tools/xenstore: remove recursion from construct_node()

In order to reduce stack usage due to recursion, switch
construct_node() to use a loop instead.

This is part of XSA-418 / CVE-2022-42321.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit da8ee25d02a5447ba39a9800ee2a710ae1f54222)

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index c676ee4e4e4f19ee5075f335274ad612ed9181fe..3907c35643e99467da08b61302b83c26bdd57b88 100644 (file)
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -1377,45 +1377,69 @@ static int add_child(const void *ctx, struct node *parent, const char *name)
 static struct node *construct_node(struct connection *conn, const void *ctx,
                                   const char *name)
 {
-       struct node *parent, *node;
-       char *parentname = get_parent(ctx, name);
+       const char **names = NULL;
+       unsigned int levels = 0;
+       struct node *node = NULL;
+       struct node *parent = NULL;
+       const char *parentname = talloc_strdup(ctx, name);
 
        if (!parentname)
                return NULL;
 
-       /* If parent doesn't exist, create it. */
-       parent = read_node(conn, parentname, parentname);
-       if (!parent && errno == ENOENT)
-               parent = construct_node(conn, ctx, parentname);
-       if (!parent)
-               return NULL;
+       /* Walk the path up until an existing node is found. */
+       while (!parent) {
+               names = talloc_realloc(ctx, names, const char *, levels + 1);
+               if (!names)
+                       goto nomem;
 
-       /* Add child to parent. */
-       if (add_child(ctx, parent, name))
-               goto nomem;
+               /*
+                * names[0] is the name of the node to construct initially,
+                * names[1] is its parent, and so on.
+                */
+               names[levels] = parentname;
+               parentname = get_parent(ctx, parentname);
+               if (!parentname)
+                       return NULL;
 
-       /* Allocate node */
-       node = talloc(ctx, struct node);
-       if (!node)
-               goto nomem;
-       node->name = talloc_strdup(node, name);
-       if (!node->name)
-               goto nomem;
+               /* Try to read parent node until we found an existing one. */
+               parent = read_node(conn, ctx, parentname);
+               if (!parent && (errno != ENOENT || !strcmp(parentname, "/")))
+                       return NULL;
 
-       /* Inherit permissions, except unprivileged domains own what they create */
-       node->perms.num = parent->perms.num;
-       node->perms.p = talloc_memdup(node, parent->perms.p,
-                                     node->perms.num * sizeof(*node->perms.p));
-       if (!node->perms.p)
-               goto nomem;
-       if (domain_is_unprivileged(conn))
-               node->perms.p[0].id = conn->id;
+               levels++;
+       }
+
+       /* Walk the path down again constructing the missing nodes. */
+       for (; levels > 0; levels--) {
+               /* Add child to parent. */
+               if (add_child(ctx, parent, names[levels - 1]))
+                       goto nomem;
+
+               /* Allocate node */
+               node = talloc(ctx, struct node);
+               if (!node)
+                       goto nomem;
+               node->name = talloc_steal(node, names[levels - 1]);
+
+               /* Inherit permissions, unpriv domains own what they create. */
+               node->perms.num = parent->perms.num;
+               node->perms.p = talloc_memdup(node, parent->perms.p,
+                                             node->perms.num *
+                                             sizeof(*node->perms.p));
+               if (!node->perms.p)
+                       goto nomem;
+               if (domain_is_unprivileged(conn))
+                       node->perms.p[0].id = conn->id;
+
+               /* No children, no data */
+               node->children = node->data = NULL;
+               node->childlen = node->datalen = 0;
+               node->acc.memory = 0;
+               node->parent = parent;
+
+               parent = node;
+       }
 
-       /* No children, no data */
-       node->children = node->data = NULL;
-       node->childlen = node->datalen = 0;
-       node->acc.memory = 0;
-       node->parent = parent;
        return node;
 
 nomem: