ECMO: Peripheral Transplantation to Rehost Embedded Linux Kernels

TL;DR

This paper introduces ECMO, a technique for transplanting device drivers into Linux kernels to enable rehosting embedded firmware in QEMU, significantly broadening support for various embedded devices.

Contribution

The paper presents a novel peripheral transplantation method and implements it in ECMO, successfully rehosting 87.1% of tested embedded Linux kernels across multiple vendors.

Findings

ECMO successfully transplanted peripherals in all 815 firmware images.

87.1% of kernels could be rehosted with user-space shell execution.

Most failures were due to unsupported root filesystem formats.

Abstract

Dynamic analysis based on the full-system emulator QEMU is widely used for various purposes. However, it is challenging to run firmware images of embedded devices in QEMU, especially the process to boot the Linux kernel (we call this process rehosting the Linux kernel.) That's because embedded devices usually use different system-on-chips (SoCs) from multiple vendors and only a limited number of SoCs are currently supported in QEMU. In this work, we propose a technique called peripheral transplantation. The main idea is to transplant the device drivers of designated peripherals into the Linux kernel. By doing so, it can replace the peripherals in the kernel that are currently unsupported in QEMU with supported ones, thus making the Linux kernel rehostable. After that, various applications can be built upon. We implemented this technique inside a prototype system called ECMO and applied it to 815 firmware images, which consist of 20 kernel versions, 37 device models, and 24 vendors. The result shows that ECMO can successfully transplant peripherals for all the 815 Linux kernels. Among them,710 kernels can be successfully rehosted, i.e., launching a user-space shell (87.1% success rate). The failed cases are mainly because the root file system format (ramfs) is not supported by the kernel. We further build three applications, i.e., kernel crash analysis, rootkit forensic analysis, and kernel fuzzing, based on the rehosted kernels to demonstrate the usage scenarios of ECMO