On Fri, Nov 21, 2014 at 8:44 AM, Seth Forshee <seth.forshee@xxxxxxxxxxxxx> wrote: > On Wed, Nov 19, 2014 at 03:09:11PM +0100, Serge E. Hallyn wrote: >> Quoting Miklos Szeredi (miklos@xxxxxxxxxx): >> > On Wed, Nov 19, 2014 at 9:50 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: >> > > On Tue, Nov 18, 2014 at 4:21 PM, Seth Forshee >> > > <seth.forshee@xxxxxxxxxxxxx> wrote: >> > >>> I asked around a bit, and it turns out there are use cases for nested >> > >> containers (i.e. a container within a container) where the rootfs for >> > >> the outer container mounts a filesystem containing the rootfs for the >> > >> inner container. If that mount is nosuid then suid utilities like ping >> > >> aren't going to work in the inner container. >> > >> >> > >> So since there's a use case for suid in a userns mount and we have what >> > >> we belive are sufficient protections against using this as a vector to >> > >> get privileges outside the container, I'm planning to move ahead without >> > >> the MNT_NOSUID restriction. Any objections? >> > > >> > > In the general case how'd we prevent suid executable being tricked to >> > > do something it shouldn't do by unprivileged mounting into sensitive >> > > places (i.e. config files) inside the container? >> >> The design of the namespaces would prevent that. You cannot manipulate your >> mounts namespace unless you own it. You cannot manipulate the mounts namespace >> for a task whose user namespace you do not own. If you can, for instance, >> bind mount $HOME/shadow onto /etc/shadow, then you already own your user >> namespace and are root there, so any suid-root program which you mount through >> fuse will only subjegate your own namespace. Any task which running in the >> parent user-ns (and therefore parent mount-ns) will not see your bind mount. >> >> > > Allowing SUID looks like a slippery slope to me. And there are plenty >> > > of solutions to the "ping" problem, AFAICS, that don't involve the >> > > suid bit. >> > >> > ping isn't even suid on my system, it has security.capability xattr instead. >> >> security.capability xattrs that will have the exact same concerns wrt >> confusion through bind mounts as suid. >> >> > Please just get rid of SUID/SGID. It's a legacy, it's a hack, not >> > worth the complexity and potential problems arising from that >> > complexity. >> >> Oh boy, I don't know which side to sit on here :) I'm all for replacing >> suid with some use of file capabilities, but realistically there are reasons >> why that hasn't happened more widely than it has - tar, package managers, >> cpio, nfs, etc. > > Miklos: I we're all generally in agreement here that suid/sgid is not > the best solution, but as Serge points out we are unfortunately not yet > in a place where it can be completely dropped in favor of capabilities. > In light of this can I convince you to reconsider your position? > I would go one step further: all the things that gain privilege on exec (suig/sgid, fscaps, and LSM transitions) are not just "not the best" but are in fact disasters. They made sense when systems had a few KB of RAM. suid/sgid is at least a /standardized/ disaster, though, and namespaced code should be able to use it. Miklos, I'm not sure whether you saw it (it was a bit buried, I think), but this series is intended to depend on a patch of mine that makes all mounts that belong to foreign namespaces act as though they're MNT_NOSUID. That means that, in order for suid/sgid to do anything, the namespace owner needs to indicate their trust in the fs by explicitly mounting it. --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
