KC Sivaramakrishnan Assistant Professor @ IIT Madras

OCaml on Baremetal Shakti RISC-V processor

It has been 3 months since I joined IIT Madras and it has been good fun so far. Along with the members of the RISE group, we’ve initiated a project to build secure applications on top of secure extensions of the open-source Shakti RISC-V processor ecosystem. Unsurprisingly, my language of choice to build the applications is OCaml. Given the availability of rich ecosystem of libraries under the MirageOS library operating system for building unikernels, we hope to minimise the amount of unsafe C code that the hardware has to contend with and protect exploits against. As a first step, we have managed to get OCaml programs to run on directly on top of the Shakti processor running in simulation under QEMU and Spike ISA simulators without an intervening operating system.

A custom bootloader performs the necessary hardware initialisation and transfers control directly to the OCaml program. We have open-sourced all of the tools necessary to build your own kernel. This handy dockerfile documents the entire process. For the impatient, an image is available in the dockerhub:

$ docker run -it iitmshakti/riscv-ocaml-baremetal:0.1.0

# Write your program
$ echo 'let _ = print_endline "A camel treads on hardware!"' > hello.ml
# Compile for Shakti
$ ocamlopt -output-obj -o payload.o hello.ml
$ file payload.o
payload.o: ELF 64-bit LSB relocatable, UCB RISC-V, version 1 (SYSV), not stripped

# Link with bootcode and build the kernel
$ make -C ../build
make: Entering directory '/root/ocaml-baremetal-riscv/build'
make[1]: Entering directory '/root/ocaml-baremetal-riscv/build'
make[2]: Entering directory '/root/ocaml-baremetal-riscv/build'
make[2]: Leaving directory '/root/ocaml-baremetal-riscv/build'
[ 64%] Built target boot
make[2]: Entering directory '/root/ocaml-baremetal-riscv/build'
make[2]: Leaving directory '/root/ocaml-baremetal-riscv/build'
[ 78%] Built target freestanding-compat
make[2]: Entering directory '/root/ocaml-baremetal-riscv/build'
make[2]: Leaving directory '/root/ocaml-baremetal-riscv/build'
[ 85%] Built target asmrun_t
make[2]: Entering directory '/root/ocaml-baremetal-riscv/build'
make[2]: Leaving directory '/root/ocaml-baremetal-riscv/build'
[ 92%] Built target nolibc_t
make[2]: Entering directory '/root/ocaml-baremetal-riscv/build'
make[2]: Leaving directory '/root/ocaml-baremetal-riscv/build'
[100%] Built target kernel
make[1]: Leaving directory '/root/ocaml-baremetal-riscv/build'
make: Leaving directory '/root/ocaml-baremetal-riscv/build'
$ file kernel 
kernel: ELF 64-bit LSB executable, UCB RISC-V, version 1 (SYSV), statically linked, with debug_info, not stripped

# Run under spike RISC-V ISA simulator
$ spike kernel
ocaml-boot: heap@0x80042be8 stack@0x8002fbc0
A camel treads on hardware!
ocaml-boot: caml runtime returned. shutting down!

# Run under QEMU
$ qemu-system-riscv64 -machine spike_v1.10 -smp 1 -m 1G -serial stdio -kernel kernel
VNC server running on 127.0.0.1:5900
ocaml-boot: heap@0x80042be8 stack@0x8002fbc0
A camel treads on hardware!
ocaml-boot: caml runtime returned. shutting down!

The immediate next step will be getting the code to run on a Shakti softcore on an FPGA. In addition to targeting high-end FPGAs, we will also be targeting the $100 Arty A7 hobbyist board and release all of the software under liberal open-source licenses.

Further along, we will port mirage libraries to Shakti following similar to the setup in Well-typed lightbulbs and implementing hardware security enhancements in Shakti for preventing spatial and temporal attacks while running unsafe C code (with the ability to dynamically turn it off when running OCaml!), hardware-assisted compartments, etc. Lots of exciting possibilities on the horizon!

Acknowledgements

Much of this work was done by the incredible Malte, who is a visiting student at IIT Madras on a semester away from Leibniz University Hannover, Arjun, Lavanya, Ambika, Chester, and the rest of the Shakti team. The RISC-V port of OCaml is developed and maintained by Nicolás Ojeda Bär.


Creative Commons License kcsrk dot info by KC Sivaramakrishnan is licensed under a Creative Commons Attribution 4.0 International License.