Fix handling of the entries-per-domain quota. Entries which are created by

the guest but deleted by dom0 were remaining accounted against the guest,
which meant that the guest would eventually run out of quota.

This patch also prevents unprivileged domains from changing the owner of a
node.  One guest could attack another by creating nodes and then transferring
them to the ownership of another, and though the accounting could be made to
work properly in this case, domains should never be transferring nodes in any
case, so it seems safer just to disallow the operation entirely.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

tools/xenstore/xenstored_core.c

@@ -791,7 +791,7 @@ static void delete_node_single(struct connection *conn, struct node *node)
 		corrupt(conn, "Could not delete '%s'", node->name);
 		return;
 	}
-	domain_entry_dec(conn);
+	domain_entry_dec(conn, node);
 }
 
 /* Must not be / */
@@ -842,7 +842,7 @@ static struct node *construct_node(struct connection *conn, const char *name)
 	node->children = node->data = NULL;
 	node->childlen = node->datalen = 0;
 	node->parent = parent;
-	domain_entry_inc(conn);
+	domain_entry_inc(conn, node);
 	return node;
 }
 
@@ -878,7 +878,7 @@ static struct node *create_node(struct connection *conn,
 	 * something goes wrong. */
 	for (i = node; i; i = i->parent) {
 		if (!write_node(conn, i)) {
-			domain_entry_dec(conn);
+			domain_entry_dec(conn, i);
 			return NULL;
 		}
 		talloc_set_destructor(i, destroy_node);
@@ -1108,6 +1108,7 @@ static void do_get_perms(struct connection *conn, const char *name)
 static void do_set_perms(struct connection *conn, struct buffered_data *in)
 {
 	unsigned int num;
+	struct xs_permissions *perms;
 	char *name, *permstr;
 	struct node *node;
 
@@ -1129,12 +1130,24 @@ static void do_set_perms(struct connection *conn, struct buffered_data *in)
 		return;
 	}
 
-	node->perms = talloc_array(node, struct xs_permissions, num);
-	node->num_perms = num;
-	if (!xs_strings_to_perms(node->perms, num, permstr)) {
+	perms = talloc_array(node, struct xs_permissions, num);
+	if (!xs_strings_to_perms(perms, num, permstr)) {
 		send_error(conn, errno);
 		return;
 	}
+
+	/* Unprivileged domains may not change the owner. */
+	if (domain_is_unprivileged(conn) &&
+	    perms[0].id != node->perms[0].id) {
+		send_error(conn, EPERM);
+		return;
+	}
+
+	domain_entry_dec(conn, node);
+	node->perms = perms;
+	node->num_perms = num;
+	domain_entry_inc(conn, node);
+
 	if (!write_node(conn, node)) {
 		send_error(conn, errno);
 		return;

tools/xenstore/xenstored_domain.c

@@ -501,18 +501,35 @@ int domain_init(void)
 	return xce_handle;
 }
 
-void domain_entry_inc(struct connection *conn)
+void domain_entry_inc(struct connection *conn, struct node *node)
 {
-	if (!conn || !conn->domain)
+	struct domain *d;
+
+	if (!conn)
 		return;
-	conn->domain->nbentry++;
+
+	if (node->perms && node->perms[0].id != conn->id) {
+		d = find_domain_by_domid(node->perms[0].id);
+		if (d)
+			d->nbentry++;
+	}
+	else if (conn->domain) {
+		conn->domain->nbentry++;
+	}
 }
 
-void domain_entry_dec(struct connection *conn)
+void domain_entry_dec(struct connection *conn, struct node *node)
 {
-	if (!conn || !conn->domain)
+	struct domain *d;
+
+	if (!conn)
 		return;
-	if (conn->domain->nbentry)
+
+	if (node->perms && node->perms[0].id != conn->id) {
+		d = find_domain_by_domid(node->perms[0].id);
+		if (d && d->nbentry)
+			d->nbentry--;
+	} else if (conn->domain && conn->domain->nbentry)
 		conn->domain->nbentry--;
 }
 

tools/xenstore/xenstored_domain.h

@@ -50,8 +50,8 @@ bool domain_can_write(struct connection *conn);
 bool domain_is_unprivileged(struct connection *conn);
 
 /* Quota manipulation */
-void domain_entry_inc(struct connection *conn);
-void domain_entry_dec(struct connection *conn);
+void domain_entry_inc(struct connection *conn, struct node *);
+void domain_entry_dec(struct connection *conn, struct node *);
 int domain_entry(struct connection *conn);
 void domain_watch_inc(struct connection *conn);
 void domain_watch_dec(struct connection *conn);