On Tue, Jun 7, 2022 at 7:24 AM Duoming Zhou <[email protected]> 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 <[email protected]>
> Signed-off-by: Duoming Zhou <[email protected]>
> 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
>