Socket path security checks can fail when the client is in a container about parsec-client-rust

[hug-dev]

Part of the resolution of this issue would have to be done with this one on Parsec.

I have just realised that our checks in the client were useless now that the socket is in /run/parsec.sock. That is because only an administrator can create the /run/parsec folder.
The same way we trusted the administrator to create the parsec user who owns the socket folder, we can now trust them to create the /run/parsec folder. I believe that is exactly the same level of trust and would allow us to remove all the checks in the client. Also, our threat model says that all users with privileges are trusted.

If we decide for now that Parsec will only be deployed with one authenticator there are two options:

1. Deployment with Direct Authentication. If the socket is not visible to the client, it means they are not in the parsec-clients group. If it visible, clients can either trust the administrator to have set the correct group permissions or do the check themselves. Those checks will not work under containers as they are now, unles we fix the parsec-clients GID. In the parsec side, we can make sure the administrator did things right and add checks for this.
2. Deployment with Unix Peer Credentials Authentication. Everybody should be able to see the socket and clients don't have to do any checks. Clients can be sure their keys can not be accessed with direct authentication because of both/either: keys are partitioned with authentication type and only one authenticator can be used by Parsec.

[ionut-arm]

I'll have a look at what changes we need to make in the threat model with the new and improved filesystem locations, and the changes required for this issue - with a focus on making the TM easier to extend for new authenticators.

[paulhowardarm]

Probably worth noting that I actually did this experiment with a build of Parsec that was still using /tmp rather than /run, but I updated the issue description to reflect the new path.

[ionut-arm]

So the overall conclusion is that we don't need the checks and can just drop them, it seems. The threat model updates should cover our backsides