Messages in this thread | | | Date | Fri, 10 Feb 2012 22:07:22 -0600 | From | Serge Hallyn <> | Subject | Re: prevent containers from turning host filesystem readonly |
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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.
> <comments about inadequacy of cgroup as an API censored - far too unprintable>
-serge
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