On Fri, Nov 21, 2014 at 12:14:19PM -0600, Eric W. Biederman wrote: > Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes: > > > 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? > > Regardless of what fuse does user namespaces must support mounting > filesystems that have the setuid and setgid bits set. Likewise we need > to handle capabilities. > > There is a parallel bit of work to the fuse patches that I think at this > point should be completed first. > > - Add s_user_ns to struct super. So we can have filesystems whose > labels are not interpreted at a global scope. > > - Tweak the file capability code to look at s_user_ns and treat it > properly. > > - Tweak the lsms to look at s_user_ns and ignore security labels that > don't come from init_user_ns. (The lsms at their discrection can > be more trusting but the default should be for them to ignore those > labels). > > - Tweak the security checks to allow setting file capabilities and > other security xattrs if we have the appropriate capabilities in > s_user_ns. > > - Update tmpfs and ramfs to set s_user_ns when being mounted. Okay. Is someone already working on these items? If not I'll work up some patches. > When those bits are done we can tweak the fuse patches to also set > s_user_ns. > > As for MNT_NO_SUID if fuse wants to enforce that in some way. I don't > particularly care, but I don't think that makes sense as a vfs property. I don't think fuse can enforce this since the flag applies to the mount and not the super block. It would have to be enforced by the vfs. Thanks, Seth -- 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
