Messages in this thread | | | Date | Sat, 11 Feb 2012 20:28:03 +0000 | From | "Serge E. Hallyn" <> | Subject | Re: prevent containers from turning host filesystem readonly |
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Quoting Eric W. Biederman (ebiederm@xmission.com): > Serge Hallyn <serge.hallyn@canonical.com> writes: > > > Quoting Al Viro (viro@ZenIV.linux.org.uk): > >> On Fri, Feb 10, 2012 at 09:19:39PM -0600, Serge Hallyn wrote: > >> > When a container shuts down, it likes to do 'mount -o remount,ro /'. > >> > That sets the superblock's readonly flag, not the mount's. So unless > >> > the mount action fails for some reason (i.e. a file is held open on > >> > the fs), if the container's rootfs is just a directory on the host's > >> > fs, the host fs will be marked readonly. > >> > > >> > Thanks to Dave Hansen for pointing out how simple the fix can be. If > >> > the devices cgroup denies the mounting task write access to the > >> > underlying superblock (as it usually does when the container's root fs > >> > is on a block device shared with the host), then it do_remount_sb should > >> > deny the right to change mount flags as well. > >> > > >> > This patch adds that check. > >> > > >> > Note that another possibility would be to have the LSM step in. We > >> > can't catch this (as is) at the LSM level because security_remount_sb > >> > doesn't get the mount flags, so we can't distinguish > >> > mount -o remount,ro > >> > from > >> > mount --bind -o remount,ro. > >> > Sending the flags to that hook would probably be a good idea in addition > >> > to this patch, but I haven't done it here. > >> > >> NAK. This is just plain wrong - what about the filesystems that are not > > > > BTW, sorry - the patch clearly should've taken non-bdevs into account, but > > I accept that wouldn't have been enough to evade a NAK. > > > >> bdev-backed or, as e.g. btrfs, sit on more than one device? > > > > btrfs is actually one of my main motivators - to quickly snapshot containers > > with btrfs means that the containers all share one fs, but that means one > > container can mark them all ro. > > Serge let me respectfully suggest that getting the user namespace done > will deal with this issue nicely. > > In the simple case you simply won't be root so remount will just be > denied. > > When/if we allow a limited form of unprivileged mounts in a user > namespace your user won't have mounted the filesystem so you should not > have the privilege to call remount on the filesystem.
Hm, that's a good point. Though note it'll require the userns code to distinguish between the a bind remount and superblock remount. The last time we seriously discussed this, that wasn't even on the roadmap. It was only going to support fully assigning the whole filesystem to a user namespace. In that case, the remount issue doesn't apply anyway as the fs isn't shared with another container.
In any case, there are other workarounds, so I wasn't in a hurry to address this - it just should be addressed eventually. I just figured that to bring up the issue I needed a patch :)
> I think I will have a set of patches ready for serious scrutiny in > the next week or so. So we aren't talking impossible pie in the sky > distance to see this happen.
Awesome.
thanks, -serge
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