[PATCH] devpts: Add ptmx_uid and ptmx_gid options

Andy Lutomirski luto at amacapital.net
Wed May 27 21:32:43 UTC 2015


On Thu, Apr 2, 2015 at 11:27 AM, Eric W. Biederman
<ebiederm at xmission.com> wrote:
> Andy Lutomirski <luto at amacapital.net> writes:
>
>> On Thu, Apr 2, 2015 at 7:29 AM, Alexander Larsson <alexl at redhat.com> wrote:
>>> On Thu, 2015-04-02 at 07:06 -0700, Andy Lutomirski wrote:
>>>> On Thu, Apr 2, 2015 at 3:12 AM, James Bottomley
>>>> <James.Bottomley at hansenpartnership.com> wrote:
>>>> > On Tue, 2015-03-31 at 16:17 +0200, Alexander Larsson wrote:
>>>> >> On tis, 2015-03-31 at 17:08 +0300, James Bottomley wrote:
>>>> >> > On Tue, 2015-03-31 at 06:59 -0700, Andy Lutomirski wrote:
>>>> >> > >
>>>> >> > > I don't think that this is correct.  That user can already create a
>>>> >> > > nested userns and map themselves as 0 inside it.  Then they can mount
>>>> >> > > devpts.
>>>> >> >
>>>> >> > I don't mind if they create a container and control the isolated ttys in
>>>> >> > that sub container in the VPS; that's fine.  I do mind if they get
>>>> >> > access to the ttys in the VPS.
>>>> >> >
>>>> >> > If you can convince me (and the rest of Linux) that the tty subsystem
>>>> >> > should be mountable by an unprivileged user generally, then what you
>>>> >> > propose is OK.
>>>> >>
>>>> >> That is controlled by the general rights to mount stuff. I.e. unless you
>>>> >> have CAP_SYS_ADMIN in the VPS container you will not be able to mount
>>>> >> devpts there. You can only do it in a subcontainer where you got
>>>> >> permissions to mount via using user namespaces.
>>>> >
>>>> > OK let me try again.  Fine, if you want to speak capabilities, you've
>>>> > given a non-root user an unexpected capability (the capability of
>>>> > creating a ptmx device).  But you haven't used a capability separation
>>>> > to do this, you've just hard coded it via a mount parameter mechanism.
>>>> >
>>>> > If you want to do this thing, do it properly, so it's acceptable to the
>>>> > whole of Linux, not a special corner case for one particular type of
>>>> > container.
>>>> >
>>>> > Security breaches are created when people code in special, little used,
>>>> > corner cases because they don't get as thoroughly tested and inspected
>>>> > as generally applicable mechanisms.
>>>> >
>>>> > What you want is to be able to use the tty subsystem as a non root user:
>>>> > fine, but set that up globally, don't hide it in containers so a lot
>>>> > fewer people care.
>>>>
>>>> I tend to agree, and not just for the tty subsystem.  This is an
>>>> attack surface issue.  With unprivileged user namespaces, unprivileged
>>>> users can create mount namespaces (probably a good thing for bind
>>>> mounts, etc), network namespaces (reasonably safe by themselves),
>>>> network interfaces and iptables rules (scary), fresh
>>>> instances/superblocks of some filesystems (scariness depends on the fs
>>>> -- tmpfs is probably fine), and more.
>>>>
>>>> I think we should have real controls for this, and this is mostly
>>>> Eric's domain.  Eric?  A silly issue that sometimes prevents devpts
>>>> from being mountable isn't a real control, though.
>
> I thought the controls for limiting how much of the userspace API
> an application could use were called seccomp and seccomp2.
>
> Do we need something like a PAM module so that we can set up these
> controls during login?
>
>>> I'm honestly surprised that non-root is allowed to mount things in
>>> general with user namespaces. This was long disabled use for non-root in
>>> Fedora, but it is now enabled.
>>>
>>> For instance, using loopback mounted files you could probably attack
>>> some of the less well tested filesystem implementations by feeding them
>>> fuzzed data.
>>>
>>
>> You actually can't do that right now.  Filesystems have to opt in to
>> being mounted in unprivileged user namespaces, and no filesystems with
>> backing stores have opted in.  devpts has, but it's buggy without this
>> patch IMO.
>
> Arguably you should use two user namespaces.  The first to do what you
> want to as root the second to run as the uid you want to run as.
>
>>> Anyway, I don't see how this affects devpts though. If you're running in
>>> a container (or uncontained), as a regular users with no mount
>>> capabilities you can already mount a devpts filesystem if you create a
>>> subbcontainer with user namespaces and map your uid to 0 in the
>>> subcontainer. Then you get a new ptmx device that you can do whatever
>>> you want with. The mount option would let you do the same, except be
>>> your regular uid in the subcontainer.
>>>
>>> The only difference outside of the subcontainer is that if the outer
>>> container has no uid 0 mapped, yet the user has CAP_SYSADMIN rights in
>>> that container. Then he can mount devpts in the outer container where he
>>> before could only mount it in an inner container.
>>>
>>
>> Agreed.  Also, devpts doesn't seem scary at all to me from a userns
>> perspective.  Regular users on normal systems can already use ptmx,
>> and AFAICS basically all of the attack surface is already available
>> through the normal /dev/ptmx node.
>
> My only real take is that there are a lot more places that you need to
> tweak beyond devpts.  So this patch seemed lacking and boring.
>
> Beyond that until I get the mount namespace sorted out things are pretty
> much in a feature freeze because I can't multitask well enough to do
> complicated patches and take feature patches.
>

Eric, do you think you have time now to take a look at this patch?

--Andy


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