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Subject[PATCH v3] net: ax25: Fix deadlock caused by skb_recv_datagram in ax25_recvmsg
Date
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. What`s more, we
add skb_free_datagram() before goto out in order to mitigate memory leak.

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 v3:
- Add skb_free_datagram() before goto out in order to mitigate memory leak.

net/ax25/af_ax25.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 95393bb2760..62aa5993093 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
@@ -1672,14 +1677,10 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
*/
if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED) {
err = -ENOTCONN;
+ skb_free_datagram(sk, skb);
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 +1726,7 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
out:
release_sock(sk);

+done:
return err;
}

--
2.17.1
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