Re: prevent containers from turning host filesystem readonly

[Serge Hallyn <serge.hallyn@canonical.com>]

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