KASLR in the age of MicroVMs

Address space layout randomization (ASLR) is a widely used component of computer security aimed at preventing code reuse and/or data-only attacks. Modern kernels utilize kernel ASLR (KASLR) and finer-grained forms, such as functional granular KASLR (FGKASLR), but do so as part of an inefficient bootstrapping process we call bootstrap self-randomization. Meanwhile, under increasing pressure to optimize their boot times, microVM architectures such as AWS Firecracker have resorted to eliminating bootstrapping steps, particularly decompression and relocation from the guest kernel boot process, leaving them without KASLR. In this paper, we present in-monitor KASLR, in which the virtual machine monitor efficiently implements KASLR for the guest kernel by skipping the expensive kernel self-relocation steps. We prototype in-monitor KASLR and FGKASLR in the open-source Firecracker virtual machine monitor demonstrating, on a microVM configured kernel, boot times 22% and 16% faster than bootstrapped KASLR and FGKASLR methods, respectively. We also show the low overhead of in-monitor KASLR, with only 4% (2 ms) increase in boot times on average compared to a kernel without KASLR. We also discuss the implications and future opportunities for in-monitor approaches.