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

From: Austin S Hemmelgarn
Date: Wed Nov 18 2015 - 07:47:23 EST

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.
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).

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