Limit the number of filters

Chaining filters is becoming an increasingly popular primitive to exploit PHP
applications. Limiting the usage of only a few of them at the time should,
if not close entirely, make it significantly less attractive.

This should close #10453

main/php_streams.h

@@ -193,6 +193,8 @@ struct _php_stream_wrapper	{
 
 #define PHP_STREAM_FLAG_WAS_WRITTEN					0x80000000
 
+#define PHP_STREAM_MAX_FILTERS	5
+
 struct _php_stream  {
 	const php_stream_ops *ops;
 	void *abstract;			/* convenience pointer for abstraction */

main/streams/filter.c

@@ -284,6 +284,12 @@ PHPAPI void php_stream_filter_free(php_stream_filter *filter)
 
 PHPAPI int php_stream_filter_prepend_ex(php_stream_filter_chain *chain, php_stream_filter *filter)
 {
+	if (chain->nb_filters >= PHP_STREAM_MAX_FILTERS) {
+	    php_error_docref(NULL, E_ERROR, "Unable to apply filter, maximum number (%d) reached", PHP_STREAM_MAX_FILTERS);
+	    return FAILURE;
+	}
+	chain->nb_filters++;
+
 	filter->next = chain->head;
 	filter->prev = NULL;
 
@@ -307,6 +313,12 @@ PHPAPI int php_stream_filter_append_ex(php_stream_filter_chain *chain, php_strea
 {
 	php_stream *stream = chain->stream;
 
+	if (chain->nb_filters >= PHP_STREAM_MAX_FILTERS) {
+	    php_error_docref(NULL, E_ERROR, "Unable to apply filter, maximum number (%d) reached", PHP_STREAM_MAX_FILTERS);
+	    return FAILURE;
+	}
+	chain->nb_filters++;
+
 	filter->prev = chain->tail;
 	filter->next = NULL;
 	if (chain->tail) {
@@ -435,6 +447,8 @@ PHPAPI int _php_stream_filter_flush(php_stream_filter *filter, int finish)
 		flags = PSFS_FLAG_NORMAL;
 	}
 
+	chain->nb_filters = 0;
+
 	/* Last filter returned data via PSFS_PASS_ON
 		Do something with it */
 
@@ -492,6 +506,7 @@ PHPAPI php_stream_filter *php_stream_filter_remove(php_stream_filter *filter, in
 	} else {
 		filter->chain->tail = filter->prev;
 	}
+	filter->chain->nb_filters--;
 
 	if (filter->res) {
 		zend_list_delete(filter->res);

main/streams/php_stream_filter_api.h

@@ -96,6 +96,7 @@ typedef struct _php_stream_filter_ops {
 
 typedef struct _php_stream_filter_chain {
 	php_stream_filter *head, *tail;
+	size_t nb_filters;
 
 	/* Owning stream */
 	php_stream *stream;

tests/security/bug10453.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Bug #10453 (using a high amount of filters for nefarious purposes)
+--FILE--
+<?php
+$fp = fopen('php://output', 'w');
+for($i=0; $i<10; $i++)
+    stream_filter_append($fp, 'string.rot13');
+fwrite($fp, "This is a test.\n");
+?>
+--EXPECTF--
+Fatal error: stream_filter_append(): Unable to apply filter, maximum number (5) reached in %s
+

tests/security/bug10453_fullchain.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Bug #10453 (using a high amount of filters for nefarious purposes via a full chain)
+--FILE--
+<?php
+$fp = fopen('php://filter/write=string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13/resource=php://output', 'w');
+fwrite($fp, "This is a test.\n");
+?>
+--EXPECTF--
+Fatal error: fopen(): Unable to apply filter, maximum number (5) reached in %s
+