Comparing Sandboxing Tools

The motivation came from here:

When programming in Node.js, a huge problem is that “npm install” downloads libraries you did not specify. It downloaded all dependencies listed in package.json, but it also downloaded their dependencies and the dependencies of their dependencies etc., which is code you did not explicitly ask for. While you can point your direct dependencies to trustworthy sources, you have no control about anything further down the line. In short: this is a (known) security hazard. A recent example is here. Auditing code in npm helps, but the whole concept is a fundamental problem.

Dart and Deno are reducing the problem significantly since you have to name all dependencies, but it does not necessarily help you if that dependency itself is compromised.

The runtime of Deno as well as wasmtime use a sandbox-approach to mitigate that: you have to enable access explicitly to anything: A Deno program has very few permissions otherwise. From a security point of view, this is much better.

Node.js nor Python have no sandbox model and when loading libraries from the Internet, which both do a lot, do you always know what you get? So I’m looking for choices how to retrofit programs with potentially questionable code.

My requirements:

  • Possible to use ad-hoc: I want to run a program with somehow limited access (e.g.: no root and no ability to become root, network access only when I allowed it, no access to files it does not need access to)
  • Protect my files from programs which run as me and thus with my normal privileges (e.g. very few program would need access to my ssh keys)

Test case:

  • Run a Node.js program which wants to read ~/.test_me and access It should not be able to do either unless it’s enabled.

Here the simple Node.js program:

const fs=require('fs');
const fetch=require('node-fetch');

async function accessStuff() {
  try {
    let f=await fs.promises.readFile(`${process.env.HOME}/.test_me`);
    console.log(`File: ${f}`);
  } catch(e) {
    console.log(`Error while accessing .test_me: ${e}`);
  fetch('', {
                method: 'get',
    .then(res => res.text())
    .then(body => console.log(body.split('\n').slice(0,1)))
    .catch(e => {
      console.error(`Error: ${e}`);

(async function () {
try {
  await accessStuff();
} catch(e) {
  console.error(`Error: ${e}`);

and a sample run without limitations:

❯ node index.js 
File: test 1

[...many more lines from]
  `<!doctype html><html itemscope="" itemtype=""
[...some HTML code from]

When it comes to security and sandboxing, those choices came up after a quick check with Google:

System-wide Mandatory Access Control (MAC)

SELinux needs special policies/contexts set up for the whole system. While this is great, it’s something root does. I see no simple way to do ad-hoc configurations to run a single command with the “correct” permissions. Plus the policy files are not easy to read nor write.

AppArmor is similar. A bit easier to read policy files, but they are all root owned, so not suitable for ad-hoc commands.

Both have a point to secure the complete system with the user explicitly not allowed to change the policies. Their purpose it not to protect the user from hurting themselves.

Sandbox Tools

Docker or containers in general provide a good way of isolation from the rest of the system and via bind mounts you can allow access to files or directories easily, but you have to create a container image first, upload it to a container registry and download and run it (Update: turns out that this is not required and a locally created image can be executed without problems). While it has its use, creating containers is a significant overhead if it’s needed for every program you are suspicious about.

minijail from Google looks good:

Minijail […] provides an executable that can be used to launch and sandbox other programs, […]

Installing on Debian was straightforward (needs kernel-headers and libcap-dev). Running a command with a specific user-definable policy is possible:

 # minijail0 -S /usr/share/minijail0/$(uname -m)/cat.policy -- \\
             /bin/cat /proc/self/seccomp_filter
but the examples/ directory was a small shock to me: a single example, and not a well explained one.

❯ cat examples/cat.policy 
# In this directory, test with:
# make LIBDIR=.
# ./minijail0 -n -S examples/cat.policy -- /bin/cat /proc/self/status
# This policy only works on x86_64.

read: 1
write: 1
restart_syscall: 1
rt_sigreturn: 1
exit_group: 1

open: 1
openat: 1
close: 1
fstat: 1
# Enforce W^X.
mmap: arg2 in ~PROT_EXEC || arg2 in ~PROT_WRITE
fadvise64: 1

While there is a tool to record the uses system calls (via strace) to create a policy (similar to what SELinux’s audit2allow tool), that means running a potentially harmful program once without restrictions. Plus the policy file is not exactly easy to understand. And the documentation does not help.

This is a dead end for my purpose.

bubblewrap is used as security layer for Ubuntu’s FlatPack installations. Using it is very command-line-option intensive, but a wrapper script will handle this. A test run:

❯ cat bwrap.test 

bwrap \
 --dev /dev \
 --ro-bind /lib /lib \
 --ro-bind /usr/bin /usr/bin \
 --ro-bind /bin /bin \
 --ro-bind /etc/resolv.conf /etc/resolv.conf \
 --ro-bind $HOME/js $HOME/js \
 --ro-bind $HOME/.test_me $HOME/.test_me \
 --tmpfs /tmp \
 --unshare-all \
 --share-net \
~/js/node/bin/node index.js

❯ ./bwrap.test 
~/.test_me contains: test 1
  `<!doctype html><html itemscope="" itemtype=""
[...some more HTML code...]

Removing the “–share-net” and removing the “–ro-bind” for .test_me stops both”

❯ ./bwrap.test 
Error while accessing .test_me: Error: ENOENT: no such file or directory, open '/home/harald/.test_me'
Error: FetchError: request to failed, reason: getaddrinfo ENOTFOUND

Note that you also need /etc/resolv.conf too to allow resolving DNS names. Also the order of “–unshare-all” and “–share-net” is important as the last one wins.

firejail is conceptually similar to bubblewrap, but beside having a large list of command line options, it also has configuration files in /etc/firejail/ and it also allows user-owned configurations (default in ~/.config/firejail):

❯ cat ~/.config/firejail/nodejs.profile
whitelist /home/harald/js
#whitelist /home/harald/.test_me
net none
include /usr/local/etc/firejail/
include /usr/local/etc/firejail/default.profile

❯ firejail --profile=~/.config/firejail/nodejs.profile node index.js
Reading profile /home/harald/.config/firejail/nodejs.profile
Reading profile /usr/local/etc/firejail/
Reading profile /usr/local/etc/firejail/default.profile
Reading profile /usr/local/etc/firejail/
Reading profile /usr/local/etc/firejail/
Reading profile /usr/local/etc/firejail/
Parent pid 231521, child pid 231522
Warning: cleaning all supplementary groups
Warning: cleaning all supplementary groups
Warning: cleaning all supplementary groups
Warning: cleaning all supplementary groups
Warning: cleaning all supplementary groups
Child process initialized in 94.84 ms
Error while accessing .test_me: Error: ENOENT: no such file or directory, open '/home/harald/.test_me'
Error: FetchError: request to failed, reason: getaddrinfo ENOTFOUND

Parent is shutting down, bye...
❯ firejail --quiet --net=none node index.js
Error while accessing .test_me: Error: EACCES: permission denied, open '/home/harald/.test_me'
Error: FetchError: request to failed, reason: getaddrinfo ENOTFOUND

The last sample shows that you don’t need to create a separate profile but similar to bwrap you can use command line options for most settings.

Uncommenting the “whitelist /home/harald/.test_me” line allows access to that file. Commenting out the “net none” allows network access. Per default network access is granted, but you can change this in /etc/firejail/default.profile. Once disabled in a profile, it cannot be re-enabled though. (Update: A “–ignore=net” option will ignore the “net none” in a profile).

After above tests I found out you can skip the “–profile=~/.config/firejail/PROFILENAME” if PROFILENAME is the binary name plus “.profile” as firejail will pick this up automatically. Very neat!

❯ firejail node index.js 
Reading profile /home/harald/.config/firejail/node.profile
Reading profile /etc/firejail/

And you can make it less verbose too and with sensible defaults you don’t even need to create any profiles. E.g. shell history files are inaccessible by default:

❯ firejail --quiet bash
$ cd
$ cat .bash_history 
cat: .bash_history: Permission denied
$ ls -la .bash_history
-r-------- 1 nobody nogroup 0 Dec 30 23:39 .bash_history

My Conclusion

SELinux and AppArmor are not something users can manage by themselves. Different scope than what I am looking for.

Using containers, especially when running as non-root works as long as you want to use containers anyway. Otherwise it’s a huge overhead: create container image, store it in a registry, and then run it. Any code changes would need a new container image to be created. Good for certain workload, especially those which will run as containers anyway later on. While I use containers extensively, a lot of programs I run are not a container.

bubblewrap works. It needs an extensive list of options to be useful. That’s not hard to put into a script. Since you have to add a lot of options and there’s no default options you can specify, it’s very explicit about permissions which makes it easier to debug since everything is configured when running your suspicious program. As the order of options is important, I can see this getting complicated quickly for non-trivial programs. Here is an example. Luckily most programs are trivial: few accesses are needed plus some capabilities like network access.

firejail got the spot between security and ease-of-use right in my opinion: sensible defaults (e.g. disabling at and crontab commands) with profiles for many programs. You can also have user-configurable profiles and they are not hard to create. The amount of extra work when using firejail is low: just adding “firejail” before the command helps a lot already out-of-the-box by hiding sensitive files and disabling miss-usable commands. Creating a specific profile makes this very configurable. And if you name the profiles like the binary you plan to use, it’s both simple to use while still being configurable.

Note that no solution is 100% secure. There’s always a trade-off between convenience and security. Unless you enforce it, if it’s inconvenient, it won’t be done. I’m guilty myself: Although I know how important backups are, I did them only occasionally (about twice a year) in the past. A year ago I finally completely automated it and only since then I have regular backups.

PS: While testing I experienced firejail to not be able run programs which have capabilities set if you use “caps.drop all” which is included in the default profile. See bug report. Can’t say yet if it’s a bug or badly worded option or lack of documentation or just unexpected behavior.

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