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Date:   Thu, 23 Apr 2020 00:10:36 -0500
From:   Pete Zaitcev <zaitcev@redhat.com>
To:     Hillf Danton <hdanton@sina.com>
Cc:     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, zaitcev@redhat.com
Subject: Re: KASAN: use-after-free Read in usblp_bulk_read
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        <20200422032323.8536-1-hdanton@sina.com>
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On Wed, 22 Apr 2020 11:23:23 +0800
Hillf Danton <hdanton@sina.com> wrote:

> Do cleanup for lp after submitted urb completes.
> 
> --- a/drivers/usb/class/usblp.c
> +++ b/drivers/usb/class/usblp.c
> @@ -1376,8 +1376,10 @@ static void usblp_disconnect(struct usb_
>  	usblp_unlink_urbs(usblp);
>  	mutex_unlock(&usblp->mut);
>  
> -	if (!usblp->used)
> +	if (!usblp->used) {
> +		wait_event(usblp->rwait, usblp->rcomplete != 0);
>  		usblp_cleanup(usblp);
> +	}
>  	mutex_unlock(&usblp_mutex);
>  }

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.

At this point I suspect the fake HCD that the test harness invokes
fails to termine the transfer properly and then a timer hits.

Here's the bot's evidence and how I read it:

> Tue, 21 Apr 2020 08:35:18 -0700
> > Reported-by: syzbot+be5b5f86a162a6c281e6@syzkaller.appspotmail.com

This is where the problem is tripped, notice that it comes
because gadget runs a timer:

> >  kasan_report+0xe/0x20 mm/kasan/common.c:641
> >  __lock_acquire+0x31af/0x3b60 kernel/locking/lockdep.c:3827
> >  lock_acquire+0x130/0x340 kernel/locking/lockdep.c:4484
> >  __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> >  _raw_spin_lock_irqsave+0x32/0x50 kernel/locking/spinlock.c:159
> >  usblp_bulk_read+0x211/0x270 drivers/usb/class/usblp.c:303
> >  __usb_hcd_giveback_urb+0x1f2/0x470 drivers/usb/core/hcd.c:1648
> >  usb_hcd_giveback_urb+0x368/0x420 drivers/usb/core/hcd.c:1713
> >  dummy_timer+0x1258/0x32ae drivers/usb/gadget/udc/dummy_hcd.c:1966
> >  call_timer_fn+0x195/0x6f0 kernel/time/timer.c:1404
> >  expire_timers kernel/time/timer.c:1449 [inline]
> >  __run_timers kernel/time/timer.c:1773 [inline]
> >  __run_timers kernel/time/timer.c:1740 [inline]

At this point the whole struct usblp is freed, including the
spinlock which we're trying to lock.

> > Allocated by task 3361:
> >  save_stack+0x1b/0x80 mm/kasan/common.c:72
> >  set_track mm/kasan/common.c:80 [inline]
> >  __kasan_kmalloc mm/kasan/common.c:515 [inline]
> >  __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:488
> >  kmalloc include/linux/slab.h:555 [inline]
> >  kzalloc include/linux/slab.h:669 [inline]
> >  usblp_probe+0xed/0x1200 drivers/usb/class/usblp.c:1104
> >  usb_probe_interface+0x310/0x800 drivers/usb/core/driver.c:374
> >  really_probe+0x290/0xac0 drivers/base/dd.c:551
> >  driver_probe_device+0x223/0x350 drivers/base/dd.c:724

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.

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?

-- Pete