Re: input/eventX permissions, force feedback

From: Dmitry Torokhov
Date: Tue Jul 18 2006 - 10:12:54 EST

On 7/18/06, Anssi Hannula <anssi.hannula@xxxxxxxxx> wrote:
Dmitry Torokhov wrote:
> Hi Anssi,
> On 7/18/06, Anssi Hannula <anssi.hannula@xxxxxxxxx> wrote:
>> Currently most distributions have /dev/input/event* strictly as 0600
>> root:root or 0640 root:root. The user logged in will not have rights to
>> the device, unlike /dev/input/js*, as he could read all passwords from
>> the keyboard device.
>> This is a problem, because /dev/input/event* is used for force feedback
>> and should therefore be user-accessible.
>> I can think of the following solutions to this problem:
>> 1. Some creative udev rule to chmod /dev/input/event* less strictly when
>> it has a /dev/input/js* and is thus a gaming device.
>> 2. Some creative udev rule to chmod /dev/input/event* more strictly when
>> it is a keyboard.
>> 3. Have another force feedback interface also in /dev/input/js*.
> You can do it in udev looking either at MODALIAS or at EV and ABS
> environment variables. I think it is pretty safe to say that a device
> with EV_ABS, EV_FF, ABS_X and ABS_Y is a force-feedback joystick-type
> device and not a keyboard.

Okay, thanks. But I think it'd be more consistant if all devices that
have js* entries would have the relaxed perms in event*. Looking at
joydev.c, that seems to be devices where EV_ABS && (ABS_X || ABS_WHEEL

OK, you can do that too.

There's another problem, too:
Some distros (Fedora, Mandriva...) don't use groups with /dev/input/jsX,
they use pam_console to chmod the device to the console owner.
Unfortunately, it allows to specify the permissions based on device file
names only.

To solve this problem, I see two solutions:

1. Have the pam_console_apply program extended so that it can perform
more complex matches (but what kind of matches would those be?).

2. Have udev create symlinks like the following case:
/dev/input/jsevent0 => event3
Then pam_console_apply could match jsevent[0-9]* and it would follow the
symlink, thus chowning event3 to the wanted user.

Unfortunately neither look too good to me. Do you have any other ideas?

I think this is really up to particular destribution to decide how
they want to handle security/granting access. One could even imagine
writing SELinux policies...

> Another solution would be to relax permissions if user is also console
> owner (home box installation).

I thought of that too, but I thought it's too big a security risk, as
it's not guaranteed that somebody else won't temporarily login on
another terminal.

That is what you are doing with pam_console_apply, don't you?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at