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Double Frees in Chrome's Partition Alloc - part 2

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Dr Silvio Cesare
@silviocesare
Summary PartitionAlloc is the hardened heap allocator used in Google's Chrome web browser. It is susceptible to a number of attacks. This blog post describes the second attack in a series of posts. I will talk about double frees and how to make an allocation request return an arbitrary pointer. This can be used with application-logic to develop an arbitrary write primitive.
Introduction In heap allocators, freelists maintain a group of free memory chunks that are available to be recycled by an allocation request. Freelist poisoning corrupts this list and injects a "fake chunk" pointer. A later allocation will return this fake chunk pointer. So it is possible to make an allocation request return an arbitrary pointer.

A double free often creates a cycle in the freelist. When a chunk is returned by an allocation request, the chunk still remains in the freelist. Thus, if an attacker writes to that chunk, it is use-after-free and akin …