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Date:   Wed, 6 May 2020 16:09:24 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Pete Zaitcev <zaitcev@...hat.com>
cc:     Oliver Neukum <oneukum@...e.com>,
        syzbot <syzbot+be5b5f86a162a6c281e6@...kaller.appspotmail.com>,
        <andreyknvl@...gle.com>, <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        <syzkaller-bugs@...glegroups.com>
Subject: Re: KASAN: use-after-free Read in usblp_bulk_read

On Wed, 6 May 2020, Pete Zaitcev wrote:

> On Wed, 06 May 2020 11:14:42 +0200
> Oliver Neukum <oneukum@...e.com> wrote:
> 
> > Very well. We are not going to find it without exceptional luck. Yet
> > there may be a real issue, too. We simply do not know. How about the
> > attached patch?
> 
> >  	usblp_unlink_urbs(usblp);
> >  	mutex_unlock(&usblp->mut);
> > +	usb_poison_anchored_urbs(&usblp->urbs);
> >  
> >  	if (!usblp->used)
> >  		usblp_cleanup(usblp);
> 
> This can't be right. Our URBs are freed by the callback, and this
> technique is not compatible with poisoning, at least with how the
> usb/core.c implements it. The usb_poison_urb() waits for URB
> to complete, and if the callback frees it, it's a problem.

That's not a problem.  URBs are reference-counted, and 
usb_poison_anchored_urbs() does usb_get_urb() before and usb_put_urb() 
after calling usb_poison_urb().

Alan Stern

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