Re: usb/gadget: another GPF in usb_gadget_unregister_driver

[Alan Stern <stern@rowland.harvard.edu>]

On Thu, 8 Jun 2017, Andrey Konovalov wrote:

> On Wed, Jun 7, 2017 at 11:20 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> > On Wed, 7 Jun 2017, Andrey Konovalov wrote:
> >
> >> On Wed, Jun 7, 2017 at 4:43 PM, Alan Stern <stern@rowland.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.
> 
> I've extract another crash log, which is a little simpler:
> https://gist.github.com/xairy/b8c814cbd731e4632e8e8fa0f51a29e8
> 
> >
> > 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.
> 
> Here's the funcgraph for usb_gadget_probe_driver:
> https://gist.github.com/xairy/3221e2cb9c59514880d24c955de30b80
> 
> The (!driver->match_existing_only) test is not executed.
> The code following "found:" is executed.
> 
> >
> > 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.
> 
> Here's the funcgraph for usb_gadget_unregister_driver:
> https://gist.github.com/xairy/887c52a12af8c9f9fe8ba3e4fa0ef1f0
> 
> What you described happens during the first call of
> usb_gadget_unregister_driver(), however there's another one after
> that, which is probably triggered by the second program.
> 
> >
> > In particular, do these subroutines get called more than once?
> 
> usb_gadget_unregister_driver() is called twice, the GPF happens during
> the second call.

Good, that's definitive.  And I feel stupid for missing this bug.  
The patch is below.

Alan Stern



Index: usb-4.x/drivers/usb/gadget/legacy/inode.c
===================================================================
--- usb-4.x.orig/drivers/usb/gadget/legacy/inode.c
+++ usb-4.x/drivers/usb/gadget/legacy/inode.c
@@ -1183,8 +1183,10 @@ dev_release (struct inode *inode, struct
 
 	/* closing ep0 === shutdown all */
 
-	if (dev->gadget_registered)
+	if (dev->gadget_registered) {
 		usb_gadget_unregister_driver (&gadgetfs_driver);
+		dev->gadget_registered = false;
+	}
 
 	/* at this point "good" hardware has disconnected the
 	 * device from USB; the host won't see it any more.