Environment variables still accessible through /proc

Impact

Environment variables can be read from procfs unless a new process is started.

PoC

use birdcage::{Birdcage, Sandbox};
use std::{env, fs};

fn main() {
    Birdcage::new().lock().unwrap();

    assert_eq!(env::var_os("SECRET"), None);

    let environ = fs::read_to_string("/proc/self/environ").unwrap();
    assert!(!environ.contains("SECRET"), "ENVIRON CONTAINS SECRET:\n{environ}");
}

$  SECRET=test cargo run
thread 'main' panicked at src/main.rs:10:5:
ENVIRON CONTAINS SECRET:
 [truncated]

Possible Solutions

The simplest solution would be relying on the ptrace isolation and always spawning a new process by changing birdcage's API to create a new command. With an additional PID namespace the guarantees could be even further reinforced.