xen: xenbus driver must not accept invalid transaction ids

When accessing Xenstore in a transaction the user is specifying a
transaction id which he normally obtained from Xenstore when starting
the transaction. Xenstore is validating a transaction id against all
known transaction ids of the connection the request came in. As all
requests of a domain not being the one where Xenstore lives share
one connection, validation of transaction ids of different users of
Xenstore in that domain should be done by the kernel of that domain
being the multiplexer between the Xenstore users in that domain and
Xenstore.

In order to prohibit one Xenstore user "hijacking" a transaction from
another user the xenbus driver has to verify a given transaction id
against all known transaction ids of the user before forwarding it to
Xenstore.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -316,7 +316,7 @@ static int xenbus_write_transaction(unsigned msg_type,
 			rc = -ENOMEM;
 			goto out;
 		}
-	} else if (msg_type == XS_TRANSACTION_END) {
+	} else if (u->u.msg.tx_id != 0) {
 		list_for_each_entry(trans, &u->transactions, list)
 			if (trans->handle.id == u->u.msg.tx_id)
 				break;