> 
> 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:

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()

> 
> 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!
>>
> 
> .