Re: [syzbot] KASAN: use-after-free Read in __usb_hcd_giveback_urb (2)

[Oliver Neukum]

On 08.12.22 18:40, Alan Stern wrote:
> On Thu, Dec 08, 2022 at 03:36:45PM +0100, Oliver Neukum wrote:
> > On 06.12.22 16:38, Alan Stern wrote:

> It's hard to tell what's really going on.  Looking at
> xpad_stop_output(), you see that it doesn't do anything if xpad->type is
> XTYPE_UNKNOWN.  Is that what happened here?

The output anchor in xpad was used. So I have to answer that in the negative.
 
> I can't figure out where the underlying race is.  Maybe it's not
> directly connected with anchors after all.
>
> > As far as I can tell the order we decrease use_count is correct. But:
> >
> > 6ec4147e7bdbd (Hans de Goede             2013-10-09 17:01:41 +0200 1674)        usb_anchor_resume_wakeups(anchor);
> > 94dfd7edfd5c9 (Ming Lei                  2013-07-03 22:53:07 +0800 1675)        atomic_dec(&urb->use_count);
> >
> > Do we need to guarantee memory ordering here?
>
> I don't think we need to do anything more.  usb_kill_urb() is careful to
> wait for completion handlers to finish, and we already have

By checking use_count

> smp_mb__after_atomic() barriers in the appropriate places to ensure
> proper memory ordering.

Do we? Looking at __usb_hcd_giveback_urb():

        usb_unanchor_urb(urb);

This is an implicit memory barrier

        if (likely(status == 0))
                usb_led_activity(USB_LED_EVENT_HOST);

        /* pass ownership to the completion handler */
        urb->status = status;
        /*
         * This function can be called in task context inside another remote
         * coverage collection section, but kcov doesn't support that kind of
         * recursion yet. Only collect coverage in softirq context for now.
         */
        kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
        urb->complete(urb);
        kcov_remote_stop_softirq();

        usb_anchor_resume_wakeups(anchor);
        atomic_dec(&urb->use_count);
        /*
         * Order the write of urb->use_count above before the read
         * of urb->reject below.  Pairs with the memory barriers in
         * usb_kill_urb() and usb_poison_urb().
         */
        smp_mb__after_atomic();

That is the latest time use_count can go to zero.
But what is the earliest time the CPU could reorder setting use_count to zero?
Try as I might the last certain memory barrier I can find in this function
is usb_unanchor_urb().
That means another CPU can complete usb_kill_urb() before usb_anchor_resume_wakeups()
runs.

        usb_anchor_resume_wakeups(anchor);

I think we need a memory barrier here, too.

        atomic_dec(&urb->use_count);

	Regards
		Oliver