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On running untrusted code in AWS Lambda
The core feature of luajit.me is running untrusted Lua code submitted via Web UI.
AWS Lambda is appealing as it would allow to drive costs down and enable auto scaling without any effort on my part. Last but not least, it supports both amd64 and aarch64!
Sandboxing is a must. Even though it is presumably possible to isolate a Lambda function from the rest of the infrastructure, an exploit could leave it in a weird state. This is concerning as Lambdas are reused if requests come in a short succession.
For luajit.me use case it is sufficient to sandbox a single process. I.e. being able to filter syscalls would be sufficient.
As it turns out, in AWS Lambda chroot, seccomp and ptrace are failing with EPERM. This makes sandboxing rather tricky.
Another option considered was QEMU.
As KVM is not available, we are running in dynamic binary translation mode. Unlike KVM-based virtualiser, it didn't undergo security audit and is not recommended. Probably still fine for a low-profile project.
QEMU can virtualise the whole system or a single user-space process. The later is utterly insecure therefore we went for a whole system virtualisation. Here's some timing data.
+ time luajit m.lua
real 0m 0.04s
user 0m 0.00s
sys 0m 0.00s
+ time luajit m.lua 1000
real 0m 0.35s
user 0m 0.10s
sys 0m 0.00s
Mandelbrot program, rendering 100x100 and 1000x1000 bitmaps.
+ vmwrap --no-kvm sh -c 'time luajit m.lua > /dev/null'
real 0m 2.64s
user 0m 0.17s
sys 0m 1.34s
+ vmwrap --no-kvm sh -c 'time luajit m.lua 1000 > /dev/null'
real 0m 7.61s
user 0m 5.06s
sys 0m 1.46s
Same program. Using vmwrap as a simple tool to run a workload in a QEMU VM.
Approx. 30 seconds.
The only working approach to sandboxing untrusted code in AWS Lambda was QEMU. The overall slowdown compared to running unsandboxed was in the range of 22-66. This is absolutely unacceptable as instant response would've been replaced with staring at a spinning wheel. I strongly believe that being blazingly fast is the important feature of luajit.me.
Long VM init time is also challenging; restoring from a QEMU snapshot could help here.
Therefore I regret to discard AWS Lambda as the prospective option for luajit.me.