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This calculation then overflowed and made the result of this function zero, instead of a value at least equal to sizeof(uint32). Using this, Pinkie was able to write eight bytes of his choice past the end of his buffer. The buffer in this case is one of the GPU transfer buffers, which are mapped in both processes’ address spaces and used to transfer data between the Native Client and GPU processes. The Windows allocator places the buffers at relatively predictable locations; and the Native Client process can directly control their size as well as certain object allocation ordering. So, this afforded quite a bit of control over exactly where an overwrite would occur in the GPU process.

The next thing Pinkie needed was a target that met two criteria: it had to be positioned within range of his overwrite, and the first eight bytes needed to be something worth changing. For this, he used the GPU buckets, which are another IPC primitive exposed from the GPU process to the Native Client process. The buckets are implemented as a tree structure, with the first eight bytes containing pointers to other nodes in the tree. By overwriting the first eight bytes of a bucket, Pinkie was able to point it to a fake tree structure he created in one of his transfer buffers. Using that fake tree, Pinkie could read and write arbitrary addresses in the GPU process. Combined with some predictable addresses in Windows, this allowed him to build a ROP chain and execute arbitrary code inside the GPU process.