[PATCH net] hv_sock: Fix hang when a connection is closed

[Dexuan Cui <decui@microsoft.com>]

hvs_do_close_lock_held() may decrease the reference count to 0 and free the
sk struct completely, and then the following release_sock(sk) may hang.

Fixes: a9eeb998c28d ("hv_sock: Add support for delayed close")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: stable@vger.kernel.org

---
With the proper kernel debugging options enabled, first a warning can
appear:

kworker/1:0/4467 is freeing memory ..., with a lock still held there!
stack backtrace:
Workqueue: events vmbus_onmessage_work [hv_vmbus]
Call Trace:
 dump_stack+0x67/0x90
 debug_check_no_locks_freed.cold.52+0x78/0x7d
 slab_free_freelist_hook+0x85/0x140
 kmem_cache_free+0xa5/0x380
 __sk_destruct+0x150/0x260
 hvs_close_connection+0x24/0x30 [hv_sock]
 vmbus_onmessage_work+0x1d/0x30 [hv_vmbus]
 process_one_work+0x241/0x600
 worker_thread+0x3c/0x390
 kthread+0x11b/0x140
 ret_from_fork+0x24/0x30

and then the following release_sock(sk) can hang:

watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [kworker/1:0:4467]
...
irq event stamp: 62890
CPU: 1 PID: 4467 Comm: kworker/1:0 Tainted: G        W         5.2.0+ #39
Workqueue: events vmbus_onmessage_work [hv_vmbus]
RIP: 0010:queued_spin_lock_slowpath+0x2b/0x1e0
...
Call Trace:
 do_raw_spin_lock+0xab/0xb0
 release_sock+0x19/0xb0
 vmbus_onmessage_work+0x1d/0x30 [hv_vmbus]
 process_one_work+0x241/0x600
 worker_thread+0x3c/0x390
 kthread+0x11b/0x140
 ret_from_fork+0x24/0x30

 net/vmw_vsock/hyperv_transport.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
index f2084e3f7aa4..efbda8ef1eff 100644
--- a/net/vmw_vsock/hyperv_transport.c
+++ b/net/vmw_vsock/hyperv_transport.c
@@ -309,9 +309,16 @@ static void hvs_close_connection(struct vmbus_channel *chan)
 {
 	struct sock *sk = get_per_channel_state(chan);
 
+	/* Grab an extra reference since hvs_do_close_lock_held() may decrease
+	 * the reference count to 0 by calling sock_put(sk).
+	 */
+	sock_hold(sk);
+
 	lock_sock(sk);
 	hvs_do_close_lock_held(vsock_sk(sk), true);
 	release_sock(sk);
+
+	sock_put(sk);
 }
 
 static void hvs_open_connection(struct vmbus_channel *chan)
-- 
2.19.1