Heap Hardening Against Hostile Spraying

Ruiqi Gong wanted to address the generic security threat of "heap spraying" in the Linux kernel. Heap spraying is when an attacker knows the memory address of a critical part of the system and makes a large number of malloc() calls trying to allocate memory at that specific address. In the event of certain types of kernel bugs, one of those malloc() calls might succeed, giving the attacker the ability to overwrite the critical part of the system with their own malicious code. Heap spraying is not a security vulnerability in itself, because user software could legitimately want to issue many malloc() calls. Rather, heap spraying is a way to exploit other bugs that may exist in the kernel.

Because heap spraying itself is not a bug or vulnerability, it poses a fascinating problem in terms of how best to reduce the ability of attackers to make use of it. For example, Ruiqi pointed out that slab caches can be shared among different subsystems and modules. A slab cache is a region of memory allocated all at once on the grounds that the calling routine knows it will need that much memory eventually.

Slab caches are more efficient than doing a bunch of piecemeal allocations as-needed, but they also present a visible target for heap spraying attacks. At the same time, as Ruiqi said, it wouldn't be realistic to disable slab caching in the kernel, because that feature is in wide use by user software, all of which would need to be rewritten and would suffer a significant performance penalty.

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