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

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Message-ID: <20151117172551.GA108807@ubuntu-hedt>
Date:	Tue, 17 Nov 2015 11:25:51 -0600
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-bcache@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v3 0/7] User namespace mount updates

On Tue, Nov 17, 2015 at 05:05:56PM +0000, Al Viro wrote:
> On Tue, Nov 17, 2015 at 10:39:03AM -0600, Seth Forshee wrote:
> > Hi Eric,
> > 
> > Here's another update to my patches for user namespace mounts, based on
> > your for-testing branch. These patches add safeguards necessary to allow
> > unprivileged mounts and update SELinux and Smack to safely handle
> > device-backed mounts from unprivileged users.
> > 
> > The v2 posting received very little in the way of feedback, so changes
> > are minimal. I've made a trivial style change to the Smack changes at
> > Casey's request, and I've added Stephen's ack for the SELinux changes.
> 
> Would you mind explaining which filesystem types do you plan to allow?
> SELinux and the rest of Linux S&M bunch do fuck-all for attacks via
> handcrafted fs image fed to the code in fs driver that does not expect
> a given kind of inconsistencies.
> 
> As it is, validation of on-disk metadata is not particularly strong;
> what's more, protection against concurrent malicious *changes* of
> fs image (via direct writes by root) is simply inexistent.
> 
> So what is that about?

The first target is fuse, which won't be vulnerable to those attacks.

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.

Seth
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