On Thu, Jul 25, 2019 at 5:09 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 25 Jul 2019, Oliver Neukum wrote:
>
> > Am Mittwoch, den 24.07.2019, 17:02 -0400 schrieb Alan Stern:
> > > On Wed, 24 Jul 2019, Oliver Neukum wrote:
> > >
> > > > drivers/hid/usbhid/hid-core.c | 13 +++++++++++++
> > > > 1 file changed, 13 insertions(+)
> > > >
> > > > diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
> > > > index c7bc9db5b192..98b996ecf4d3 100644
> > > > --- a/drivers/hid/usbhid/hid-core.c
> > > > +++ b/drivers/hid/usbhid/hid-core.c
> > > > @@ -1229,6 +1229,17 @@ static int usbhid_power(struct hid_device *hid, int lvl)
> > > > struct usbhid_device *usbhid = hid->driver_data;
> > > > int r = 0;
> > > >
> > > > + spin_lock_irq(&usbhid->lock);
> > > > + if (test_bit(HID_DISCONNECTED, &usbhid->iofl)) {
> > > > + r = -ENODEV;
> > > > + spin_unlock_irq(&usbhid->lock);
> > > > + goto bail_out;
> > > > + } else {
> > > > + /* protect against disconnect */
> > > > + usb_get_dev(interface_to_usbdev(usbhid->intf));
> > > > + spin_unlock_irq(&usbhid->lock);
> > > > + }
> > > > +
> > > > switch (lvl) {
> > > > case PM_HINT_FULLON:
> > > > r = usb_autopm_get_interface(usbhid->intf);
> > > > @@ -1238,7 +1249,9 @@ static int usbhid_power(struct hid_device *hid, int lvl)
> > > > usb_autopm_put_interface(usbhid->intf);
> > > > break;
> > > > }
> > > > + usb_put_dev(interface_to_usbdev(usbhid->intf));
> > > >
> > > > +bail_out:
> > > > return r;
> > > > }
> > >
> > > Isn't this treating the symptom instead of the cause?
> >
> > Sort of. Holding a reference for the whole time would have merit,
> > but I doubt it is strictly necessary.
>
> Just to be crystal clear, I was talking about a device reference --
> usb_{get,put}_dev or usb_{get,put}_intf -- not a runtime PM reference.
>
> (Incidentally, your patch could be simplified by using usb_get_intf
> instead of usb_get_dev.)
>
> > > Shouldn't the hid_device hold a reference to usbhid->intf throughout
> > > its lifetime? That way this sort of problem wouldn't arise in any
> > > routine, not just usbhid_power().
> >
> > Unfortunately the semantics would still be wrong without the check
> > in corner cases. In case disconnect() is called without a physical
> > unplug, we must not touch the power state.
> > I am indeed afraid that in that case my putative fix is still racy.
> > But I don't to just introduce a mutex just for this. Any ideas?
>
> That's a separate issue. USB drivers -- indeed, all drivers -- are
> required to balance their runtime PM gets and puts (although in the
> case of a physical disconnection it doesn't matter). Are you asking
> about the best way to do this?
>
> Normally a driver's release or disconnect routine will stop all
> asynchronous accesses to the device (interrupt handlers, work queues,
> URBs, and so on). At that point the only remaining runtime PM activity
> will be whatever the routine itself does. So it can see if any extra
> runtime PM gets or puts are needed, and do whatever is necessary.
>
> Does that answer your question? I can't tell for sure...
>
> Note: I did not try to track down the reason for the invalid access
> reported by syzbot. It looked like a simple use-after-free, which
> would normally be fixed by taking the appropriate reference. Which is
> what your patch does, except that it holds the reference only for a
> short time instead of over the entire lifetime of the private data
> structure (the usbhid structure), which is what normally happens.
This report looks like very similar to these two:
https://syzkaller.appspot.com/bug?extid=b156665cf4d1b5e00c76
https://syzkaller.appspot.com/bug?extid=3cbe5cd105d2ad56a1df
Maybe we should mark those two as duplicates.
Hillf suggested a fix on one of them, but it looks different from what
you propose:
https://groups.google.com/d/msg/syzkaller-bugs/xW7LvKfpyn0/SpKbs5ZLEAAJ
