Add max_depth option to unserialize()

Add a max_depth option to unserialize and an unserialize_max_depth
ini setting, which can be used to control the depth limit. The
default value is 4096.

This option is intended to prevent stack overflows during the
unserialization of deeply nested structures.

This fixes bug #78549 and addresses oss-fuzz #17581, #17589, #17664,
and #17788.

NEWS

@@ -12,6 +12,9 @@ PHP                                                                        NEWS
   . Fixed bug #78579 (mb_decode_numericentity: args number inconsistency).
     (cmb)
 
+- Standard:
+  . Fixed bug #78549 (Stack overflow due to nested serialized input). (Nikita)
+
 19 Sep 2019, PHP 7.4.0RC2
 
 - Core:

UPGRADING

@@ -316,6 +316,12 @@ PHP 7.4 UPGRADE NOTES
 
     RFC: https://wiki.php.net/rfc/custom_object_serialization
 
+  . A new 'max_depth' option for unserialize(), as well as a
+    unserialize_max_depth ini setting have been added. These control the
+    maximum depth of structures permitted during unserialization, and are
+    intended to prevent stack overflows. The default depth limit is 4096 and
+    can be disabled by setting unserialize_max_depth=0.
+
   . array_merge() and array_merge_recursive() may now be called without any
     arguments, in which case they will return an empty array. This is useful
     in conjunction with the spread operator, e.g. array_merge(...$arrays).

ext/standard/basic_functions.c

@@ -3667,6 +3667,7 @@ PHP_MINIT_FUNCTION(basic) /* {{{ */
 	register_html_constants(INIT_FUNC_ARGS_PASSTHRU);
 	register_string_constants(INIT_FUNC_ARGS_PASSTHRU);
 
+	BASIC_MINIT_SUBMODULE(var)
 	BASIC_MINIT_SUBMODULE(file)
 	BASIC_MINIT_SUBMODULE(pack)
 	BASIC_MINIT_SUBMODULE(browscap)

ext/standard/basic_functions.h

@@ -235,6 +235,7 @@ typedef struct _php_basic_globals {
 #endif
 
 	int umask;
+	zend_long unserialize_max_depth;
 } php_basic_globals;
 
 #ifdef ZTS

ext/standard/php_var.h

@@ -22,6 +22,7 @@
 #include "ext/standard/basic_functions.h"
 #include "zend_smart_str_public.h"
 
+PHP_MINIT_FUNCTION(var);
 PHP_FUNCTION(var_dump);
 PHP_FUNCTION(var_export);
 PHP_FUNCTION(debug_zval_dump);
@@ -50,6 +51,10 @@ PHPAPI php_unserialize_data_t php_var_unserialize_init(void);
 PHPAPI void php_var_unserialize_destroy(php_unserialize_data_t d);
 PHPAPI HashTable *php_var_unserialize_get_allowed_classes(php_unserialize_data_t d);
 PHPAPI void php_var_unserialize_set_allowed_classes(php_unserialize_data_t d, HashTable *classes);
+PHPAPI void php_var_unserialize_set_max_depth(php_unserialize_data_t d, zend_long max_depth);
+PHPAPI zend_long php_var_unserialize_get_max_depth(php_unserialize_data_t d);
+PHPAPI void php_var_unserialize_set_cur_depth(php_unserialize_data_t d, zend_long cur_depth);
+PHPAPI zend_long php_var_unserialize_get_cur_depth(php_unserialize_data_t d);
 
 #define PHP_VAR_SERIALIZE_INIT(d) \
 	(d) = php_var_serialize_init()

ext/standard/tests/serialize/max_depth.phpt

@@ -0,0 +1,159 @@
+--TEST--
+Bug #78549: Stack overflow due to nested serialized input
+--FILE--
+<?php
+
+function create_nested_data($depth, $prefix, $suffix, $inner = 'i:0;') {
+    return str_repeat($prefix, $depth) . $inner . str_repeat($suffix, $depth);
+}
+
+echo "Invalid max_depth:\n";
+var_dump(unserialize('i:0;', ['max_depth' => 'foo']));
+var_dump(unserialize('i:0;', ['max_depth' => -1]));
+
+echo "Array:\n";
+var_dump(unserialize(
+    create_nested_data(128, 'a:1:{i:0;', '}'),
+    ['max_depth' => 128]
+) !== false);
+var_dump(unserialize(
+    create_nested_data(129, 'a:1:{i:0;', '}'),
+    ['max_depth' => 128]
+));
+
+echo "Object:\n";
+var_dump(unserialize(
+    create_nested_data(128, 'O:8:"stdClass":1:{i:0;', '}'),
+    ['max_depth' => 128]
+) !== false);
+var_dump(unserialize(
+    create_nested_data(129, 'O:8:"stdClass":1:{i:0;', '}'),
+    ['max_depth' => 128]
+));
+
+// Default depth is 4096
+echo "Default depth:\n";
+var_dump(unserialize(create_nested_data(4096, 'a:1:{i:0;', '}')) !== false);
+var_dump(unserialize(create_nested_data(4097, 'a:1:{i:0;', '}')));
+
+// Depth can also be adjusted using ini setting
+echo "Ini setting:\n";
+ini_set("unserialize_max_depth", 128);
+var_dump(unserialize(create_nested_data(128, 'a:1:{i:0;', '}')) !== false);
+var_dump(unserialize(create_nested_data(129, 'a:1:{i:0;', '}')));
+
+// But an explicitly specified depth still takes precedence
+echo "Ini setting overridden:\n";
+var_dump(unserialize(
+    create_nested_data(256, 'a:1:{i:0;', '}'),
+    ['max_depth' => 256]
+) !== false);
+var_dump(unserialize(
+    create_nested_data(257, 'a:1:{i:0;', '}'),
+    ['max_depth' => 256]
+));
+
+// Reset ini setting to a large value,
+// so it's clear that it won't be used in the following.
+ini_set("unserialize_max_depth", 4096);
+
+class Test implements Serializable {
+    public function serialize() {
+        return '';
+    }
+    public function unserialize($str) {
+        // Should fail, due to combined nesting level
+        var_dump(unserialize(create_nested_data(129, 'a:1:{i:0;', '}')));
+        // Should succeeed, below combined nesting level
+        var_dump(unserialize(create_nested_data(128, 'a:1:{i:0;', '}')) !== false);
+    }
+}
+echo "Nested unserialize combined depth limit:\n";
+var_dump(is_array(unserialize(
+    create_nested_data(128, 'a:1:{i:0;', '}', 'C:4:"Test":0:{}'),
+    ['max_depth' => 256]
+)));
+
+class Test2 implements Serializable {
+    public function serialize() {
+        return '';
+    }
+    public function unserialize($str) {
+        // If depth limit is overridden, the depth should be counted
+        // from zero again.
+        var_dump(unserialize(
+            create_nested_data(257, 'a:1:{i:0;', '}'),
+            ['max_depth' => 256]
+        ));
+        var_dump(unserialize(
+            create_nested_data(256, 'a:1:{i:0;', '}'),
+            ['max_depth' => 256]
+        ) !== false);
+    }
+}
+echo "Nested unserialize overridden depth limit:\n";
+var_dump(is_array(unserialize(
+    create_nested_data(64, 'a:1:{i:0;', '}', 'C:5:"Test2":0:{}'),
+    ['max_depth' => 128]
+)));
+
+?>
+--EXPECTF--
+Invalid max_depth:
+
+Warning: unserialize(): max_depth should be int in %s on line %d
+bool(false)
+
+Warning: unserialize(): max_depth cannot be negative in %s on line %d
+bool(false)
+Array:
+bool(true)
+
+Warning: unserialize(): Maximum depth of 128 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 1157 of 1294 bytes in %s on line %d
+bool(false)
+Object:
+bool(true)
+
+Warning: unserialize(): Maximum depth of 128 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 2834 of 2971 bytes in %s on line %d
+bool(false)
+Default depth:
+bool(true)
+
+Warning: unserialize(): Maximum depth of 4096 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 36869 of 40974 bytes in %s on line %d
+bool(false)
+Ini setting:
+bool(true)
+
+Warning: unserialize(): Maximum depth of 128 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 1157 of 1294 bytes in %s on line %d
+bool(false)
+Ini setting overridden:
+bool(true)
+
+Warning: unserialize(): Maximum depth of 256 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 2309 of 2574 bytes in %s on line %d
+bool(false)
+Nested unserialize combined depth limit:
+
+Warning: unserialize(): Maximum depth of 256 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 1157 of 1294 bytes in %s on line %d
+bool(false)
+bool(true)
+bool(true)
+Nested unserialize overridden depth limit:
+
+Warning: unserialize(): Maximum depth of 256 exceeded. The depth limit can be changed using the max_depth unserialize() option or the unserialize_max_depth ini setting in %s on line %d
+
+Notice: unserialize(): Error at offset 2309 of 2574 bytes in %s on line %d
+bool(false)
+bool(true)
+bool(true)

ext/standard/var.c

@@ -1174,7 +1174,7 @@ PHP_FUNCTION(serialize)
 }
 /* }}} */
 
-/* {{{ proto mixed unserialize(string variable_representation[, array allowed_classes])
+/* {{{ proto mixed unserialize(string variable_representation[, array options])
    Takes a string representation of variable and recreates it */
 PHP_FUNCTION(unserialize)
 {
@@ -1182,9 +1182,10 @@ PHP_FUNCTION(unserialize)
 	size_t buf_len;
 	const unsigned char *p;
 	php_unserialize_data_t var_hash;
-	zval *options = NULL, *classes = NULL;
+	zval *options = NULL;
 	zval *retval;
 	HashTable *class_hash = NULL, *prev_class_hash;
+	zend_long prev_max_depth, prev_cur_depth;
 
 	ZEND_PARSE_PARAMETERS_START(1, 2)
 		Z_PARAM_STRING(buf, buf_len)
@@ -1200,12 +1201,16 @@ PHP_FUNCTION(unserialize)
 	PHP_VAR_UNSERIALIZE_INIT(var_hash);
 
 	prev_class_hash = php_var_unserialize_get_allowed_classes(var_hash);
+	prev_max_depth = php_var_unserialize_get_max_depth(var_hash);
+	prev_cur_depth = php_var_unserialize_get_cur_depth(var_hash);
 	if (options != NULL) {
-		classes = zend_hash_str_find(Z_ARRVAL_P(options), "allowed_classes", sizeof("allowed_classes")-1);
+		zval *classes, *max_depth;
+
+		classes = zend_hash_str_find_deref(Z_ARRVAL_P(options), "allowed_classes", sizeof("allowed_classes")-1);
 		if (classes && Z_TYPE_P(classes) != IS_ARRAY && Z_TYPE_P(classes) != IS_TRUE && Z_TYPE_P(classes) != IS_FALSE) {
 			php_error_docref(NULL, E_WARNING, "allowed_classes option should be array or boolean");
-			PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
-			RETURN_FALSE;
+			RETVAL_FALSE;
+			goto cleanup;
 		}
 
 		if(classes && (Z_TYPE_P(classes) == IS_ARRAY || !zend_is_true(classes))) {
@@ -1225,12 +1230,29 @@ PHP_FUNCTION(unserialize)
 
 			/* Exception during string conversion. */
 			if (EG(exception)) {
-				zend_hash_destroy(class_hash);
-				FREE_HASHTABLE(class_hash);
-				PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
+				goto cleanup;
 			}
 		}
 		php_var_unserialize_set_allowed_classes(var_hash, class_hash);
+
+		max_depth = zend_hash_str_find_deref(Z_ARRVAL_P(options), "max_depth", sizeof("max_depth") - 1);
+		if (max_depth) {
+			if (Z_TYPE_P(max_depth) != IS_LONG) {
+				php_error_docref(NULL, E_WARNING, "max_depth should be int");
+				RETVAL_FALSE;
+				goto cleanup;
+			}
+			if (Z_LVAL_P(max_depth) < 0) {
+				php_error_docref(NULL, E_WARNING, "max_depth cannot be negative");
+				RETVAL_FALSE;
+				goto cleanup;
+			}
+
+			php_var_unserialize_set_max_depth(var_hash, Z_LVAL_P(max_depth));
+			/* If the max_depth for a nested unserialize() call has been overridden,
+			 * start counting from zero again (for the nested call only). */
+			php_var_unserialize_set_cur_depth(var_hash, 0);
+		}
 	}
 
 	if (BG(unserialize).level > 1) {
@@ -1254,13 +1276,16 @@ PHP_FUNCTION(unserialize)
 		gc_check_possible_root(ref);
 	}
 
+cleanup:
 	if (class_hash) {
 		zend_hash_destroy(class_hash);
 		FREE_HASHTABLE(class_hash);
 	}
 
-	/* Reset to previous allowed_classes in case this is a nested call */
+	/* Reset to previous options in case this is a nested call */
 	php_var_unserialize_set_allowed_classes(var_hash, prev_class_hash);
+	php_var_unserialize_set_max_depth(var_hash, prev_max_depth);
+	php_var_unserialize_set_cur_depth(var_hash, prev_cur_depth);
 	PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
 
 	/* Per calling convention we must not return a reference here, so unwrap. We're doing this at
@@ -1299,3 +1324,13 @@ PHP_FUNCTION(memory_get_peak_usage) {
 	RETURN_LONG(zend_memory_peak_usage(real_usage));
 }
 /* }}} */
+
+PHP_INI_BEGIN()
+	STD_PHP_INI_ENTRY("unserialize_max_depth", "4096", PHP_INI_ALL, OnUpdateLong, unserialize_max_depth, php_basic_globals, basic_globals)
+PHP_INI_END()
+
+PHP_MINIT_FUNCTION(var)
+{
+	REGISTER_INI_ENTRIES();
+	return SUCCESS;
+}

ext/standard/var_unserializer.re

@@ -49,6 +49,8 @@ struct php_unserialize_data {
 	var_dtor_entries *last_dtor;
 	HashTable        *allowed_classes;
 	HashTable        *ref_props;
+	zend_long         cur_depth;
+	zend_long         max_depth;
 	var_entries       entries;
 };
 
@@ -61,6 +63,8 @@ PHPAPI php_unserialize_data_t php_var_unserialize_init() {
 		d->first_dtor = d->last_dtor = NULL;
 		d->allowed_classes = NULL;
 		d->ref_props = NULL;
+		d->cur_depth = 0;
+		d->max_depth = BG(unserialize_max_depth);
 		d->entries.used_slots = 0;
 		d->entries.next = NULL;
 		if (!BG(serialize_lock)) {
@@ -92,6 +96,20 @@ PHPAPI void php_var_unserialize_set_allowed_classes(php_unserialize_data_t d, Ha
 	d->allowed_classes = classes;
 }
 
+PHPAPI void php_var_unserialize_set_max_depth(php_unserialize_data_t d, zend_long max_depth) {
+	d->max_depth = max_depth;
+}
+PHPAPI zend_long php_var_unserialize_get_max_depth(php_unserialize_data_t d) {
+	return d->max_depth;
+}
+
+PHPAPI void php_var_unserialize_set_cur_depth(php_unserialize_data_t d, zend_long cur_depth) {
+	d->cur_depth = cur_depth;
+}
+PHPAPI zend_long php_var_unserialize_get_cur_depth(php_unserialize_data_t d) {
+	return d->cur_depth;
+}
+
 static inline void var_push(php_unserialize_data_t *var_hashx, zval *rval)
 {
 	var_entries *var_hash = (*var_hashx)->last;
@@ -438,6 +456,18 @@ static int php_var_unserialize_internal(UNSERIALIZE_PARAMETER, int as_key);
 
 static zend_always_inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTable *ht, zend_long elements, zend_object *obj)
 {
+	if (var_hash) {
+		if ((*var_hash)->max_depth > 0 && (*var_hash)->cur_depth >= (*var_hash)->max_depth) {
+			php_error_docref(NULL, E_WARNING,
+				"Maximum depth of " ZEND_LONG_FMT " exceeded. "
+				"The depth limit can be changed using the max_depth unserialize() option "
+				"or the unserialize_max_depth ini setting",
+				(*var_hash)->max_depth);
+			return 0;
+		}
+		(*var_hash)->cur_depth++;
+	}
+
 	while (elements-- > 0) {
 		zval key, *data, d, *old_data;
 		zend_ulong idx;
@@ -447,7 +477,7 @@ static zend_always_inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTab
 
 		if (!php_var_unserialize_internal(&key, p, max, NULL, 1)) {
 			zval_ptr_dtor(&key);
-			return 0;
+			goto failure;
 		}
 
 		data = NULL;
@@ -477,7 +507,7 @@ numeric_key:
 				}
 			} else {
 				zval_ptr_dtor(&key);
-				return 0;
+				goto failure;
 			}
 		} else {
 			if (EXPECTED(Z_TYPE(key) == IS_STRING)) {
@@ -492,7 +522,7 @@ string_key:
 
 					if (UNEXPECTED(zend_unmangle_property_name_ex(Z_STR(key), &unmangled_class, &unmangled_prop, &unmangled_prop_len) == FAILURE)) {
 						zval_ptr_dtor(&key);
-						return 0;
+						goto failure;
 					}
 
 					unmangled = zend_string_init(unmangled_prop, unmangled_prop_len, 0);
@@ -559,13 +589,13 @@ string_key:
 				goto string_key;
 			} else {
 				zval_ptr_dtor(&key);
-				return 0;
+				goto failure;
 			}
 		}
 
 		if (!php_var_unserialize_internal(data, p, max, var_hash, 0)) {
 			zval_ptr_dtor(&key);
-			return 0;
+			goto failure;
 		}
 
 		if (UNEXPECTED(info)) {
@@ -573,7 +603,7 @@ string_key:
 				zval_ptr_dtor(data);
 				ZVAL_UNDEF(data);
 				zval_dtor(&key);
-				return 0;
+				goto failure;
 			}
 			if (Z_ISREF_P(data)) {
 				ZEND_REF_ADD_TYPE_SOURCE(Z_REF_P(data), info);
@@ -587,11 +617,20 @@ string_key:
 
 		if (elements && *(*p-1) != ';' && *(*p-1) != '}') {
 			(*p)--;
-			return 0;
+			goto failure;
 		}
 	}
 
+	if (var_hash) {
+		(*var_hash)->cur_depth--;
+	}
 	return 1;
+
+failure:
+	if (var_hash) {
+		(*var_hash)->cur_depth--;
+	}
+	return 0;
 }
 
 static inline int finish_nested_data(UNSERIALIZE_PARAMETER)

php.ini-development

@@ -284,6 +284,13 @@ implicit_flush = Off
 ; callback-function.
 unserialize_callback_func =
 
+; The unserialize_max_depth specifies the default depth limit for unserialized
+; structures. Setting the depth limit too high may result in stack overflows
+; during unserialization. The unserialize_max_depth ini setting can be
+; overridden by the max_depth option on individual unserialize() calls.
+; A value of 0 disables the depth limit.
+;unserialize_max_depth = 4096
+
 ; When floats & doubles are serialized, store serialize_precision significant
 ; digits after the floating point. The default value ensures that when floats
 ; are decoded with unserialize, the data will remain the same.

php.ini-production

@@ -284,6 +284,13 @@ implicit_flush = Off
 ; callback-function.
 unserialize_callback_func =
 
+; The unserialize_max_depth specifies the default depth limit for unserialized
+; structures. Setting the depth limit too high may result in stack overflows
+; during unserialization. The unserialize_max_depth ini setting can be
+; overridden by the max_depth option on individual unserialize() calls.
+; A value of 0 disables the depth limit.
+;unserialize_max_depth = 4096
+
 ; When floats & doubles are serialized, store serialize_precision significant
 ; digits after the floating point. The default value ensures that when floats
 ; are decoded with unserialize, the data will remain the same.