Mitigating UAF Exploits with Delay Free for Internet Explorer
After introducing the “isolated heap” in June security patch for Internet Explorer, Microsoft has once again introduced several improvements in the July patch for Internet Explorer. The most interesting and smart improvement is one which we will call “delay free.” This improvement is designed to mitigate Use After Free (UAF) vulnerability exploits by making sure Internet Explorer does not free object‘s heap space immediately when its reference count is zero.
Take Internet Explorer 11, for example. We randomly selected the class CDivElement. Before the latest Microsoft patch, the class’s operator delete function deletes the object’s heap space immediately. The previous code was as follows:
Figure 1. Previous code
After the latest patch rollout, the code has been changed to the following:
Figure 2. New code
It calls the function MemoryProtection::CMemoryProtector::ProtectedFree. The function is newly introduced in this patch. In the function , we can see that it saves the object address and length to an array which is a member of CMemoryProtector. The CMemoryProtector instance address is recorded in the thread local storage. Thus, the object heap space doesn’t have to be freed and later codes in the same thread can still access the object heap space.
When is the object space freed? It happens at two points:
- In the beginning of MSHTML!GlobalWndProc, it will call CMemoryProtector::ProtectCurrentThread. The function will call CMemoryProtector::ReclaimMemoryWithoutProtection to really free the all items in the array which is saved in this thread local storage.
- When an object deletes and calls CMemoryProtector::ProtectedFree and if the total hold waiting delay free objects size is over a threshold, it will start free process.
How can the “delay free” mitigate UAF vulnerability exploits?
Google Chrome has implementation that does memory protection to mitigate exploits. It divides the heap in several partitions. The object is allocated in the corresponding partition. For example, DOM node objects are allocated in a specific partition and Array buffer objects are allocated to another partition. This part can be taken as the corresponding section of “isolate heap.” However, there doesn’t seem to be corresponding section for “delay free.”
While other types of vulnerabilities exploits (such as type confusion) aren’t addressed by this improvement, we are pleased to see the continuous efforts of Microsoft to address UAF exploits. This new improvement, coupled with the isolated heap, will make it more difficult for attackers to exploit UAF vulnerabilities.
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