Re: [PATCH v2] net: ax25: Fix deadlock caused by skb_recv_datagram in ax25_recvmsg

[Eric Dumazet <edumazet@google.com>]

On Tue, Jun 7, 2022 at 7:24 AM Duoming Zhou <duoming@zju.edu.cn> wrote:
>
> The skb_recv_datagram() in ax25_recvmsg() will hold lock_sock
> and block until it receives a packet from the remote. If the client
> doesn`t connect to server and calls read() directly, it will not
> receive any packets forever. As a result, the deadlock will happen.
>
> The fail log caused by deadlock is shown below:
>
> [  369.606973] INFO: task ax25_deadlock:157 blocked for more than 245 seconds.
> [  369.608919] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  369.613058] Call Trace:
> [  369.613315]  <TASK>
> [  369.614072]  __schedule+0x2f9/0xb20
> [  369.615029]  schedule+0x49/0xb0
> [  369.615734]  __lock_sock+0x92/0x100
> [  369.616763]  ? destroy_sched_domains_rcu+0x20/0x20
> [  369.617941]  lock_sock_nested+0x6e/0x70
> [  369.618809]  ax25_bind+0xaa/0x210
> [  369.619736]  __sys_bind+0xca/0xf0
> [  369.620039]  ? do_futex+0xae/0x1b0
> [  369.620387]  ? __x64_sys_futex+0x7c/0x1c0
> [  369.620601]  ? fpregs_assert_state_consistent+0x19/0x40
> [  369.620613]  __x64_sys_bind+0x11/0x20
> [  369.621791]  do_syscall_64+0x3b/0x90
> [  369.622423]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> [  369.623319] RIP: 0033:0x7f43c8aa8af7
> [  369.624301] RSP: 002b:00007f43c8197ef8 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
> [  369.625756] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f43c8aa8af7
> [  369.626724] RDX: 0000000000000010 RSI: 000055768e2021d0 RDI: 0000000000000005
> [  369.628569] RBP: 00007f43c8197f00 R08: 0000000000000011 R09: 00007f43c8198700
> [  369.630208] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff845e6afe
> [  369.632240] R13: 00007fff845e6aff R14: 00007f43c8197fc0 R15: 00007f43c8198700
>
> This patch moves the skb_recv_datagram() before lock_sock() in order
> that other functions that need lock_sock could be executed.
>
> Suggested-by: Thomas Osterried <thomas@osterried.de>
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> Reported-by: Thomas Habets <thomas@@habets.se>
> ---
> Changes in v2:
>   - Make commit messages clearer.
>
>  net/ax25/af_ax25.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> index 95393bb2760..02cd6087512 100644
> --- a/net/ax25/af_ax25.c
> +++ b/net/ax25/af_ax25.c
> @@ -1665,6 +1665,11 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
>         int copied;
>         int err = 0;
>
> +       /* Now we can treat all alike */
> +       skb = skb_recv_datagram(sk, flags, &err);
> +       if (!skb)
> +               goto done;
> +

So at this point we have skb=something. This means that the following
branch will leak it.

if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED) {
    err =  -ENOTCONN;
    goto out;    // skb will be leaked
}


>         /*
>          *      This works for seqpacket too. The receiver has ordered the
> @@ -1675,11 +1680,6 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
>                 goto out;
>         }
>
> -       /* Now we can treat all alike */
> -       skb = skb_recv_datagram(sk, flags, &err);
> -       if (skb == NULL)
> -               goto out;
> -
>         if (!sk_to_ax25(sk)->pidincl)
>                 skb_pull(skb, 1);               /* Remove PID */
>
> @@ -1725,6 +1725,7 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
>  out:
>         release_sock(sk);
>
> +done:
>         return err;
>  }
>
> --
> 2.17.1
>