tools/xenstore: fix checking node permissions

Today chk_domain_generation() is being used to check whether a node
permission entry is still valid or whether it is referring to a domain
no longer existing. This is done by comparing the node's and the
domain's generation count.

In case no struct domain is existing for a checked domain, but the
domain itself is valid, chk_domain_generation() assumes it is being
called due to the first node created for a new domain and it will
return success.

This might be wrong in case the checked permission is related to an
old domain, which has just been replaced with a new domain using the
same domid.

Fix that by letting chk_domain_generation() fail in case a struct
domain isn't found. In order to cover the case of the first node for
a new domain try to allocate the needed struct domain explicitly when
processing the related SET_PERMS command. In case a referenced domain
isn't existing, flag the related permission to be ignored right away.

This is XSA-417 / CVE-2022-42320.

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

tools/xenstore/xenstored_core.c

@@ -1753,6 +1753,11 @@ static int do_set_perms(const void *ctx, struct connection *conn,
 	if (!xs_strings_to_perms(perms.p, perms.num, permstr))
 		return errno;
 
+	if (domain_alloc_permrefs(&perms) < 0)
+		return ENOMEM;
+	if (perms.p[0].perms & XS_PERM_IGNORE)
+		return ENOENT;
+
 	/* First arg is node name. */
 	if (strstarts(in->buffer, "@")) {
 		if (set_perms_special(conn, in->buffer, &perms))

tools/xenstore/xenstored_domain.c

@@ -907,7 +907,6 @@ int domain_entry_inc(struct connection *conn, struct node *node)
  * count (used for testing whether a node permission is older than a domain).
  *
  * Return values:
- * -1: error
  *  0: domain has higher generation count (it is younger than a node with the
  *     given count), or domain isn't existing any longer
  *  1: domain is older than the node
@@ -915,20 +914,38 @@ int domain_entry_inc(struct connection *conn, struct node *node)
 static int chk_domain_generation(unsigned int domid, uint64_t gen)
 {
 	struct domain *d;
-	xc_dominfo_t dominfo;
 
 	if (!xc_handle && domid == 0)
 		return 1;
 
 	d = find_domain_struct(domid);
-	if (d)
-		return (d->generation <= gen) ? 1 : 0;
 
-	if (!get_domain_info(domid, &dominfo))
-		return 0;
+	return (d && d->generation <= gen) ? 1 : 0;
+}
 
-	d = alloc_domain(NULL, domid);
-	return d ? 1 : -1;
+/*
+ * Allocate all missing struct domain referenced by a permission set.
+ * Any permission entries for not existing domains will be marked to be
+ * ignored.
+ */
+int domain_alloc_permrefs(struct node_perms *perms)
+{
+	unsigned int i, domid;
+	struct domain *d;
+	xc_dominfo_t dominfo;
+
+	for (i = 0; i < perms->num; i++) {
+		domid = perms->p[i].id;
+		d = find_domain_struct(domid);
+		if (!d) {
+			if (!get_domain_info(domid, &dominfo))
+				perms->p[i].perms |= XS_PERM_IGNORE;
+			else if (!alloc_domain(NULL, domid))
+				return ENOMEM;
+		}
+	}
+
+	return 0;
 }
 
 /*
@@ -941,8 +958,6 @@ int domain_adjust_node_perms(struct connection *conn, struct node *node)
 	int ret;
 
 	ret = chk_domain_generation(node->perms.p[0].id, node->generation);
-	if (ret < 0)
-		return errno;
 
 	/* If the owner doesn't exist any longer give it to priv domain. */
 	if (!ret) {
@@ -959,8 +974,6 @@ int domain_adjust_node_perms(struct connection *conn, struct node *node)
 			continue;
 		ret = chk_domain_generation(node->perms.p[i].id,
 					    node->generation);
-		if (ret < 0)
-			return errno;
 		if (!ret)
 			node->perms.p[i].perms |= XS_PERM_IGNORE;
 	}

tools/xenstore/xenstored_domain.h

@@ -63,6 +63,7 @@ bool domain_is_unprivileged(struct connection *conn);
 
 /* Remove node permissions for no longer existing domains. */
 int domain_adjust_node_perms(struct connection *conn, struct node *node);
+int domain_alloc_permrefs(struct node_perms *perms);
 
 /* Quota manipulation */
 int domain_entry_inc(struct connection *conn, struct node *);