netdev - [PATCH v2 net] hv_sock: Fix hang when a connection is closed

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Message-ID: <PU1P153MB01696DDD3A3F601370701DD2BFDF0@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM>
Date:   Wed, 31 Jul 2019 01:25:45 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Sunil Muthuswamy <sunilmut@...rosoft.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        vkuznets <vkuznets@...hat.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>
Subject: [PATCH v2 net] hv_sock: Fix hang when a connection is closed

There is a race condition for an established connection that is being closed
by the guest: the refcnt is 4 at the end of hvs_release() (Note: here the
'remove_sock' is false):

1 for the initial value;
1 for the sk being in the bound list;
1 for the sk being in the connected list;
1 for the delayed close_work.

After hvs_release() finishes, __vsock_release() -> sock_put(sk) *may*
decrease the refcnt to 3.

Concurrently, hvs_close_connection() runs in another thread:
  calls vsock_remove_sock() to decrease the refcnt by 2;
  call sock_put() to decrease the refcnt to 0, and free the sk;
  next, the "release_sock(sk)" may hang due to use-after-free.

In the above, after hvs_release() finishes, if hvs_close_connection() runs
faster than "__vsock_release() -> sock_put(sk)", then there is not any issue,
because at the beginning of hvs_close_connection(), the refcnt is still 4.

The issue can be resolved if an extra reference is taken when the
connection is established.

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

---

Changes in v2: 
  Changed the location of the sock_hold() call. 
  Updated the changelog accordingly.

  Thanks Sunil for the suggestion!

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 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
index f2084e3f7aa4..9d864ebeb7b3 100644
--- a/net/vmw_vsock/hyperv_transport.c
+++ b/net/vmw_vsock/hyperv_transport.c
@@ -312,6 +312,11 @@ static void hvs_close_connection(struct vmbus_channel *chan)
 	lock_sock(sk);
 	hvs_do_close_lock_held(vsock_sk(sk), true);
 	release_sock(sk);
+
+	/* Release the refcnt for the channel that's opened in
+	 * hvs_open_connection().
+	 */
+	sock_put(sk);
 }

 static void hvs_open_connection(struct vmbus_channel *chan)
@@ -407,6 +412,9 @@ static void hvs_open_connection(struct vmbus_channel *chan)
 	}

 	set_per_channel_state(chan, conn_from_host ? new : sk);
+
+	/* This reference will be dropped by hvs_close_connection(). */
+	sock_hold(conn_from_host ? new : sk);
 	vmbus_set_chn_rescind_callback(chan, hvs_close_connection);

 	/* Set the pending send size to max packet size to always get
-- 
2.19.1