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

From: Seth Forshee
Date: Wed Nov 18 2015 - 09:31:24 EST

On Wed, Nov 18, 2015 at 07:46:53AM -0500, Austin S Hemmelgarn wrote:
> On 2015-11-17 17:01, Seth Forshee wrote:
> >On Tue, Nov 17, 2015 at 09:05:42PM +0000, Al Viro wrote:
> >>On Tue, Nov 17, 2015 at 03:39:16PM -0500, Austin S Hemmelgarn wrote:
> >>
> >>>>This is absolutely insane, no matter how much LSM snake oil you slatter on
> >>>>the whole thing. All of a sudden you are exposing a huge attack surface
> >>>>in the place where it would hurt most and as the consolation we are offered
> >>>>basically "Ted is willing to fix holes when they are found".
> >
> >None of the LSM changes are intended to protect against attacks from
> >these sorts of attacks at all, so that's irrelevant.
> >
> >As I said before, I'm also working to find holes up front. That plus a
> >commitment from the maintainer seems like a good start at least. What
> >bar would you set for a given filesystem to be considered "safe enough"?
> >
> >>>For the context of static image attacks, anything that's foun
> >>>_needs_ to be fixed regardless, and unless you can find some way to
> >>>actually prevent attacks on mounted filesystems that doesn't involve
> >>>a complete re-write of the filesystem drivers, then there's not much
> >>>we can do about it. Yes, unprivileged mounts expose an attack
> >>>surface, but so does userspace access to the network stack, and so
> >>>do a lot of other features that are considered essential in a modern
> >>>general purpose operating system.
> >>
> >>"X is exposes an attack surface. Y exposes a diferent attack surface.
> >>Y is considered important. Therefore X is important enough to implement it"
> >>
> >>Right...
> >
> >That isn't the argument he made. I would summarize the argument as,
> >"Saying that X exposes an attack surface isn't by itself enough to
> >reject X, otherwise we wouldn't expose anything (such as example Y)."
> It's good to see someone understood my meaning...
> >
> >You believe that the attack surface is too large, and that's
> >understandable. Is it your opinion that this is a fundamental problem
> >for an in-kernel filesystem driver, i.e. that we can never be confident
> >enough in an in-kernel filesystem parser to allow untrusted data? If
> >not, what would it take to establish a level of confidence that you
> >would be comfortable with?
> While I can't speak for Al's opinion on this, I would like to point
> out my earlier comment:
> > It's unfeasible from a practical standpoint to expect filesystems
> to > assume that stuff they write might change under them due to
> malicious > intent of a third party.

So maybe the first requirement is that the user cannot modify the
backing store directly while the device is mounted.

> We can't protect against everything, not without making the system
> completely unusable for general purpose computing. There is always
> some degree of trust involved in usage of a computer, the OS has to
> trust that the hardware works correctly, the administrator has to
> trust the OS to behave correctly, and the users have to trust the
> administrator. The administrator also needs to have at least some
> trust in the users, otherwise he shouldn't be allowing them to use
> the system.
> Perhaps we should have an option that can only be enabled on
> creation of the userns that would allow it to use regular kernel
> mounts, and without that option we default to only allowing FUSE and
> a couple of virtual filesystems (like /proc and devtmpfs).

I've considered the idea of something more global like a sysctl, or a
per-filesystem knob in sysfs. I guess a per-container knob is another
option, I'm not sure what interface we use to expose it though.
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