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Date:   Mon, 12 Dec 2022 10:52:20 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Oliver Neukum <oneukum@suse.com>
Cc:     syzbot <syzbot+712fd0e60dda3ba34642@syzkaller.appspotmail.com>,
        WeitaoWang-oc@zhaoxin.com, arnd@arndb.de,
        gregkh@linuxfoundation.org, khalid.masum.92@gmail.com,
        kishon@ti.com, linux-kernel@vger.kernel.org,
        linux-usb@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] KASAN: use-after-free Read in __usb_hcd_giveback_urb (2)
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On Mon, Dec 12, 2022 at 01:29:24PM +0100, Oliver Neukum wrote:
> 
> 
> 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);

Okay, how about if this is changed to atomic_dec_return?  That puts a 
full memory barrier both before and after the atomic decrement, so as a 
bonus we could remove the smp_mb__after_atomic call.

Alan Stern