On Mon, 2014-05-26 at 00:24 +0200, Serge E. Hallyn wrote: > Quoting James Bottomley (james.bottom...@hansenpartnership.com): > > On Sat, 2014-05-24 at 22:25 +0000, Serge Hallyn wrote: > > > Quoting James Bottomley (james.bottom...@hansenpartnership.com): > > > > On Fri, 2014-05-23 at 11:20 +0300, Marian Marinov wrote: > > > > > On 05/20/2014 05:19 PM, Serge Hallyn wrote: > > > > > > Quoting Andy Lutomirski (l...@amacapital.net): > > > > > >> On May 15, 2014 1:26 PM, "Serge E. Hallyn" <se...@hallyn.com> > > > > > >> wrote: > > > > > >>> > > > > > >>> Quoting Richard Weinberger (rich...@nod.at): > > > > > >>>> Am 15.05.2014 21:50, schrieb Serge Hallyn: > > > > > >>>>> Quoting Richard Weinberger (richard.weinber...@gmail.com): > > > > > >>>>>> On Thu, May 15, 2014 at 4:08 PM, Greg Kroah-Hartman > > > > > >>>>>> <gre...@linuxfoundation.org> wrote: > > > > > >>>>>>> Then don't use a container to build such a thing, or fix the > > > > > >>>>>>> build scripts to not do that :) > > > > > >>>>>> > > > > > >>>>>> I second this. To me it looks like some folks try to (ab)use > > > > > >>>>>> Linux containers for purposes where KVM > > > > > >>>>>> would much better fit in. Please don't put more complexity > > > > > >>>>>> into containers. They are already horrible > > > > > >>>>>> complex and error prone. > > > > > >>>>> > > > > > >>>>> I, naturally, disagree :) The only use case which is > > > > > >>>>> inherently not valid for containers is running a > > > > > >>>>> kernel. Practically speaking there are other things which > > > > > >>>>> likely will never be possible, but if someone > > > > > >>>>> offers a way to do something in containers, "you can't do that > > > > > >>>>> in containers" is not an apropos response. > > > > > >>>>> > > > > > >>>>> "That abstraction is wrong" is certainly valid, as when vpids > > > > > >>>>> were originally proposed and rejected, > > > > > >>>>> resulting in the development of pid namespaces. "We have to > > > > > >>>>> work out (x) first" can be valid (and I can > > > > > >>>>> think of examples here), assuming it's not just trying to hide > > > > > >>>>> behind a catch-22/chicken-egg problem. > > > > > >>>>> > > > > > >>>>> Finally, saying "containers are complex and error prone" is > > > > > >>>>> conflating several large suites of userspace > > > > > >>>>> code and many kernel features which support them. Being more > > > > > >>>>> precise would, if the argument is valid, lend > > > > > >>>>> it a lot more weight. > > > > > >>>> > > > > > >>>> We (my company) use Linux containers since 2011 in production. > > > > > >>>> First LXC, now libvirt-lxc. To understand the > > > > > >>>> internals better I also wrote my own userspace to create/start > > > > > >>>> containers. There are so many things which can > > > > > >>>> hurt you badly. With user namespaces we expose a really big > > > > > >>>> attack surface to regular users. I.e. Suddenly a > > > > > >>>> user is allowed to mount filesystems. > > > > > >>> > > > > > >>> That is currently not the case. They can mount some virtual > > > > > >>> filesystems and do bind mounts, but cannot mount > > > > > >>> most real filesystems. This keeps us protected (for now) from > > > > > >>> potentially unsafe superblock readers in the > > > > > >>> kernel. > > > > > >>> > > > > > >>>> Ask Andy, he found already lots of nasty things... > > > > > >> > > > > > >> I don't think I have anything brilliant to add to this discussion > > > > > >> right now, except possibly: > > > > > >> > > > > > >> ISTM that Linux distributions are, in general, vulnerable to all > > > > > >> kinds of shenanigans that would happen if an > > > > > >> untrusted user can cause a block device to appear. That user > > > > > >> doesn't need permission to mount it > > > > > > > > > > > > Interesting point. This would further suggest that we absolutely > > > > > > must ensure that a loop device which shows up in > > > > > > the container does not also show up in the host. > > > > > > > > > > Can I suggest the usage of the devices cgroup to achieve that? > > > > > > > > Not really ... cgroups impose resource limits, it's namespaces that > > > > impose visibility separations. In theory this can be done with the > > > > device namespace that's been proposed; however, a simpler way is simply > > > > to rm the device node in the host and mknod it in the guest. I don't > > > > really see host visibility as a huge problem: in a shared OS > > > > virtualisation it's not really possible securely to separate the guest > > > > from the host (only vice versa). > > > > > > > > But I really don't think we want to do it this way. Giving a container > > > > the ability to do a mount is too dangerous. What we want to do is > > > > intercept the mount in the host and perform it on behalf of the guest as > > > > host root in the guest's mount namespace. If you do it that way, it > > > > > > That doesn't help the problem of guests being able to provide bad input > > > for (basically fuzz) the in-kernel filesystem code. So apparently I'm > > > suffering a failure of the imagination - what problem exactly does it > > > solve? > > > > Well, there's two types of fuzzing, one is on sys_mount, which this > > would help with because the host filters the mount including all > > parameters and may even redo the mount (from direct to bind etc). > > Sorry - I'm not *trying* to be dense, but am still not seeing it. > > Let's assume that we continue to be strict about what a container may > mount - let's say they can only mount using loopdev from blockdev images. > They have to own the file, as well as the mount target. Whatever they > do with sys_mount, the only danger I see is the one where the filesystem > data is bad and causes a DOS or privilege escalation in some bad fs > reading code in the kernel. > > What else is there? Are you thinking of the sys_mount flags? I guess > the void *data? (Though I see that as the same problem; we're just > not trusting the fs code to deal with badly formed data)
OK, so the problem you're worrying about is allowing the user to modify a block device and then mount it? In that case, I agree, it doesn't matter who does the mount, because a hostile user is looking to exploit bad data on the device. By and large, filesystems are tolerant to this type of fuzzing, but the strict solution is not to allow a container to mount any block devices it has direct access to. James -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
