linux-kernel - Re: prevent containers from turning host filesystem readonly

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Message-ID: <20120211035705.GA5563@sergelap>
Date:	Fri, 10 Feb 2012 21:57:06 -0600
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andy Whitcroft <apw@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dave Hansen <haveblue@...ibm.com>,
	linux-security-module@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>,
	St?phane Graber <stgraber@...ntu.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>
Subject: Re: prevent containers from turning host filesystem readonly

Quoting Al Viro (viro@...IV.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
> 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>

Drat.

Would passing the mount flags from do_remount() to security_sb_remount()
be acceptable?  Then at least the LSM could distinguish and act
accordingly.

Thanks for looking.

-serge
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