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!