Message-ID: <679a376739b99_132e08294f3@willemb.c.googlers.com.notmuch>
Date: Wed, 29 Jan 2025 09:12:55 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: stsp <stsp2@...dex.ru>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Ondrej Mosnacek <omosnace@...hat.com>
Cc: Willem de Bruijn <willemb@...gle.com>, 
 Jason Wang <jasowang@...hat.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 network dev <netdev@...r.kernel.org>, 
 Linux Security Module list <linux-security-module@...r.kernel.org>, 
 SElinux list <selinux@...r.kernel.org>
Subject: Re: Possible mistake in commit 3ca459eaba1b ("tun: fix group
 permission check")

stsp wrote:
> 29.01.2025 01:59, Willem de Bruijn пишет:
> > stsp wrote:
> >> By doing that you indeed avoid
> >> the problem of "completely
> >> inaccessible tap". However, that
> >> breaks my setup, as I really
> >> intended to provide tap to the
> >> owner and the unrelated group.
> >> This is because, eg when setting
> >> a CI job, you can add the needed
> >> user to the needed group, but
> >> you also need to re-login, which
> >> is not always possible. :(
> > Could you leave tun->owner unset?
> 
> That's exactly the problem: when
> the user is not in the needed group,
> then you need to unset _both_.
> Unsetting only owner is not enough.
> Adding the user to the group is not
> enough because then you need to
> re-login (bad for CI jobs).

At some point we can question whether the issue is with the setup,
rather than the kernel mechanism.

Why does your setup have an initial user that lacks the group
permissions of the later processes, and a tun instance that has both
owner and group constraints set?

Can this be fixed in userspace, rather than allow this odd case in the
kernel. Is it baked deeply into common containerization tools, say?
 
> I actually tried to address the
> supplementary groups problem:
> https://lore.kernel.org/lkml/20241108204102.1752206-1-stsp2@yandex.ru/T/
> but nothing came out, so I have
> to walk around multiple projects,
> talking them into a new semantics
> and representing the problems
> like this one. If people instead
> concentrate on solving the inability
> to change the supplementary group
> list, nothing like this would ever
> happen. :)
> 
> >> Also completely ignoring group
> >> when the user is set, is somewhat
> >> questionable. At the very least,
> >> perhaps then you need to explicitly
> >> clear the group when the user
> >> is set, to avoid the confusion.
> >> Having "either user or group"
> >> sounds like a sensible semantic,
> >> but its a different semantic.
> > True. I think that would have satisfied the intent of adding the
> > group check at the time, and would have avoided this situation.
> >
> > But we indeed cannot retroactively restrict allowed behavior.
> > As that will break users.
> >
> > Conversely, it might be that an existing user out there depends on
> > the prior behavior that only a process that matches both user and
> > group can use the device. Which might be reason for reverting the
> > patch entirely.
> But this is not an option too, let
> me remind the previous situation:
> 1. If the user is in the group, then
>     the group doesn't have any effect
>     at all.
> 2. if the user is not in the group -
>     no one can access the device.
> 
> "either-or" semantic is a direct fix
> to that, as it represents case 1 and
> fixes case 2. My semantic covers the
> real-world situation of inability to
> change the group list, but it needs
> further tweaking and discussing.
> Applying "either-or" may be feasible,
> but the complete revert looks like
> returning to a quite broken state.
> 

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