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

[Eric W. Biederman]

Maciej Żenczykowski <zenczykowski at gmail.com> writes:

>> 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.
>
>> 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...
>
>> 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.
>
>> 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. :-(

>> 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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