Securing non-root X input

[Dave Airlie]

On Sun, Jan 31, 2010 at 5:13 PM, Dmitry Torokhov
<dmitry.torokhov at gmail.com> wrote:
> On Sat, Jan 30, 2010 at 06:35:47PM -0700, Matthew Wilcox wrote:
>> On Fri, Jan 29, 2010 at 11:45:46PM -0800, Dmitry Torokhov wrote:
>> > Hi Matthew,
>> >
>> > On Fri, Jan 29, 2010 at 04:24:38PM -0700, Matthew Wilcox wrote:
>> > > This tiny patch allows the X server to ask how many times the device has
>> > > been opened.  If it's more than one, the X server can ask the user what
>> > > they want to do about it.  For bonus points, the X server can also run
>> > > programs like lsof or fuser to find out which other processes have the
>> > > device open, and tell the user that information too.  At that point,
>> > > the sysadmin can call in the ICBM strike on the offending user.
>> > >
>> > > Does this approach work for everyone?
>> >
>> > I do not think so. What about the cases when event devices are
>> > legitimately opened by several processes, like this:
>> >
>> > [dtor at dtor-d630 work]$ ps aux | grep hald-addon-input
>> > root      1132  0.0  0.0  22200   824 ?        S    Jan22   0:29
>> > hald-addon-input: Listening on /dev/input/event7 /dev/input/event2 /dev/input/event1 /dev/input/event6 /dev/input/event0 /dev/input/event12 /dev/input/event4
>> > dtor     30424  0.0  0.0 102736   808 pts/3    S+   23:23   0:00 grep hald-addon-input
>> > [dtor at dtor-d630 work]$
>> >
>> > It might not be hald but some other daemon monitoring key presses
>> > (sleep, hibernate, wifi keys and switches, etc).
>> >
>> > If it was just about ensuring that only oneprocess accesses the device
>> > then we could just use EVIOCGRAB but as experience shows it is not a
>> > workable solution.
>>
>> Yes, that's right.  I didn't quite go far enough in my explanation
>> above ...  the X server can look around the system to see what trusted
>> daemons (running as either root or the same user as the one running X)
>> currently have the device open, and notify the user if there's additional
>> openers that it isn't expecting.
>>
>
> Then it will be constant race between X and the rest of the world with X
> pretty much always behind. Kind of like SELinux - as soon as try moving
> left or right the thing starts screaming at you...
>
>> Maybe we don't need a kernel patch to make this work after all, just
>> a suid helper for X that uses the code from lsof/fuser to list all the
>> current openers of /dev/input/eventN.
>>
>
> But what about the case where malicious user opens the devices after the
> X done its scan?

That can't happen since we remove privs from the previous users of the
node before starting the new X server via ConsoleKit or at least thats the plan,

The problem is only a user holding open the evdev device after they've lost
perms on the device.

Dave.

> mknod is a privileged operation, requiring CAP_MKNOD. Otherwise evcen
> current setup would be completely insecure if any user could just mknod
> in his home directory and snoop root's keypresses at console.

Its more the other devices the kernel might make, or udev. Not sure if
that ever happens though.

Dave.