Re: [PATCH 3/7] fs: Ignore file caps in mounts from other user namespaces

Seth Forshee
2015-07-16T13:14:16Z

On Thu, Jul 16, 2015 at 12:44:49AM -0500, Eric W. Biederman wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
>
> > On Wed, Jul 15, 2015 at 10:04 PM, Eric W. Biederman
> > <ebiederm@xmission.com> wrote:
> >> Andy Lutomirski <luto@amacapital.net> writes:
> >>
> >>>
> >>> So here's the semantic question:
> >>>
> >>> Suppose an unprivileged user (uid 1000) creates a user namespace and a
> >>> mount namespace. They stick a file (owned by uid 1000 as seen by
> >>> init_user_ns) in there and mark it setuid root and give it fcaps.
> >>
> >> To make this make sense I have to ask, is this file on a filesystem
> >> where uid 1000 as seen by the init_user_ns stored as uid 1000 on
> >> the filesystem? Or is this uid 0 as seen by the filesystem?
> >>
> >> I assume this is uid 0 on the filesystem in question or else your
> >> unprivileged user would not have sufficient privileges over the
> >> filesystem to setup fcaps.
> >
> > I was thinking uid 0 as seen by the filesystem. But even if it were
> > uid 1000, the unprivileged user can still set whatever mode and xattrs
> > they want -- they control the backing store.
>
> Yes. And that is what I was really asking. Are we taking about a
> filesystem where the user controls the backing store?
>
> >>> Then global root gets an fd to this filesystem. If they execve the
> >>> file directly, then, with my patch 4, it won't act as setuid 1000 and
> >>> the fcaps will be ignored. Even with my patch 4, though, if they bind
> >>> mount the fs and execve the file from their bind mount, it will act as
> >>> setuid 1000. Maybe this is odd. However, with Seth's patch 3, the
> >>> fcaps will (correctly) not be honored.
> >>
> >> With patch 3 you can also think of it as fcaps being honored and you
> >> get all the caps in the appropriate user namespace, but since you are
> >> not in that user namespace and so don't have a place to store them
> >> in struct cred you don't get the file caps.
> >>
> >> From the philosophy of interpreting the file as defined by the
> >> filesystem in principle we could extend struct cred so you actually
> >> get the creds just in uid 1000s user namespace, but that is very
> >> unlikely to be worth it.
> >
> > I agree.
> >
> >>
> >>> I tend to thing that, if we're not honoring the fcaps, we shouldn't be
> >>> honoring the setuid bit either. After all, it's really not a trusted
> >>> file, even though the only user who could have messed with it really
> >>> is the apparent owner.
> >>
> >> For the file caps we can't honor them because you don't have the bits
> >> in struct cred.
> >>
> >> For setuid we can honor it, and setuid is something that the user
> >> namespace allows.
> >>
> >
> > We certainly *can* honor it. But why should we? I'd be more
> > comfortable with this if the contents of an untrusted filesystem were
> > really treated as just data.
>
> In these weird bleed through situtations I don't know that we should.
> But extending nosuid protections in this way is a bit like yama
> a bit gratuitious stomping don't care cases in the semantics to
> make bugs harder to exploit.
>
> >>> And, if we're going to say we don't trust the file and shouldn't honor
> >>> setuid or fcaps, then merging all the functionality into mnt_may_suid
> >>> could make sense. Yes, these two things do different things, but they
> >>> could hook in to the same place.
> >>
> >> There are really two separate questions:
> >> - Do we trust this filesystem?
> >> - Do you have the bits to implement this concept?
> >>
> >> Even if in this specific context the two questions wind up looking
> >> exactly the same. I think it makes a lot of sense to ask the two
> >> questions separately. As future maintenance changes may cause the
> >> implementation of the questions to diverge.
> >>
> >
> > Agreed.
> >
> > Unless someone thinks of an argument to the contrary, I'd say "no, we
> > don't trust this filesystem". I could be convinced otherwise.
>
> But this is context dependent. From the perspective of the container
> we really do want to trust the filesystem. As the container root set it
> up, and if he isn't being hostile likely has a use for setfcaps files
> and setuid files and all of the rest.
>
> Perhaps I should phrase it as:
> - In this context do we trust the code? AKA mnt_may_suid?
> - What do these bits mean in this context? (Usually something more complicated).
>
> Which says to me we want both patches 3 and 4 (even if 4 uses s_user_ns)
> because 3 is different than 4.

So what I'll do is:

- Add a s_user_ns check to mnt_may_suid
- Keep the (now redundant) s_user_ns check in get_file_caps

I'm on the fence about having both the mnt and user ns checks in
mnt_may_suid - it might be overkill, but it still adds the protection
against clearing MNT_NOSUID in a bind mount. So I guess I'll keep the
mnt ns check.

Seth