linux-kernel - Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL

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Date:   Thu, 15 Nov 2018 13:06:53 +0100
From:   David Herrmann <dh.herrmann@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Dmitry Torokhov <dtor@...gle.com>, ebiggers@...nel.org,
        jannh@...gle.com, Jiri Kosina <jikos@...nel.org>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot+72473edc9bf4eb1c6556@...kaller.appspotmail.com,
        stable <stable@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or
 elevated privileges

Hey

On Thu, Nov 15, 2018 at 9:14 AM Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
>
> On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <dtor@...gle.com> wrote:
> > > I think it's best not to make
> > > assumptions about how the interface will be used and to be consistent with how
> > > other ->write() methods in the kernel handle the misfeature where a __user
> > > pointer in the write() or read() payload is dereferenced.
> >
> > I can see that you might want to check credentials, etc, if interface
> > can be accessed by unprivileged process, however is it a big no no for
> > uhid/userio/uinput devices.
>
> Yep, any sane distribution would restrict the permissions of
> uhid/userio/uinput to only be accessed by root. If that ever changes,
> there is already an issue with the system and it was compromised
> either by a terribly dizzy sysadmin.

UHID is safe to be used by a non-root user. This does not imply that
you should open up access to the world, but you are free to have a
dedicated group or user with access to uhid. I agree that in most
common desktop-scenarios you should not grant world-access to it,
though.

> >
> > > Temporarily switching
> > > to USER_DS would only avoid one of the two problems.
> >
> > So because of the above there is only one problem. If your system
> > opened access to uhid to random processes you have much bigger
> > problems than exposing some data from a suid binary. You can spam "rm
> > -rf .; rm -rf /" though uhid if there is interactive session
> > somewhere.
> >
> > >
> > > Do you think the proposed restrictions would actually break anything?
> >
> > It would break if someone uses UHID_CREATE with sendpage. I do not
> > know if anyone does. If we were certain there are no users we'd simply
> > removed UHID_CREATE altogether.
>
> AFAICT, there are 2 users of uhid:
> - bluez for BLE devices (using HID over GATT)
> - hid-replay for debugging.

There are several more (eg., android bt-broadcom), and UHID_CREATE is
actively used. Dropping support for it will break these use-cases.

Thanks
David