DAMN: Overhead-Free IOMMU Protection for Networking

DMA operations can access memory buffers only if they are "mapped" in the IOMMU, so operating systems protect themselves against malicious/errant network DMAs by mapping and unmapping each packet immediately before/after it is DMAed. This approach was recently found to be riskier and less performant than keeping packets non-DMAable and instead copying their content to/from permanently-mapped buffers. Still, the extra copy hampers performance of multi-gigabit networking. We observe that achieving protection at the DMA (un)map boundary is needlessly constraining, as devices must be prevented from changing the data only after the kernel reads it. So there is no real need to switch ownership of buffers between kernel and device at the DMA (un)mapping layer, as opposed to the approach taken by all existing IOMMU protection schemes. We thus eliminate the extra copy by (1)~implementing a new allocator called DMA-Aware Malloc for Networking (DAMN), which (de)allocates packet buffers from a memory pool permanently mapped in the IOMMU; (2)~modifying the network stack to use this allocator; and (3)~copying packet data only when the kernel needs it, which usually morphs the aforementioned extra copy into the kernel's standard copy operation performed at the user-kernel boundary. DAMN thus provides full IOMMU protection with performance comparable to that of an unprotected system.