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

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Date:	Tue, 17 Nov 2015 15:32:55 -0600
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	"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 03:54:50PM -0500, Austin S Hemmelgarn wrote:
> On 2015-11-17 14:16, Seth Forshee wrote:
> >On Tue, Nov 17, 2015 at 02:02:09PM -0500, Austin S Hemmelgarn wrote:
> >>On 2015-11-17 12:55, 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?
> >>To properly protect against attacks on mounted filesystems, we'd
> >>need some new concept of a userspace immutable file (that is, one
> >>where nobody can write to it except the kernel, and only the kernel
> >>can change it between regular access and this new state), and then
> >>have the kernel set an image (or block device) to this state when a
> >>filesystem is mounted from it (this introduces all kinds of other
> >>issues too however, for example stuff that allows an online fsck on
> >>the device will stop working, as will many un-deletion tools).
> >
> >Yeah, Serge and I were just tossing that idea around on irc. If we can
> >make that work then it's probably the best solution.
> >
> > From a naive perspective it seems like all we really have to do is make
> >the block device inode immutable to userspace when it is mounted. And
> >the parent block device if it's a partition, which might be a bit
> >troublesome. We'd have to ensure that writes couldn't happen via any fds
> >already open when the device was mounted too.
> >
> >We'd need some cooperation from the loop driver too I suppose, to make
> >sure the file backing the loop device is also immutable.
> >
> From a completeness perspective, you'd also need to hook into DM,
> MD, and bcache to handle their backing devices.  There's not much we
> could do about iSCSI/ATAoE/NBD devices, and I think being able to

But really no one would be able to mount any of those without
intervention from a privileged user anyway. The same is true today of
loop devices, but I have some patches to change that.

> restrict stuff calling mount from a userns to only be able to use
> FUSE would still be useful (FWIW, GRUB2 has a tool to use FUSE for
> testing it's own filesystem drivers, which I use regularly when I
> just need a read-only mount).

Agreed, fuse alone is very useful, though there is a performance
penalty.
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