Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761190Ab2BKEJs (ORCPT ); Fri, 10 Feb 2012 23:09:48 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:60288 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760799Ab2BKEH2 (ORCPT ); Fri, 10 Feb 2012 23:07:28 -0500 Date: Fri, 10 Feb 2012 22:07:22 -0600 From: Serge Hallyn To: Al Viro Cc: lkml , Andy Whitcroft , Andrew Morton , "Eric W. Biederman" , Dave Hansen , linux-security-module@vger.kernel.org, Linux Containers , St?phane Graber , Daniel Lezcano Subject: Re: prevent containers from turning host filesystem readonly Message-ID: <20120211040722.GA5891@sergelap> References: <20120211031939.GA4772@sergelap> <20120211033732.GK23916@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120211033732.GK23916@ZenIV.linux.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2021 Lines: 44 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/