[systemd-devel] Supporting U2F over HID on Linux?

[Benjamin Tissoires]

On Sun, Nov 2, 2014 at 5:49 PM, Andy Lutomirski <luto at amacapital.net> wrote:
> On Sun, Nov 2, 2014 at 2:45 PM, Benjamin Tissoires
> <benjamin.tissoires at gmail.com> wrote:
>> On Sun, Nov 2, 2014 at 4:40 PM, Jiri Kosina <jkosina at suse.cz> wrote:
>>> On Sun, 2 Nov 2014, Jiri Kosina wrote:
>>>
>>>> Alternatively, you can just write udev rule which triggers on HID devices,
>>>> issues HIDIOCGRDESC ioctl() on the just-created hidraw node, and decides
>>>> afterwards whether node permissions need to be altered ... right?
>>>
>>> Just to make myself clear here -- this is basically alternative to your
>>> 1st option, but for cases where you want to make yourself independent on
>>> sysfs existence (I don't what is the craziness level of setups you are
>>> considering).
>>>
>>
>> I am a little bit concerned with the user space retrieving the
>> descriptor, parsing it and then deciding how to set the permissions.
>> It's not that it is super complex, but it will duplicate the parsing
>> we already do in the kernel. Wacom tried to do that in their usb
>> driver, and they never managed to fully implement one.
>>
>> I think the HID_GROUP solution is super trivial:
>> - add a match on the U2F input report and set the group to
>> HID_GROUP_U2F (2 lines in hid_scan_input_usage() in hid-core.c)
>> - allow hid-generic to also match HID_GROUP_U2F (1 line in hid-generic.c).
>>
>> Then, the udev rule would be super trivial.
>
> I'm confused.  What would the user-visible effect of this be?  Would
> the hidraw node still show up?  What is user code supposed to match to
> detect a U2F device or to otherwise set permissions?
>

The only change with what we currently have is that the modalias of
the device will be something like
MODALIAS=hid:b0003gHID_GROUP_U2Fv0000XXXXp0000XXXX. (replace
HID_GROUP_U2F with the proper hex value). So matching against this
modalias is trivial, and you can just put the hidraw node rw for
users.

The device will still be handled by hid-generic, so there will be no
changes besides the modalias.

So yes, you still need a udev rule to set the proper permission, but
you will not have to parse the report descriptors.

Cheers,
Benjamin

PS: some time spent and I realized that the modalias would be the one
of the HID node, which the hidraw is a son. Not the hidraw node
directly :/

> --Andy
>
>>
>> Also, if we want to further extend the kernel API for U2F, the group
>> will already be in place.
>>
>> Cheers,
>> Benjamin
>
>
>
> --
> Andy Lutomirski
> AMA Capital Management, LLC