The Linux kernel extensively uses the Berkeley Packet Filter (BPF) to allow user-written BPF applications to execute in the kernel space. The BPF employs a verifier to check the security of user-supplied BPF code statically. 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 novel hardware-assisted, cross-platform isolation framework designed to protect the kernel from malicious BPF programs. MOAT introduces a two-layer memory isolation scheme that leverages hardware features such as Intel MPK and Arm Stage-2 translation to enforce isolation. Our design overcomes several key challenges, including the limited scalability of available hardware isolation mechanisms and the risk of helper function abuse. We implement MOAT for Intel x86 and Arm on Linux (ver. 6.1.38), and our evaluation shows that MOAT delivers low-cost isolation of BPF programs under mainstream use cases, such as isolating a BPF packet filter with only 3% throughput loss.