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

From: Seth Forshee
Date: Wed Nov 18 2015 - 09:23:36 EST

On Wed, Nov 18, 2015 at 07:23:48AM -0500, Austin S Hemmelgarn wrote:
> On 2015-11-17 16:32, Seth Forshee wrote:
> >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.
> Um, no, depending on how your system is configured, it's fully
> possible to mount those as a regular user with no administrative
> interaction required. All that needs done is some udev rules (or
> something equivalent) to add ACL's to the device nodes allowing
> regular users to access them (and there are systems I've seen that
> are naive enough to do this). And I can almost assure you that
> there will be someone out there who decides to directly expose such
> block devices to a user namespace.

But it still requires the admin set it up that way, no? And aren't
privileges required to set up those devices in the first place?

I'm not saying that it wouldn't be a good idea to lock down the backing
stores for those types of devices too, just that it isn't something that
a regular user could exploit without an admin doing something to
facilitate it.
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