Re: [PATCH v2] usb: devio: Add ioctl to disallow detaching kernel USB drivers.

[Emilio LÃpez]

Hi Alan,

El 22/01/16 a las 13:10, Alan Stern escribiÃ:
> On Thu, 21 Jan 2016, Emilio LÃpez wrote:
>
> > From: Reilly Grant <reillyg@xxxxxxxxxxxx>
> >
> > The new USBDEVFS_DROP_PRIVILEGES ioctl allows a process to voluntarily
> > relinquish the ability to issue other ioctls that may interfere with
> > other processes and drivers that have claimed an interface on the
> > device.
> >
> > Signed-off-by: Reilly Grant <reillyg@xxxxxxxxxxxx>
> > Signed-off-by: Emilio LÃpez <emilio.lopez@xxxxxxxxxxxxxxx>
>
>
> >   static int proc_resetdevice(struct usb_dev_state *ps)
> >   {
> > +	struct usb_host_config *actconfig = ps->dev->actconfig;
> > +	struct usb_interface *interface;
> > +	int i, number;
> > +
> > +	/* Don't touch the device if any interfaces are claimed. It
> > +	 * could interfere with other drivers' operations and this
> > +	 * process has dropped its privileges to do such things.
> > +	 */
>
> This comment should be rephrased.  It should say something like:
> "Don't allow if the process has dropped its privilege to do such
> things and any of the interfaces are claimed."

I have replaced it with the following now

        /* Don't allow a device reset if the process has dropped the
         * privilege to do such things and any of the interfaces are
         * currently claimed.
         */

> You also might consider allowing the reset if the interfaces are
> claimed only by the current process (or more precisely, by ps).
>
> > +static int proc_drop_privileges(struct usb_dev_state *ps, void __user *arg)
> > +{
> > +	struct usbdevfs_drop_privs data;
> > +
> > +	if (copy_from_user(&data, arg, sizeof(data)))
> > +		return -EFAULT;
> > +
> > +	/* This is a one way operation. Once privileges were dropped,
> > +	 * you cannot do it again (Otherwise unprivileged processes
> > +	 * would be able to change their allowed interfaces mask)
> > +	 */
>
> If you're going to keep a mask of claimable interfaces then there's no
> reason this has to be a one-time operation.  Processes should always be
> allowed to shrink the mask, just not to grow it.

Good point, I've changed this to look like the following

        /* This is an one way operation. Once privileges are
         * dropped, you cannot regain them. You may however reissue
         * this ioctl to shrink the allowed interfaces mask.
         */
        if (ps->privileges_dropped)
                ps->interface_allowed_mask &= data.interface_allowed_mask;
        else
                ps->interface_allowed_mask = data.interface_allowed_mask;

        ps->privileges_dropped = true;

Or maybe I could change the default mask to ~0 and simplify this a bit, hm.

Thank you for the review!

Emilio