tools/xenstore: drop watch event messages exceeding maximum size

By setting a watch with a very large tag it is possible to trick
xenstored to send watch event messages exceeding the maximum allowed
payload size. This might in turn lead to a crash of xenstored as the
resulting error can cause dereferencing a NULL pointer in case there
is no active request being handled by the guest the watch event is
being sent to.

Fix that by just dropping such watch events. Additionally modify the
error handling to test the pointer to be not NULL before dereferencing
it.

This is XSA-324.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 505560a5dea65e516deba99b3facaa61cf86f952..56b5e4578b1346e69ac066322ec6ce3993c8403b 100644 (file)
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -682,6 +682,9 @@ void send_reply(struct connection *conn, enum xsd_sockmsg_type type,
        /* Replies reuse the request buffer, events need a new one. */
        if (type != XS_WATCH_EVENT) {
                bdata = conn->in;
+               /* Drop asynchronous responses, e.g. errors for watch events. */
+               if (!bdata)
+                       return;
                bdata->inhdr = true;
                bdata->used = 0;
                conn->in = NULL;

diff --git a/tools/xenstore/xenstored_watch.c b/tools/xenstore/xenstored_watch.c
index 71c108ea99f1785c46e6f416f94576e547716404..9ff20690c0003e3d152115ce28cb2ebf80d34216 100644 (file)
--- a/tools/xenstore/xenstored_watch.c
+++ b/tools/xenstore/xenstored_watch.c
@@ -92,6 +92,10 @@ static void add_event(struct connection *conn,
        }
 
        len = strlen(name) + 1 + strlen(watch->token) + 1;
+       /* Don't try to send over-long events. */
+       if (len > XENSTORE_PAYLOAD_MAX)
+               return;
+
        data = talloc_array(ctx, char, len);
        if (!data)
                return;