Re: [PATCH 0/7] Initial support for user namespace owned mounts

Casey Schaufler
2015-07-15T22:39:05Z

On 7/15/2015 2:06 PM, Eric W. Biederman wrote:
> Casey Schaufler <casey@schaufler-ca.com> writes:
>
>> On 7/15/2015 12:46 PM, Seth Forshee wrote:
>>> These are the first in a larger set of patches that I've been working on
>>> (with help from Eric Biederman) to support mounting ext4 and fuse
>>> filesystems from within user namespaces. I've pushed the full series to:
>>>
>>> git://kernel.ubuntu.com/sforshee/linux.git userns-mounts
>>>
>>> Taking the series as a whole, the strategy is to handle as much of the
>>> heavy lifting as possible in the vfs so the filesystems don't have to
>>> handle weird edge cases. If you look at the full series you'll find that
>>> the changes in ext4 to support user namespace mounts turn out to be
>>> fairly minimal (fuse is a bit more complicated though as it must deal
>>> with translating ids for a userspace process which is running in pid and
>>> user namespaces).
>>>
>>> The patches I'm sending today lay some of the groundwork in the vfs and
>>> related code. They fall into two broad groups:
>>>
>>> 1. Patches 1-2 add s_user_ns and simplify MNT_NODEV handling. These are
>>> pretty straightforward, and Eric has expressed interest in merging
>>> these patches soon. Note that patch 2 won't apply cleanly without
>>> Eric's noexec patches for proc and sys [1].
>>>
>>> 2. Patches 2-7 tighten down security for mounts with s_user_ns !=
>>> &init_user_ns. This includes updates to how file caps and suid are
>>> handled and LSM updates to ignore security labels on superblocks
>>> from non-init namespaces.
>>>
>>> The LSM changes in particular may not be optimal, as I don't have a
>>> lot of familiarity with this code, so I'd be especially appreciative
>>> of review of these changes and suggestions on how to improve them.
>> Lukasz Pawelczyk <l.pawelczyk@samsung.com> proposed
>> LSM support in user namespaces ([RFC] lsm: namespace hooks)
>> that make a whole lot more sense than just turning off
>> the option of using labels on files. Gutting the ability
>> to use MAC in a namespace is a step down the road of
>> making MAC and namespaces incompatible.
> This is not "turning off the option to use labels on files".

It gives an unprivileged user the ability to ignore
the Smack labels that are on files and to create files
with labels that do not match the rules laid down by the
security module.

> This is supporting mounting filesystems like ext4 by unprivileged users
> and not trusting the labels they set in the same way as we trust labels
> on filesystems mounted by privileged users.

OK, you don't trust the metadata on a filesystem mounted by an untrusted
user. That's fair.

> The first step needs to be not trusting those labels and treating such
> filesystems as filesystems without label support. I hope that is Seth
> has implemented.

A filesystem with Smack labels gets mounted in a namespace. The labels
are ignored. Instead, the filesystem defaults (potentially specified as
mount options smackfsdef="something", but usually the floor label ("_"))
are used, giving the user the ability to read everything and (usually)
change nothing. This is both dangerous (unintended read access to files)
and pointless (can't make changes).

I can't speak authoritatively for SELinux, but it looks to me like you
may have similar issues there.

> In the long run we can do more interesting things with such filesystems
> once the appropriate LSM policy is in place.

The problem is not that the short term behavior is uninteresting,
it's that it is broken. Mounting a filesystem with xattrs and ignoring
those xattrs results in incorrect access control decisions.

> Getting s_user_ns present on struct super, properly set, and all of the
> appropriate checks against it present in the vfs so that filesystems
> don't need to duplicate logic is important if we are going do more
> interesting things with user namespaces (as users have been asking for).

OK, but the fact that someone wants to do something they shouldn't
doesn't mean you get to break things that work now to accommodate
them. There are reasons why mounting filesystems requires privilege!

> It is important for things as small as making it safe to allow
> truly unprivileged users to mount fuse filesystems.

If it isn't safe you shouldn't be doing it, even if it's "small"
and something that would make life easier for some set of users.

> I am on the fence with Lukasz Pawelczyk's patches. Some parts I liked
> some parts I had issues with. As I recall one of my issues was that
> those patches conflicted in detail if not in principle with this
> appropach.
>
> If these patches do not do a good job of laying the ground work for
> supporting security labels that unprivileged users can set than Seth
> could really use some feedback. Figuring out how to properly deal with
> the LSMs has been one of his challenges.

The feedback is that you can't pick and
choose when you are going to pay attention to the security attributes
on a filesystem. It's possible that it will work out the way you want
it, but it probably won't. Smack doesn't allow you to choose if you're
using xattrs. SELinux does, but certainly doesn't expect you to be
flipping it on and off. I'm not convinced that it's safe to do for
capability sets, either, but I'm not up to arguing PIxFE+ vector
calculations just now.

> I am hoping I can finishing working through the patches to fix the
> semantics of rename and bind mounts before the next merge window opens,
> so I can have enough cycles to lift the feature freeze on user
> namespaces. Except for maybe his first two patches (which fix a small
> userspace API breakage) none of Seth's patches get to go in until I lift
> the freeze.

Thanks. I know (believe me, I know) how frustrating it can be when
you get the big NAK on something that seems like it's addressed.
Unfortunately, the proposed approach (not just the specifics of
implementation) does not work.

> Which is probably too much information but I hope this makes it clear
> that the point of this work is as an enabler for future developments,
> not as something to make user namespaces and LSMs incompatible.

I am paranoid, but not to the extent that I think anyone
is trying to break the interaction between security modules
and namespaces. Having worked with Lukasz on his security
namespace patches it is clear to me that this is not a simple
problem and that it is unlikely to have the simple solution
everyone would like to see. I also don't see an intermediate
state that works while the "real" solution is being refined.
As always, I'm willing to be proven wrong.

> Eric