netdev - Re: [PATCH net] can: isotp: isotp_rcv

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Date:   Thu, 20 Jan 2022 09:23:06 +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 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.

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?

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!
>