From c8057cb483abf2cd4060b39616423e19283fbd0a Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@suse.com>
Date: Tue, 13 Sep 2022 07:35:08 +0200
Subject: tools/xenstore: don't buffer multiple identical watch events

A guest not reading its Xenstore response buffer fast enough might
pile up lots of Xenstore watch events buffered. Reduce the generated
load by dropping new events which already have an identical copy
pending.

The special events "@..." are excluded from that handling as there are
known use cases where the handler is relying on each event to be sent
individually.

This is part of XSA-326.

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

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 488d540f3a32..f1fa97b8cf50 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -916,6 +916,7 @@ void send_reply(struct connection *conn, enum xsd_sockmsg_type type,
 	bdata->inhdr = true;
 	bdata->used = 0;
 	bdata->timeout_msec = 0;
+	bdata->watch_event = false;
 
 	if (len <= DEFAULT_BUFFER_SIZE)
 		bdata->buffer = bdata->default_buffer;
@@ -948,7 +949,7 @@ void send_reply(struct connection *conn, enum xsd_sockmsg_type type,
 void send_event(struct buffered_data *req, struct connection *conn,
 		const char *path, const char *token)
 {
-	struct buffered_data *bdata;
+	struct buffered_data *bdata, *bd;
 	unsigned int len;
 
 	len = strlen(path) + 1 + strlen(token) + 1;
@@ -970,12 +971,29 @@ void send_event(struct buffered_data *req, struct connection *conn,
 	bdata->hdr.msg.type = XS_WATCH_EVENT;
 	bdata->hdr.msg.len = len;
 
+	/*
+	 * Check whether an identical event is pending already.
+	 * Special events are excluded from that check.
+	 */
+	if (path[0] != '@') {
+		list_for_each_entry(bd, &conn->out_list, list) {
+			if (bd->watch_event && bd->hdr.msg.len == len &&
+			    !memcmp(bdata->buffer, bd->buffer, len)) {
+				trace("dropping duplicate watch %s %s for domain %u\n",
+				      path, token, conn->id);
+				talloc_free(bdata);
+				return;
+			}
+		}
+	}
+
 	if (timeout_watch_event_msec && domain_is_unprivileged(conn)) {
 		bdata->timeout_msec = get_now_msec() + timeout_watch_event_msec;
 		if (!conn->timeout_msec)
 			conn->timeout_msec = bdata->timeout_msec;
 	}
 
+	bdata->watch_event = true;
 	bdata->pend.req = req;
 	if (req)
 		req->pend.ref.event_cnt++;
diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index db09f463a657..b9b50e81c7b4 100644
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -62,6 +62,9 @@ struct buffered_data
 	/* Are we still doing the header? */
 	bool inhdr;
 
+	/* Is this a watch event? */
+	bool watch_event;
+
 	/* How far are we? */
 	unsigned int used;