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    SubjectRe: [PATCH] devpts: Add ptmx_uid and ptmx_gid options
    Alexander Larsson <alexl@redhat.com> writes:

    > On Thu, 2015-05-28 at 11:44 -0500, Eric W. Biederman wrote:
    >> Andy Lutomirski <luto@amacapital.net> writes:
    >>
    >> > On Thu, Apr 2, 2015 at 11:27 AM, Eric W. Biederman
    >> > <ebiederm@xmission.com> wrote:
    >> > > Andy Lutomirski <luto@amacapital.net> writes:
    >> > >
    >> > > > On Thu, Apr 2, 2015 at 7:29 AM, Alexander Larsson <
    >> > > > alexl@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@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?
    >>
    >> I am much closer. Escaping bind mounts is still not yet fixed but I
    >> have code that almost works.
    >>
    >> My gut feel still says that two user namespaces one where your 0 is
    >> mapped to your uid and a second where your uid is identity mapped is
    >> the
    >> preferrable configuration, and makes this patch unnecessary.
    >
    > I don't really understand this. My usecase is that I want a desktop app
    > sandbox, it should run as the actual user that is running the graphical
    > session mapped to its real uid. In this namespace i want a /dev/pts so
    > that i can e.g. shell out to ssh and feed it a password on the tty
    > prompt or similar. And i don't want to bind-mount in the host /dev/pts,
    > because then the sandbox can read from the ttys of other apps.
    >
    > Where does the second namespace enter into this?

    Step a. Create create a user namespace where uid 0 is mapped to your
    real uid, and set up your sandbox (aka mount /dev/pts and everything
    else).

    Step b. Create a nested user namespace where your uid is identity
    mapped and run your desktop application. You can even drop all caps in
    your namespace.

    Or basically:
    unshare(CLONE_NEWUSER)

    map 0 to real_uid
    set things up.

    unshare(CLONE_NEWUSER)
    map real_uid to 0 (Because I am assuming we are
    single threaded in the nested context)

    drop caps
    exec /path/to/my/sandboxed/application

    Eric


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