Re: [PATCH v3 0/7] User namespace mount updates

From: Seth Forshee <seth.forshee@canonical.com>
To: Theodore Ts'o <tytso@mit.edu>, Al Viro <viro@ZenIV.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-bcache@vger.kernel.org, dm-devel@redhat.com,
	linux-raid@vger.kernel.org, linux-mtd@lists.infradead.org,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, selinux@tycho.nsa.gov,
	Serge Hallyn <serge.hallyn@canonical.com>,
	Andy Lutomirski <luto@amacapital.net>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/7] User namespace mount updates
Date: Wed, 18 Nov 2015 13:28:55 -0600	[thread overview]
Message-ID: <20151118192855.GH134139@ubuntu-hedt> (raw)
In-Reply-To: <20151118191045.GB3434@thunk.org>

On Wed, Nov 18, 2015 at 02:10:45PM -0500, Theodore Ts'o wrote:
> On Tue, Nov 17, 2015 at 12:34:44PM -0600, Seth Forshee wrote:
> > On Tue, Nov 17, 2015 at 05:55:06PM +0000, Al Viro wrote:
> > > On Tue, Nov 17, 2015 at 11:25:51AM -0600, Seth Forshee wrote:
> > > 
> > > > Shortly after that I plan to follow with support for ext4. I've been
> > > > fuzzing ext4 for a while now and it has held up well, and I'm currently
> > > > working on hand-crafted attacks. Ted has commented privately (to others,
> > > > not to me personally) that he will fix bugs for such attacks, though I
> > > > haven't seen any public comments to that effect.
> > > 
> > > _Static_ attacks, or change-image-under-mounted-fs attacks?
> > 
> > Right now only static attacks, change-image-under-mounted-fs attacks
> > will be next.
> 
> I will fix bugs about static attacks.  That is, it's interesting to me
> that a buggy file system (no matter how it is created), not cause the
> kernel to crash --- and privilege escalation attacks tend to be
> strongly related to those bugs where we're not doing strong enough
> checking.
> 
> Protecting against a malicious user which changes the image under the
> file system is a whole other kettle of fish.  I am not at all user you
> can do this without completely sacrificing performance or making the
> code impossible to maintain.  So my comments do *not* extend to
> protecting against a malicious user who is changing the block device
> underneath the kernel.
> 
> If you want to submit patches to make the kernel more robust against
> these attacks, I'm certainly willing to look at the patches.  But I'm
> certainly not guaranteeing that they will go in, and I'm certainly not
> promising to fix all vulnerabilities that you might find that are
> caused by a malicious block device.  Sorry, that's too much buying a
> pig in a poke....

Thanks Ted. My plan right now is to explore the possibility of blocking
writes to the backing store from userspace while it's mounted.