linux-kernel - Re: usb/gadget: another GPF in usb_gadget_unregister

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Date:   Wed, 7 Jun 2017 17:20:35 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrey Konovalov <andreyknvl@...gle.com>
cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Chen <peter.chen@....com>,
        Krzysztof Opasiak <k.opasiak@...sung.com>,
        Colin Ian King <colin.king@...onical.com>,
        Felix Hädicke <felixhaedicke@....de>,
        Roger Quadros <rogerq@...com>,
        USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: usb/gadget: another GPF in usb_gadget_unregister_driver

On Wed, 7 Jun 2017, Andrey Konovalov wrote:

> On Wed, Jun 7, 2017 at 4:43 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Wed, 7 Jun 2017, Andrey Konovalov wrote:
> >
> >> Hi,
> >>
> >> I've got the following error report while fuzzing the kernel with syzkaller.
> >>
> >> On commit b29794ec95c6856b316c2295904208bf11ffddd9 (4.12-rc4+).
> >>
> >> This looks quite similar to
> >> https://groups.google.com/forum/#!topic/syzkaller/HDawLBeeORI
> >
> > It does look very similar, but that problem was supposed to have been
> > fixed by commit 7b0173811260 ("usb: gadget: udc: core: fix return code
> > of usb_gadget_probe_driver()").
> >
> >> I'm able to reproduce this, so I can collect some debug traces if needed.
> >
> > Can you provide an strace or the equivalent?
> 
> Here's the syzkaller program (which is actually two programs executed
> consequently):
> https://gist.github.com/xairy/fe0a7531e00df5e8bc23e2e56e413510
> 
> Here's the strace log:
> https://gist.github.com/xairy/5fadc3b5d8b2b80c97e566538de08bc4

Do you know which of the two programs got the GPF?  I can't tell from 
the strace log.

> Unfortunately there's a lot of unrelated garbage, but I can't extract
> a simple C reproducer.

That's okay, it's easy enough to see what's going on.  One program 
opens /dev/gadget/dummy_udc, writes an invalid setup string, then 
writes a valid setup string, and then exits.  The other program just 
opens the file and then exits.

> I can also apply patches with debug printk's, run the reproducer and
> send you the result if that will help.

Maybe you can patch usb_gadget_probe_driver() in 
drivers/usb/gadget/udc/core.c.  Find out whether the "if 
(!driver->match_existing_only)" test is executed and whether it 
succeeds, and find out whether the code following "found:" is executed.
I would expect that the test is not executed and the jump to "found:" 
is taken, so udc_bind_to_driver() is called and returns 0.  Thus, 
udc->driver should be set to driver.

Also, in usb_gadget_unregister_driver(), in the list_for_each_entry() 
loop, we should have udc->driver == driver and therefore ret should get 
set to 0.  Consequently, the list_del() near the end should not be 
executed and so the GPF should not occur.

In particular, do these subroutines get called more than once?

Alan Stern