Re: Suspending access to opened/active /dev/nodes during application runtime

Hi

On Fri, Mar 7, 2014 at 7:45 PM, Lukasz Pawelczyk [off-list ref] wrote:

Problem:
Has anyone thought about a mechanism to limit/remove an access to a
device during an application runtime? Meaning we have an application
that has an open file descriptor to some /dev/node and depending on
*something* it gains or looses the access to it gracefully (with or
without a notification, but without any fatal consequences).

Example:
LXC. Imagine we have 2 separate containers. Both running full operating
systems. Specifically with 2 X servers. Both running concurrently of
course. Both need the same input devices (e.g. we have just one mouse).
This creates a security problem when we want to have completely separate
environments. One container is active (being displayed on a monitor and
controlled with a mouse) while the other container runs evtest
/dev/input/something and grabs the secret password user typed in the
other.

Solutions:
The complete solution would comprise of 2 parts:
- a mechanism that would allow to temporally "hide" a device from an
open file descriptor.
- a mechanism for deciding whether application/process/namespace should
have an access to a specific device at a specific moment

Let's focus on the first problem only, as it would need to be solved
first anyway.  I haven't found anything that would allow me to do
it. There are a lot mechanisms that make it possible to restrict an
access during open():
- DAC
- ACL (controlled by hand or with uaccess)
- LSM (in general)
- device cgroups
But all of those can't do a thing when the device is already opened and
an application has a file descriptor.  I don't see such mechanism in
kernel sources either.

I do imagine that it would not be possible for every device to handle
such a thing (dri comes to mind) without breaking something (graphics
card state in dri example). But there is class of simple input/output
devices that would handle this without problems.

I did implement some proof-of-concept solution for an evdev driver by
allowing or disallowing events that go to evdev_client structure using
some arbitrary condition. But this is far from a generic solution.

My proof-of-concept is somewhat similar to this (I just found it):
http://www.spinics.net/lists/linux-input/msg25547.html
Though a little bit wider in scope. But neither is flawless nor
generic.

Has anyone had any thoughts about a similar problem?

Lennart and Greg have already answered most of this, few notes from me:
 * EVIOCREVOKE and DRM_SET_MASTER/DROP_MASTER are real. We use them.
They solve your problem for gfx and input devices.
 * EVIOCMUTE is *bad*. It is a privileged ioctl compared to
EVIOCREVOKE, so we've never merged it. It neither has major advantages
over revoke. So use EVIOCREVOKE.
 * A generic frevoke() syscall would solve all is, but is unlikely to
ever appear upstream.

Cheers
David