Re: prevent containers from turning host filesystem readonly

[Al Viro]

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
bdev-backed or, as e.g. btrfs, sit on more than one device?

<comments about inadequacy of cgroup as an API censored - far too unprintable>
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