There is 1 issue with this though and that’s that we need unprivileged user namespaces enabled for it to work. bubblewrap sets the no_new_privs flag which prevents executing setuid binaries (which Firefox/Chromium fallback to when there are no user namespaces). So, we need to set the kernel.unprivileged_userns_clone=1 sysctl for sandbox-app-launcher to work with the Firefox/Chromium sandbox.
Chromium fails with an error when starting it without user namespaces:
The setuid sandbox is not running as root. Common causes:
* An unprivileged process using ptrace on it, like a debugger.
* A parent process set prctl(PR_SET_NO_NEW_PRIVS, ...)
Failed to move to new namespace: PID namespaces supported, Network namespace supported, but failed: errno = Operation not permitted
[9:9:0528/021452.679452:FATAL:zygote_host_impl_linux.cc(186)] Check failed: ReceiveFixedMessage(fds[0], kZygoteBootMessage, sizeof(kZygoteBootMessage), &boot_pid).
Firefox however goes on fine while the sandbox silently fails and it pretends that it’s active.
I don’t know how to workaround this other than setting the sysctl above (which exposes tons of kernel attack surface so it’s not a good solution at all).