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Sauron

CPUID hooking

sauron is a type-2 Intel VT-x hypervisor for blue-pilling a booted 64-bit Linux system and a bad LOTR reference. It utilises Intel's virtualisation extensions (VMX) to place itself above the kernel (ring -1) then intercepts operations/events from the guest.

sauron is not for practical applications, it is an educational project to learn about VMX; the kernel module is unstable which will crash your system.

Features

  • CPUID emulation and subversion

Description

The kernel module initialises the logical processor state by modifying CR0 and CR4. Then the module allocates a VMCS, VMXON, and MSR bitmap prior to executing VMXON, VMPTRLD, and VMCLEAR. Once the VMX CPU structures are loaded, the VMCS is fully initialised with the current system state. Finally, the logical processor executes VMLAUNCH which enables the hypervisor (implicitly placing the kernel module above the kernel itself).

During execution, several events and instructions cause VM-exits which the hypervisor can trace and modify without the guest's knowledge. This allows the hypervisor to subvert the guest by modifying values indicating the presence of a hypervisor (e.g. CR4.VMXE[bit 13], CPUID.1:ECX.VMX[bit 5], etc.). The hypervisor is disabled by executing CPUID with a magic leaf and subleaf which is intercepted by the hypervisor.

Built With

  • ia32-doc which provides usable C definitions of numerous structures and constants defined in the Intel manuals.

Getting Started

Prerequisites

  • 64-bit Intel processor with VT-x (VMX) support.
$ grep "vmx" /proc/cpuinfo
  • 64-bit Linux kernel 5.7.8 (or above).
$ uname -r
X.Y.Z-<distrubution>
$ grep "module.sig_enforce=1" /proc/cmdline

sauron was tested and developed on 64-bit 5.7.12-arch1-1 on an i5-6500 under VMware

Installation

  1. Clone the repository.
$ git clone https://github.com/birb007/sauron.git
  1. Compile the kernel module.
$ make clean && make

Usage

All the following commands must be ran as root. Firstly, the kernel module must be loaded.

$ insmod sauron.ko

Then the hypervisor can be disabled/enabled with the below commands.

$ echo 1 > /sys/kernel/sauron/enable # enable
$ echo 0 > /sys/kernel/sauron/enable # disable

Use dmesg to view output from the kernel module.

$ dmesg -k | grep sauron

Troubleshooting

If an issue occurs, attempt to replicate it with a debug build to provide more context. To build a debug release, run the below command.

make debug

Then create an issue on GitHub with the output and any other relevant information (no guarantees it will be fixed).

Issues

  • KVM and VirtualBox do not properly implement XSAVE under nested VMX which causes sauron to immediately crash the host.
  • Prolonged execution will result in a crash.
  • Not all error cases are accounted for which can result in crashes or unexpected behaviour in untested environments.

Future Development

  • Implement EPT shadow hooks
  • Implement tracing handlers for events causing a VM-exit.

Acknowledgements

  • Gbhv is a hypervisor for Windows which inspired significant aspects of the overall design.
  • KSM is a hypervisor for Linux and Windows which provided a reference while troubleshooting.
  • SimpleVisor provided inspiration for several aspects of the design.
  • Intel SDM 3C (Part 3, Chapter 23) is the authoritative resource on VMX.
  • Reverse Engineering Discord answered all of my questions which was crucial during development.