A major caveat in tools like sudo and doas for that matter is that they rely on setuid binaries and privilege escalation in order to run commands as root.

The design is not ideal, and also drags in a few limitations:

1. The whole user session needs to retain capabilities to perform privilege escalation.
2. They don’t work when running an entire user session in a restricted user namespace.
3. setuid binaries limitations on how the whole system is secured.

An interesting alternative with a is s6-sudod, which splits the program into two parts: a privileged server and an unprivileged client.

This is a summary of an experiment from a few weeks ago where I experimented with using ssh locally to perform the same role as sudo, without exposing this sshd instance to the network.

Goals

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• Enable authorised users (and only authorised users) to run commands as root.
• Don’t use privilege escalation.

Implementation

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First, I configured a dedicated SSH key that will be authorised for authentication as root. This key is not in the regular authorized_keys file, but in a separate file which will only be used for this purpose:

mkdir /root/.ssh/
echo ssh-ed25519 AAAAC3Nza... > /root/.ssh/local_keys

I then ran an sshd server instance bound to a unix domain socket. Permissions are tightened so unauthorised users cannot even access the socket. This instance overrides the PermitRootLogin option to enable logging in as root¹ and uses the newly created /root/.ssh/local_keys as a source for authorised keys:

mkdir /run/sshd/
chown root:wheel /run/sshd/
chmod 750 /run/sshd/
s6-ipcserver /run/sshd/sshd.sock sshd -ie -o AuthorizedKeysFile=/root/.ssh/local_keys -o PermitRootLogin=yes

The root account was locked to disallow logging in via any mechanism. This was done by prefixing the password’s hash with ! (so no password’s hash can ever match this value). sshd interprets this special prefix as the account being locked and won’t allow logging in as root.

I changed the root password in /etc/passwd and replaced the ! with an *. sshd won’t give new value any special interpretation, and will allow logging in as root. The value * will never match the hash of any password either, so logging in via password remains effectively disabled.²

I then needed to connect to the local sshd instance. While sshd has a -i flag which allows passing an existing socket to it, ssh has no equivalent flag. The ProxyCommand option can be (ab)used for this:

ssh -o ProxyCommand='socat STDIO UNIX-CONNECT:/run/sshd/sshd.sock' \
    -i .ssh/root-key.pub \
    -t \
    root@root \
    "cd $(pwd); '$SHELL' --login"

I’m using a hardware-bound SSH key in this case, which means that I need to tap the physical device to authorise this connection. I could also use an ssh-agent that requires explicit approval before disclosing keys (e.g.: hissh-agent).

A little caveat here is that socat will read all input from ssh, and then write it into the socket, effectively duplicating the overhead of the connection. I read the relevant manual pages a few more times, and couldn’t find a solution. I came across ProxyUseFdpass, but wasn’t entirely sure how to make it work.

After some more research online (and some major frustration), I found a clear usage example from 2016. It turnes out that ProxyUseFdpass was quite straightforward and allows me to specify a command that sends the socket file descriptor (via stdout) to ssh, and ssh then connects over this socket.

I saved the following script into /home/hugo/tmp/passfd.py:

#!/usr/bin/env python3
# From: https://www.gabriel.urdhr.fr/2016/08/07/openssh-proxyusefdpass/

import sys
import socket
import array

# Create the file descriptor:
s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
s.connect("/run/sshd/sshd.sock")

# Pass the file descriptor:
fds = array.array("i", [s.fileno()])
ancdata = [(socket.SOL_SOCKET, socket.SCM_RIGHTS, fds)]
socket.socket(fileno = 1).sendmsg([b'\0'], ancdata)

And the command to connect to ssh now becomes:

ssh -o ProxyCommand='/home/hugo/tmp/passfd.py' \
    -i .ssh/root-key.pub \
    -o ProxyUseFdpass=yes \
    -t \
    root@root \
    "cd $(pwd); '$SHELL' --login"

The linked article mentioned using nc (for a somewhat different use case). Initially, it would seem that nc -FU /run/sshd/sshd.sock would work, but the manual page actually specifies that this is not supported:

-F		Pass the first connected socket using sendmsg(2) to stdout and exit. This
		is useful in conjunction with -X to have nc perform connection setup
		with a proxy but then leave the rest of the connection to another
		program (e.g. ssh(1) using the ssh_config(5) ProxyUseFdpass option).
		Cannot be used with -c or -U.

Conclusion

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This technique works. It relies mainly on OpenSSH for all the sensitive security details. Not only does OpenSSH have a great track record, but it also enables various forms of authentication including using a hardware-based SSH key.

Configuring this on a new host has no complex steps, and the above ipcserver command can just be executed via the system’s service manager.

The above passfd.py script is a quick hack to move the experiment forward; for daily usage it would be best to write a tiny executable that does the same thing and put it into /usr/local/bin. The whole ssh command could also be placed in a tiny wrapper.