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S3K - Simply Secure Separation Kernel

Repository moved to https://github.com/kth-step/s3k

Simply Secure Separation Kernel (S3K) is a real-time separation kernel for embedded RISC-V applications. S3K aims to spatially and temporally isolate all partitions running on the RISC-V processor. S3K relies on RISC-V's PMP mechanism to isolate the partitions, no virtual memory is required.

  • Formal Verification. A separation kernel should be completely verified. We consider the following two approaches:

    1. We formally verify the C code with respect to a formal model of the kernel and the RISC-V specification. This allows us to compile the kernel with formally verified compiler CompCert, and get a fully formally verified kernel.

    2. We formally verify the kernel binary using HolBA, allowing us to use any compiler and thus get better optimization.

  • Security. A separation kernel should secure processes with spatial and temporal isolation.

  • Performance. Fast context switches and IPC calls are essential for a separation kernel.

  • Dynamic. The kernel should have dynamic memory protection and scheduling, allowing the system to be updated during runtime.

  • Small footprint. A small footprint is important for most embedded systems.

  • Extensible. RISC-V has a lot of extensions (official and custom) and these extension can invalidate the kernel verification. We want to synthesize kernel code that extends the kernel functionality and verification.

  • Real-time properties. Real-time properties makes the kernel suitable for some RT applications. Exact properties TBD.

Basic API

System calls (Basic)

  • void s3k_get_pid() - Get the process's PID.
  • uint64_t s3k_read_reg(register_number) - Read a virtual register.
  • void s3k_write_reg(register_number, value) - Write to a virtual register.
  • void s3k_yield() - Yield the remainder of the time slice (TODO: rename the function?).
  • cap_t s3k_read_cap(i) - Read capability from slot i.
  • uint64_t s3k_move_cap(i, j) - Move a capability in slot i to slot j.
  • uint64_t s3k_delete_cap(i) - Delete capability in slot i.
  • uint64_t s3k_revoke_cap(i) - Delete all children of capability in slot i`.
  • uint64_t s3k_derive_cap(i, j, cap) - Derive capability capfrom capability in slotiand place in slotj`.

System calls (Capability invocation)

The following system calls are pseudo system calls implemented on s3k_invoke_cap(i, a1, a2, a3, a4, a5, a6, a7).

Supervisor Invocations, the i of the following system calls should point at a supervisor capability.

  • uint64_t s3k_supervisor_suspend(i, pid) - Suspend process pid.
  • uint64_t s3k_supervisor_resume(i, pid) - Resume process pid.
  • uint64_t s3k_supervisor_get_state(i, pid) - Get state of process pid.
  • uint64_t s3k_supervisor_read_reg(i, pid, register_number) - Reads a virtual register of process pid. (Req. process pid suspended).
  • uint64_t s3k_supervisor_write_reg(i, pid, register_number, value) - Write to virtual register of process pid. (Req. process pid suspended).
  • uint64_t s3k_supervisor_read_cap(i, pid, j) - Read capability j of process pid. (Req. process pid suspended).
  • uint64_t s3k_supervisor_give_cap(i, pid, j, k) - Give capability j to process pid, placing it in slot k. (Req. process pid suspended).
  • uint64_t s3k_supervisor_take_cap(i, pid, j, k) - Take capability j from process pid, placing it in slot k. (Req. process pid suspended).

Virtual registers

TODO: Fix constants for virtual registers.

User guide

Prerequisites:

  • python3 with pyyaml
  • riscv-gnu-toolchain : compiled with unknown-elf or similar (i.e., not linux)

The following commands will compile the kernel, s3k.elf, with a dummy payload:

git clone git@github.com:kth-step/separation-kernel.git
cd separation-kernel
make CC=riscv64-unknown-elf-gcc # or CC=riscv64-elf-gcc for AUR

Custom payload, kernel config and platform:

  • Copy config.h and configure.
  • Configure the variables in your config file.
  • Copy bsp/virt.h or bsp/sifive_u.h and configure it for your platform.
  • make CC=riscv64-unknown-elf-gcc CONFIG=path/to/my/config.h PLATFORM=path/to/my/platform.h [PAYLOAD=path/to/my/payload]

Check https://github.com/kth-step/separation-kernel-examples (WIP) for sample application.

Coding style

  • Functions variables should use snake_case.
  • Function names should be prefixed with the file name, e.g., proc_init() if the function resides in file.c
  • Structs, enum, and union must be defined with snake_case and have a typedef in the form snake_case_t.
  • C files should be formatted using clang-format (make format), style file (.clang-format) is provided.

Progress

  • Implement the foundation of the scheduler.
  • Implement message passing so we can send basic messages with no capabilities.
  • Implement message passing so we can send time slices, allowing us to dynamically change the scheduling.
  • Implement the supervisor capability allowing process 0 to manage other process's capabilities.
  • Implement memory protection (memory slice, memory frame).
  • Implement exception handling.
  • Separate kernel and user binaries. Implement a basic bootloader in process 0 for loading the user binaries.
  • Design and build applications, benchmarks and tests.
  • Start on formal verification.

Related Projects

  • OpenMZ - A lightweight open-source implementation of MultiZone Security.
  • seL4 - A verified and secure µ-kernel, suitable for more capable hardware.
  • eChronos - a real-time operating systems.
  • CertiKOS - A verified, secure and modularized kernel for more capable hardware.
  • Keystone Enclave - A lightweight TEE for RISC-V.
  • MultiZone Security - A lightweight TEE for RISC-V.
  • STH - SICS Thin Hypervisor, a research kernel part of the PROSPER project.

Copyright (MIT License)

Copyright 2021 Henrik Karlsson henrik10@kth.se

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.