x509: excessive resource use verifying policy constraints

A security vulnerability has been identified in all supported versions
of OpenSSL related to the verification of X.509 certificate chains
that include policy constraints.  Attackers may be able to exploit this
vulnerability by creating a malicious certificate chain that triggers
exponential use of computational resources, leading to a denial-of-service
(DoS) attack on affected systems.

Fixes CVE-2023-0464

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/20571)

3 files changed, 42 insertions, 14 deletions

diff --git a/crypto/x509/pcy_local.h b/crypto/x509/pcy_local.h
index 18b53cc09e..cba107ca03 100644
--- a/crypto/x509/pcy_local.h
+++ b/crypto/x509/pcy_local.h
@@ -111,6 +111,11 @@ struct X509_POLICY_LEVEL_st {
 };
 
 struct X509_POLICY_TREE_st {
+    /* The number of nodes in the tree */
+    size_t node_count;
+    /* The maximum number of nodes in the tree */
+    size_t node_maximum;
+
     /* This is the tree 'level' data */
     X509_POLICY_LEVEL *levels;
     int nlevel;
@@ -157,7 +162,8 @@ X509_POLICY_NODE *ossl_policy_tree_find_sk(STACK_OF(X509_POLICY_NODE) *sk,
 X509_POLICY_NODE *ossl_policy_level_add_node(X509_POLICY_LEVEL *level,
                                              X509_POLICY_DATA *data,
                                              X509_POLICY_NODE *parent,
-                                             X509_POLICY_TREE *tree);
+                                             X509_POLICY_TREE *tree,
+                                             int extra_data);
 void ossl_policy_node_free(X509_POLICY_NODE *node);
 int ossl_policy_node_match(const X509_POLICY_LEVEL *lvl,
                            const X509_POLICY_NODE *node, const ASN1_OBJECT *oid);
diff --git a/crypto/x509/pcy_node.c b/crypto/x509/pcy_node.c
index 5bee8c2a0b..a0ccbd01d4 100644
--- a/crypto/x509/pcy_node.c
+++ b/crypto/x509/pcy_node.c
@@ -59,16 +59,21 @@ X509_POLICY_NODE *ossl_policy_level_find_node(const X509_POLICY_LEVEL *level,
 X509_POLICY_NODE *ossl_policy_level_add_node(X509_POLICY_LEVEL *level,
                                              X509_POLICY_DATA *data,
                                              X509_POLICY_NODE *parent,
-                                             X509_POLICY_TREE *tree)
+                                             X509_POLICY_TREE *tree,
+                                             int extra_data)
 {
     X509_POLICY_NODE *node;
 
+    /* Verify that the tree isn't too large.  This mitigates CVE-2023-0464 */
+    if (tree->node_maximum > 0 && tree->node_count >= tree->node_maximum)
+        return NULL;
+
     node = OPENSSL_zalloc(sizeof(*node));
     if (node == NULL)
         return NULL;
     node->data = data;
     node->parent = parent;
-    if (level) {
+    if (level != NULL) {
         if (OBJ_obj2nid(data->valid_policy) == NID_any_policy) {
             if (level->anyPolicy)
                 goto node_error;
@@ -88,7 +93,7 @@ X509_POLICY_NODE *ossl_policy_level_add_node(X509_POLICY_LEVEL *level,
         }
     }
 
-    if (tree) {
+    if (extra_data) {
         if (tree->extra_data == NULL)
             tree->extra_data = sk_X509_POLICY_DATA_new_null();
         if (tree->extra_data == NULL) {
@@ -101,6 +106,7 @@ X509_POLICY_NODE *ossl_policy_level_add_node(X509_POLICY_LEVEL *level,
         }
     }
 
+    tree->node_count++;
     if (parent)
         parent->nchild++;
 
diff --git a/crypto/x509/pcy_tree.c b/crypto/x509/pcy_tree.c
index 4b954b0776..95fb46f4ec 100644
--- a/crypto/x509/pcy_tree.c
+++ b/crypto/x509/pcy_tree.c
@@ -14,6 +14,17 @@
 
 #include "pcy_local.h"
 
+/*
+ * If the maximum number of nodes in the policy tree isn't defined, set it to
+ * a generous default of 1000 nodes.
+ *
+ * Defining this to be zero means unlimited policy tree growth which opens the
+ * door on CVE-2023-0464.
+ */
+#ifndef OPENSSL_POLICY_TREE_NODES_MAX
+# define OPENSSL_POLICY_TREE_NODES_MAX 1000
+#endif
+
 static void expected_print(BIO *channel,
                            X509_POLICY_LEVEL *lev, X509_POLICY_NODE *node,
                            int indent)
@@ -161,6 +172,9 @@ static int tree_init(X509_POLICY_TREE **ptree, STACK_OF(X509) *certs,
     if ((tree = OPENSSL_zalloc(sizeof(*tree))) == NULL)
         return X509_PCY_TREE_INTERNAL;
 
+    /* Limit the growth of the tree to mitigate CVE-2023-0464 */
+    tree->node_maximum = OPENSSL_POLICY_TREE_NODES_MAX;
+
     /*
      * http://tools.ietf.org/html/rfc5280#section-6.1.2, figure 3.
      *
@@ -177,7 +191,7 @@ static int tree_init(X509_POLICY_TREE **ptree, STACK_OF(X509) *certs,
     if ((data = ossl_policy_data_new(NULL,
                                      OBJ_nid2obj(NID_any_policy), 0)) == NULL)
         goto bad_tree;
-    if (ossl_policy_level_add_node(level, data, NULL, tree) == NULL) {
+    if (ossl_policy_level_add_node(level, data, NULL, tree, 1) == NULL) {
         ossl_policy_data_free(data);
         goto bad_tree;
     }
@@ -236,7 +250,8 @@ static int tree_init(X509_POLICY_TREE **ptree, STACK_OF(X509) *certs,
  * Return value: 1 on success, 0 otherwise
  */
 static int tree_link_matching_nodes(X509_POLICY_LEVEL *curr,
-                                    X509_POLICY_DATA *data)
+                                    X509_POLICY_DATA *data,
+                                    X509_POLICY_TREE *tree)
 {
     X509_POLICY_LEVEL *last = curr - 1;
     int i, matched = 0;
@@ -246,13 +261,13 @@ static int tree_link_matching_nodes(X509_POLICY_LEVEL *curr,
         X509_POLICY_NODE *node = sk_X509_POLICY_NODE_value(last->nodes, i);
 
         if (ossl_policy_node_match(last, node, data->valid_policy)) {
-            if (ossl_policy_level_add_node(curr, data, node, NULL) == NULL)
+            if (ossl_policy_level_add_node(curr, data, node, tree, 0) == NULL)
                 return 0;
             matched = 1;
         }
     }
     if (!matched && last->anyPolicy) {
-        if (ossl_policy_level_add_node(curr, data, last->anyPolicy, NULL) == NULL)
+        if (ossl_policy_level_add_node(curr, data, last->anyPolicy, tree, 0) == NULL)
             return 0;
     }
     return 1;
@@ -265,7 +280,8 @@ static int tree_link_matching_nodes(X509_POLICY_LEVEL *curr,
  * Return value: 1 on success, 0 otherwise.
  */
 static int tree_link_nodes(X509_POLICY_LEVEL *curr,
-                           const X509_POLICY_CACHE *cache)
+                           const X509_POLICY_CACHE *cache,
+                           X509_POLICY_TREE *tree)
 {
     int i;
 
@@ -273,7 +289,7 @@ static int tree_link_nodes(X509_POLICY_LEVEL *curr,
         X509_POLICY_DATA *data = sk_X509_POLICY_DATA_value(cache->data, i);
 
         /* Look for matching nodes in previous level */
-        if (!tree_link_matching_nodes(curr, data))
+        if (!tree_link_matching_nodes(curr, data, tree))
             return 0;
     }
     return 1;
@@ -304,7 +320,7 @@ static int tree_add_unmatched(X509_POLICY_LEVEL *curr,
     /* Curr may not have anyPolicy */
     data->qualifier_set = cache->anyPolicy->qualifier_set;
     data->flags |= POLICY_DATA_FLAG_SHARED_QUALIFIERS;
-    if (ossl_policy_level_add_node(curr, data, node, tree) == NULL) {
+    if (ossl_policy_level_add_node(curr, data, node, tree, 1) == NULL) {
         ossl_policy_data_free(data);
         return 0;
     }
@@ -367,7 +383,7 @@ static int tree_link_any(X509_POLICY_LEVEL *curr,
     /* Finally add link to anyPolicy */
     if (last->anyPolicy &&
             ossl_policy_level_add_node(curr, cache->anyPolicy,
-                                       last->anyPolicy, NULL) == NULL)
+                                       last->anyPolicy, tree, 0) == NULL)
         return 0;
     return 1;
 }
@@ -550,7 +566,7 @@ static int tree_calculate_user_set(X509_POLICY_TREE *tree,
             extra->flags = POLICY_DATA_FLAG_SHARED_QUALIFIERS
                 | POLICY_DATA_FLAG_EXTRA_NODE;
             node = ossl_policy_level_add_node(NULL, extra, anyPolicy->parent,
-                                              tree);
+                                              tree, 1);
         }
         if (!tree->user_policies) {
             tree->user_policies = sk_X509_POLICY_NODE_new_null();
@@ -577,7 +593,7 @@ static int tree_evaluate(X509_POLICY_TREE *tree)
 
     for (i = 1; i < tree->nlevel; i++, curr++) {
         cache = ossl_policy_cache_set(curr->cert);
-        if (!tree_link_nodes(curr, cache))
+        if (!tree_link_nodes(curr, cache, tree))
             return X509_PCY_TREE_INTERNAL;
 
         if (!(curr->flags & X509_V_FLAG_INHIBIT_ANY)