From 77acc8a069acbdd0e4ab0ac43f7d676a23e413a1 Mon Sep 17 00:00:00 2001
From: Nikita Popov <nikita.ppv@gmail.com>
Date: Tue, 7 Jul 2020 16:24:13 +0200
Subject: [PATCH] Fixed bug #79793

Make sure the string key is not released while throwing the
undefined index warning.
---
 NEWS                     |  2 ++
 Zend/tests/bug79793.phpt | 32 ++++++++++++++++++++++++++++++++
 Zend/zend_execute.c      |  7 ++++++-
 3 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 Zend/tests/bug79793.phpt

diff --git a/NEWS b/NEWS
index bfe7b596eb..6c35059e5e 100644
--- a/NEWS
+++ b/NEWS
@@ -23,6 +23,8 @@ PHP                                                                        NEWS
     (Nikita)
   . Fixed bug #79784 (Use after free if changing array during undef var during
     array write fetch). (Nikita)
+  . Fixed bug #79793 (Use after free if string used in undefined index warning
+    is changed). (Nikita)
 
 - Fileinfo:
   . Fixed bug #79756 (finfo_file crash (FILEINFO_MIME)). (cmb)
diff --git a/Zend/tests/bug79793.phpt b/Zend/tests/bug79793.phpt
new file mode 100644
index 0000000000..9e4e2e20be
--- /dev/null
+++ b/Zend/tests/bug79793.phpt
@@ -0,0 +1,32 @@
+--TEST--
+Bug #79793: Use after free if string used in undefined index warning is changed
+--FILE--
+<?php
+
+$key = "foo";
+$key .= "bar";
+set_error_handler(function($_, $m) use (&$key) {
+    echo "$m\n";
+    $key .= "baz";
+});
+
+$ary = [];
+$ary[$key]++;
+var_dump($ary);
+$ary[$key] += 1;
+var_dump($ary);
+
+?>
+--EXPECT--
+Undefined index: foobar
+array(1) {
+  ["foobar"]=>
+  int(1)
+}
+Undefined index: foobarbaz
+array(2) {
+  ["foobar"]=>
+  int(1)
+  ["foobarbaz"]=>
+  int(1)
+}
diff --git a/Zend/zend_execute.c b/Zend/zend_execute.c
index 5aed92ff45..59c151fe66 100644
--- a/Zend/zend_execute.c
+++ b/Zend/zend_execute.c
@@ -2181,10 +2181,15 @@ str_index:
 					retval = &EG(uninitialized_zval);
 					break;
 				case BP_VAR_RW:
+					/* Key may be released while throwing the undefined index warning. */
+					zend_string_addref(offset_key);
 					if (UNEXPECTED(zend_undefined_index_write(ht, offset_key) == FAILURE)) {
+						zend_string_release(offset_key);
 						return NULL;
 					}
-					/* break missing intentionally */
+					retval = zend_hash_add_new(ht, offset_key, &EG(uninitialized_zval));
+					zend_string_release(offset_key);
+					break;
 				case BP_VAR_W:
 					retval = zend_hash_add_new(ht, offset_key, &EG(uninitialized_zval));
 					break;
-- 
2.49.0