LFI Runtime

The LFI runtime is responsible for loading sandboxes and handling host calls. It is split into two sub-components:

• core: provides functionality for reserving virtual address space, mapping sandbox memory, transferring control to/from the sandbox. It does not impose any runtime API on the sandbox beyond the minimum necessary.
• linux: provides a Linux emulation layer on top of the core library. By default this is a minimal runtime that only services the system calls needed for startup and brk-based memory allocation.
• linux/full: the full Linux configuration, built only with -Dlinux_full=true. It extends the minimal runtime through the hooks in linux/hooks.h (per-proc and per-thread state, plus first look at every syscall) and currently adds support for mmap and clone. Sandboxed libraries built for it can use the libc allocator instead of minialloc, and host threads that call into a sandbox exporting _lfi_thread_create get a sandbox-created thread with TLS. It also provides a passthrough mode for testing and benchmarking (see below).
• sbox: a C++ library for calling sandboxed functions with a lightweight tainting API. See sbox/README.md for details.

Installation

meson setup build
cd build
ninja

This produces liblfi, along with the lfi-run tool that can be used to run binaries at the command-line.

See core/include/lfi_core.h and linux/include/lfi_linux.h for the API provided by liblfi.

Platform Support

The runtime currently targets Linux for Arm64 and x86-64. There is also experimental support for RISC-V that is in-progress.

Usage

Usage: lfi-run [OPTION...] INPUT...

  -h, --help             show help
  -V, --verbose          verbose output
  --perf                 enable perf support
  -v, --verify           enable verification
  --no-debug             disable debugging support
  -p, --sys-passthrough  pass file, time, and resource syscalls to the host (UNSAFE, requires linux_full) (default)
  --no-sys-passthrough   disable --sys-passthrough
  --no-aslr              disable address space layout randomization
  --pagesize=<int>       system page size
  --env=<var=val>        set environment variable
  <input>                input command

Threat Model

LFI is currently being used for library isolation, where the library has been written by an honest party. In addition, LFI is only currently being used with the default minimal Linux runtime, which vastly reduces the number of system calls available to sandboxed programs.

Work to harden the full configuration (-Dlinux_full=true) and the runtime for arbitrarily malicious programs is ongoing. The full configuration's passthrough mode is unsafe and intended only for testing and benchmarking.

Sandboxed programs are able to cause faults or cause the runtime to fault/abort. Work to allow the runtime to gracefully recover by handling the signal is ongoing.