[PATCH net-next] ax25: Fix deadlock caused by skb_recv_datagram in ax25_recvmsg

[Duoming Zhou]

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:

[  861.122612] INFO: task ax25_deadlock:148 blocked for more than 737 seconds.
[  861.124543] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  861.127764] Call Trace:
[  861.129688]  <TASK>
[  861.130743]  __schedule+0x2f9/0xb20
[  861.131526]  schedule+0x49/0xb0
[  861.131640]  __lock_sock+0x92/0x100
[  861.131640]  ? destroy_sched_domains_rcu+0x20/0x20
[  861.131640]  lock_sock_nested+0x6e/0x70
[  861.131640]  ax25_sendmsg+0x46/0x420
[  861.134383]  ? ax25_recvmsg+0x1e0/0x1e0
[  861.135658]  sock_sendmsg+0x59/0x60
[  861.136791]  __sys_sendto+0xe9/0x150
[  861.137212]  ? __schedule+0x301/0xb20
[  861.137710]  ? __do_softirq+0x4a2/0x4fd
[  861.139153]  __x64_sys_sendto+0x20/0x30
[  861.140330]  do_syscall_64+0x3b/0x90
[  861.140731]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
[  861.141249] RIP: 0033:0x7fdf05ee4f64
[  861.141249] RSP: 002b:00007ffe95772fc0 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
[  861.141249] RAX: ffffffffffffffda RBX: 0000565303a013f0 RCX: 00007fdf05ee4f64
[  861.141249] RDX: 0000000000000005 RSI: 0000565303a01678 RDI: 0000000000000005
[  861.141249] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[  861.141249] R10: 0000000000000000 R11: 0000000000000246 R12: 0000565303a00cf0
[  861.141249] R13: 00007ffe957730e0 R14: 0000000000000000 R15: 0000000000000000

This patch moves the skb_recv_datagram() before lock_sock() in order
that other functions that need lock_sock could be executed.

Reported-by: Thomas Habets <thomas@@habets.se>
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
 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;
+
 	lock_sock(sk);
 	/*
 	 * 	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