On Thu, May 28, 2015 at 10:01 AM, Alexander Larsson <alexl@xxxxxxxxxx> wrote: > On Thu, 2015-05-28 at 11:44 -0500, Eric W. Biederman wrote: >> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >> >> > On Thu, Apr 2, 2015 at 11:27 AM, Eric W. Biederman >> > <ebiederm@xxxxxxxxxxxx> wrote: >> > > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >> > > >> > > > On Thu, Apr 2, 2015 at 7:29 AM, Alexander Larsson < >> > > > alexl@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx> 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? > I think Eric is suggesting making a user namespace that maps your uid as 0, then making a mount namespace and mounting devpts, then making *another* user namespace that maps your uid (seen as 0) back to whatever nonzero number you want. That would probably work, but I think it's really ugly. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
