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@microsoft.com>
Reviewed-by: Sunil Muthuswamy <sunilmut@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
index f2084e3..9d864eb 100644 (file)
--- 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