libceph: validate con->state at the top of try_write()

ceph_con_workfn() validates con->state before calling try_read() and
then try_write().  However, try_read() temporarily releases con->mutex,
notably in process_message() and ceph_con_in_msg_alloc(), opening the
window for ceph_con_close() to sneak in, close the connection and
release con->sock.  When try_write() is called on the assumption that
con->state is still valid (i.e. not STANDBY or CLOSED), a NULL sock
gets passed to the networking stack:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
  IP: selinux_socket_sendmsg+0x5/0x20

Make sure con->state is valid at the top of try_write() and add an
explicit BUG_ON for this, similar to try_read().

Cc: stable@vger.kernel.org
Link: https://tracker.ceph.com/issues/23706
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>

net/ceph/messenger.c

@@ -2569,6 +2569,11 @@ static int try_write(struct ceph_connection *con)
 	int ret = 1;
 
 	dout("try_write start %p state %lu\n", con, con->state);
+	if (con->state != CON_STATE_PREOPEN &&
+	    con->state != CON_STATE_CONNECTING &&
+	    con->state != CON_STATE_NEGOTIATING &&
+	    con->state != CON_STATE_OPEN)
+		return 0;
 
 more:
 	dout("try_write out_kvec_bytes %d\n", con->out_kvec_bytes);
@@ -2594,6 +2599,8 @@ more:
 	}
 
 more_kvec:
+	BUG_ON(!con->sock);
+
 	/* kvec data queued? */
 	if (con->out_kvec_left) {
 		ret = write_partial_kvec(con);