc_skb() and is 4096 after alloc_skb() in isotp_rcv_cf(). The following
>>> skb_put() will trigger panic.
>>>
>>> The following log is my reproducing log with Marc's commit and my debug modification in isotp_rcv_cf().
>>>
>>> [ 150.605776][ C6] isotp_rcv_cf: before alloc_skb so->rc.len: 0, after alloc_skb so->rx.len: 4096
>>
>>
>> But so->rx_len is not a value that is modified by alloc_skb():
>>
>> nskb = alloc_skb(so->rx.len, gfp_any());
>> if (!nskb)
>> return 1;
>>
>> memcpy(skb_put(nskb, so->rx.len), so->rx.buf,
>> so->rx.len);
>>
>>
>> Can you send your debug modification changes please?
>
> My reproducing debug as attachment and following:
>
> diff --git a/net/can/isotp.c b/net/can/isotp.c
> index df6968b28bf4..8b12d63b4d59 100644
> --- a/net/can/isotp.c
> +++ b/net/can/isotp.c
> @@ -119,8 +119,8 @@ enum {
> };
>
> struct tpcon {
> - int idx;
> - int len;
> + unsigned int idx;
> + unsigned int len;
> u32 state;
> u8 bs;
> u8 sn;
> @@ -505,6 +505,7 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
> struct isotp_sock *so = isotp_sk(sk);
> struct sk_buff *nskb;
> int i;
> + bool unexpection = false;
>
> if (so->rx.state != ISOTP_WAIT_DATA)
> return 0;
> @@ -562,11 +563,13 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
> sk_error_report(sk);
> return 1;
> }
> -
> + if (so->rx.len == 0)
> + unexpection = true;
> nskb = alloc_skb(so->rx.len, gfp_any());
> if (!nskb)
> return 1;
> -
> + if (unexpection)
> + printk("%s: before alloc_skb so->rc.len: 0, after alloc_skb so->rx.len: %u\n", __func__, so->rx.len);
> memcpy(skb_put(nskb, so->rx.len), so->rx.buf,
> so->rx.len);
>
>
>>
>> Best regards,
>> Oliver
>>
>>> [ 150.611477][ C6] skbuff: skb_over_panic: text:ffffffff881ff7be len:4096 put:4096 head:ffff88807f93a800 data:ffff88807f93a800 tail:0x1000 end:0xc0 dev:<NULL>
>>> [ 150.615837][ C6] ------------[ cut here ]------------
>>> [ 150.617238][ C6] kernel BUG at net/core/skbuff.c:113!
>>>
>>
>> .
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