[PATCH] userns: honour no_new_privs for cap_bset during user ns creation/switch

Maciej ?enczykowski [off-list ref] writes:

quoted

Good point about CAP_DAC_OVERRIDE on files you own.

I think there is an argument that you are playing dangerous games with
the permission system there, as it isn't effectively a file you own if
you can't read it, and you can't change it's permissions.

Append-only files are useful - particularly for logging.
It could also simply be a non-readable file on a R/O filesystem.

quoted

Given little things like that I can completely see no_new_privs meaning
you can't create a user namespace.  That seems consistent with the
meaning and philosophy of no_new_privs.  So simple it is hard to get
wrong.

Yes, I could totally buy the argument that no_new_privs should prevent
creating a user ns.

However, there's also setns() and that's a fair bit harder to reason about.
Entirely deny it?  But that actually seems potentially useful...
Allow it but cap it?  That's what this does...

quoted

We could do more clever things like plug this whole in user namespaces,
and that would not hurt my feelings.

Sure, this particular one wouldn't be all that easy I think... and how
many such holes are there?
I found this particular one *after* your first reply in this thread.

quoted

However unless that is our only
choice to avoid badly breaking userspace I would have to have to depend
on user namespaces being perfect for no_new_privs to be a proper jail.

This stuff is ridiculously complex to get right from userspace. :-(

quoted

As a general rule user namespaces are where we tackle the subtle scary
things that should work, and no_new_privs is where we implement a simple
hard to get wrong jail.  Most of the time the effect is the same to an
outside observer (bounded permissions), but there is a real difference
in difficulty of implementation.

So, where to now...

Would you accept patches that:

- make no_new_priv block user ns creation?

- make no_new_priv block user ns transition?

Yes.

The approach will need to be rethought if there is anything deliberately
combining user namespaces and no_new_privs.  As regressions are a no-no.
So we need wide spread testing, to avoid that.

But as much as possible I want no_new_privs to be simple and doing it's
job.

I will also take and encourage patches that close this minor privilege
escalation from the user namespace side.  As ideally creating a user
namespace should be as safe as no_new_privs.

Or perhaps we can assume that lack of create privs is sufficient, and
if there's a pre-existing user ns for you to enter, then that's
acceptable...
Although this implies you probably always want to combine no_new_privs
with a leaf user ns, or no_new_privs isn't all that useful for root in
root ns...
This added complexity, probably means it should be blocked...

Yes.

- inherits bset across user ns creation/transition based on X?
[this is the one we care about, because there are simply too many bugs
in the kernel wrt. certain caps]

That was my suspicion, and attack surface reduction is a different
discussion.  Would no_new_privs preventing a userns transition be enough
for the cases you care about?

Otherwise this is a different conversation because it is not about
semantics but about making the code safer to use.  In general if code is
simply not safe to user in a user namespace I would prefer to tighten
the permission checks, and just not allow that code.

Mostly what I have seen in previous conversations is simply concerns
about code that is not used or needed, being a problem.

X could be:
- a new flag similar to no_new_priv
- a new securebit flag (w/lockbit)  [provided securebits survive a
userns transition, haven't checked]
- or perhaps a new capability
- something else?

How do we make forward progress?

We start by causing no_new_privs to block userns creation and entering.

Eric
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