tools/xenstore: alloc new memory in domain_adjust_node_perms()

In order to avoid modifying the node data in the data base in case a
domain is gone, let domain_adjust_node_perms() allocate new memory for
the permissions in case they need to be modified. As this should
happen only in very rare cases, it is fine to do this even when having
copied the node data already.

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

tools/xenstore/xenstored_core.c

@@ -752,6 +752,11 @@ struct node *read_node(struct connection *conn, const void *ctx,
 		goto error;
 	}
 
+	/* Data is binary blob (usually ascii, no nul). */
+	node->data = node->perms + hdr->num_perms;
+	/* Children is strings, nul separated. */
+	node->children = node->data + node->hdr.datalen;
+
 	if (domain_adjust_node_perms(node))
 		goto error;
 
@@ -759,11 +764,6 @@ struct node *read_node(struct connection *conn, const void *ctx,
 	if (node->acc.domid != get_node_owner(node))
 		node->acc.memory = 0;
 
-	/* Data is binary blob (usually ascii, no nul). */
-	node->data = node->perms + hdr->num_perms;
-	/* Children is strings, nul separated. */
-	node->children = node->data + node->hdr.datalen;
-
 	if (access_node(conn, node, NODE_ACCESS_READ, NULL))
 		goto error;
 

tools/xenstore/xenstored_domain.c

@@ -1334,13 +1334,29 @@ int domain_alloc_permrefs(struct node_perms *perms)
 int domain_adjust_node_perms(struct node *node)
 {
 	unsigned int i;
+	struct xs_permissions *perms = node->perms;
+	bool copied = false;
 
 	for (i = 1; i < node->hdr.num_perms; i++) {
-		if (node->perms[i].perms & XS_PERM_IGNORE)
+		if ((perms[i].perms & XS_PERM_IGNORE) ||
+		    chk_domain_generation(perms[i].id, node->hdr.generation))
 			continue;
-		if (!chk_domain_generation(node->perms[i].id,
-					   node->hdr.generation))
-			node->perms[i].perms |= XS_PERM_IGNORE;
+
+		/*
+		 * Don't do a in-place modification, as the node might
+		 * reference data directly in the data base, which we don't
+		 * want to modify.
+		 */
+		if (!copied) {
+			perms = talloc_memdup(node, node->perms,
+					node->hdr.num_perms * sizeof(*perms));
+			if (!perms)
+				return ENOMEM;
+			node->perms = perms;
+			copied = true;
+		}
+
+		perms[i].perms |= XS_PERM_IGNORE;
 	}
 
 	return 0;

tools/xenstore/xenstored_domain.h

@@ -90,8 +90,14 @@ void ignore_connection(struct connection *conn, unsigned int err);
 /* Returns the implicit path of a connection (only domains have this) */
 const char *get_implicit_path(const struct connection *conn);
 
-/* Remove node permissions for no longer existing domains. */
+/*
+ * Remove node permissions for no longer existing domains.
+ * In case of a change of permissions the related array is reallocated in
+ * order to avoid a data base change when operating on a node directly
+ * referencing the data base contents.
+ */
 int domain_adjust_node_perms(struct node *node);
+
 int domain_alloc_permrefs(struct node_perms *perms);
 
 /* Quota manipulation */