MOAT: Towards Safe BPF Kernel Extension

The Linux kernel makes considerable use of Berkeley Packet Filter (BPF) to allow user-written BPF applications to execute in the kernel space. BPF employs a verifier to statically check the security of user-supplied BPF code. Recent attacks show that BPF programs can evade security checks and gain unauthorized access to kernel memory, indicating that the verification process is not flawless. In this paper, we present MOAT, a system that isolates potentially malicious BPF programs using Intel Memory Protection Keys (MPK). Enforcing BPF program isolation with MPK is not straightforward; MOAT is carefully designed to alleviate technical obstacles, such as limited hardware keys and supporting a wide variety of kernel BPF helper functions. We have implemented MOAT in a prototype kernel module, and our evaluation shows that MOAT delivers low-cost isolation of BPF programs under various real-world usage scenarios, such as the isolation of a packet-forwarding BPF program for the memcached database with an average throughput loss of 6%.