Subject: Bug#1465: [X2Go-Dev] Bug#1465: Bug#1465: Allow running with restricted shell (rbash), or limit applications that can be run.

From: Stefan Baur <[email-masked]>
Date: Mon, 4 May 2020 14:30:26 +0200

Am 04.05.20 um 14:06 schrieb Ulrich Sibiller:
> On Mon, May 4, 2020 at 1:15 PM Stefan Baur <[email-masked]> wrote:
>> You need to realize the truth: What a user can see (as in "access"),
>> they can copy.
> 
> Well, I basically agree with what you wrote. But the OP was mentioning
> he just wants to provide _one_ single published application.
> 
> Now let us assume some pre-conditions:
> - the application is unable to display the data you want to protect.
> If not, all the ways you mocked up above could be used and the
> approach will not work

And that's the catch: If the application can't display the data - then
why would the user need access to it at all?  chmod/chown it away from
them and you're good to go.  But obviously the data is needed *somehow*,
or else they wouldn't have the problem of wanting to hide it from the user.

If it's raw data vs. processed data (e.g. points of data in a CSV file
vs. a graph generated from it), then (and only then) the solution is simple:
You need an app (a daemon, probably - though it might be done per-call
with passwordless sudo or the suid bit set on that app) that processes
the raw data and runs under another account, and another app that is
able to talk to this app, which can only display the processed data.
That way, you can use file system permissions to "wall off" the raw data
so only the processing app can access that data.

> - the application cannot start other apps like an xterm or a shell on
> user request

And that is particularly hard, as experience shows.  Even a "file
chooser" dialog from a standard GUI toolkit that you use for selecting a
data file will usually offer you a right-click option with "execute".
Navigate to the xterm binary (path traversal via ../../../ and the like
is another problem to keep in mind, even if you're able to hide critical
directories from view) and there's your shell.
One such item that you forget about, and boom - you've been exploited.

> - there's only ssh access
> - the x2go scripts are sane and secure

That's something I wouldn't want to bet my money on without an extensive
audit.  And even then there's the risk that the audit missed to spot
something.

> Then all we'd need was
> - a restricted ssh-key that only allows for the commands that are
> required for the x2go session handling

Which doesn't work out of the box.  You can specify exactly one command.
To be able to use more than one, you need a wrapper script on the host
that is set as forced command, which then parses $SSH_ORIGINAL_COMMAND.
These scripts are notoriously bad to maintain, error-prone, and while
they work with scripted commands (e.g. running an automated rsync job
with varying target directories), they suck hard for interactive use.

> - ensuring the x2go session handling will only start that single
> application and no other user specified command.
> 
> The user then can still configure arbitrary sessions but they will
> either always fail or ignore the user's command and run the one
> application in question. We could also provide a server side setting
> that only allows published application connects.
> 
> It will not work out of the box but I am pretty sure it could be implemented.

To me, it sounds like a horrible kludge that is bound to collapse rather
sooner than later, and it would only offer a false sense of security.
Making false promises is something we should leave to the Microsoft
Windows world, I'd say. ;)

> Also, IIRC Mihai added an explicit bash call into certain commands to
> make it work fur users with a different login shell. And obviously the
> original rbash instructions worked before. So you could also try to
> set that up and do some research where to remove the explicit bash
> calls.

Given that bash is enforced there for a reason, it doesn't sound like a
good idea to replace it with something else.

-Stefan

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