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Date:   Thu, 23 Apr 2020 12:29:15 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Oliver Neukum <oneukum@suse.com>
cc:     Pete Zaitcev <zaitcev@redhat.com>, Hillf Danton <hdanton@sina.com>,
        syzbot <syzbot+be5b5f86a162a6c281e6@syzkaller.appspotmail.com>,
        <andreyknvl@google.com>, <gregkh@linuxfoundation.org>,
        <linux-kernel@vger.kernel.org>, <linux-usb@vger.kernel.org>,
        <syzkaller-bugs@googlegroups.com>
Subject: Re: KASAN: use-after-free Read in usblp_bulk_read
In-Reply-To: <1587640413.23108.7.camel@suse.com>
Message-ID: <Pine.LNX.4.44L0.2004231213260.20147-100000@netrider.rowland.org>
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On Thu, 23 Apr 2020, Oliver Neukum wrote:

> Am Donnerstag, den 23.04.2020, 00:10 -0500 schrieb Pete Zaitcev:
> > 
> > I do not agree with this kind of workaround. The model we're following
> > is for usb_kill_urb() to cancel the transfer. The usblp invokes it
> > through usb_kill_anchored_urbs() and usblp_unlink_urbs(), as seen
> > above. There can be no timer hitting anything once it returns.
> 
> Right. It seems to me that the problem is not killing an existing
> transfer but a failure to check in case of new transfers whether
> the device has been disconnected.
> 
> > 1104 is kzalloc for struct usblp.
> > 
> > > > Freed by task 12266:
> > > >  save_stack+0x1b/0x80 mm/kasan/common.c:72
> > > >  set_track mm/kasan/common.c:80 [inline]
> > > >  kasan_set_free_info mm/kasan/common.c:337 [inline]
> > > >  __kasan_slab_free+0x117/0x160 mm/kasan/common.c:476
> > > >  slab_free_hook mm/slub.c:1444 [inline]
> > > >  slab_free_freelist_hook mm/slub.c:1477 [inline]
> > > >  slab_free mm/slub.c:3034 [inline]
> > > >  kfree+0xd5/0x300 mm/slub.c:3995
> > > >  usblp_disconnect.cold+0x24/0x29 drivers/usb/class/usblp.c:1380
> > > >  usb_unbind_interface+0x1bd/0x8a0 drivers/usb/core/driver.c:436
> > > >  __device_release_driver drivers/base/dd.c:1137 [inline]
> > > >  device_release_driver_internal+0x42f/0x500 drivers/base/dd.c:1168
> > > >  bus_remove_device+0x2eb/0x5a0 drivers/base/bus.c:533
> > 
> > 1380 is an inlined call to usblp_cleanup, which is just
> > a bunch of kfree.
> 
> But that must never happen while while the device is open.
> If that ever happens something is wrong with usblp->used.
> 
> > The bug report is still a bug report, but I'm pretty sure the
> > culprit is the emulated HCD and/or the gadget layer. Unfortunately,
> > I'm not up to speed in that subsystem. Maybe Alan can look at it?
> 
> I doubt it. Operation by a timer triggering a timeout must work.

The timer is not the issue.  usb_kill_anchored_urbs() waits until all 
the URBs have completed, and those completions happen when the timer 
fires.

The only suspicious thing I see is that usblp_resume() calls 
handle_bidir() without first acquiring any mutex.  But resume shouldn't 
race with disconnect.

The only other place where read URBs get submitted is under
usblp_read(), which does acquire the mutex and checks for disconnection
while holding it.

So I'm baffled.

Alan Stern