linux-kernel - RE: [PATCH] usb/core: fix NULL pointer dereference in recursively_mark

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Date:	Tue, 24 Dec 2013 16:53:18 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Du, ChangbinX" <changbinx.du@...el.com>
cc:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"sarah.a.sharp@...ux.intel.com" <sarah.a.sharp@...ux.intel.com>,
	"Lan, Tianyu" <tianyu.lan@...el.com>,
	"burzalodowa@...il.com" <burzalodowa@...il.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb/core: fix NULL pointer dereference in
 recursively_mark_NOTATTACHED

On Tue, 24 Dec 2013, Du, ChangbinX wrote:

> > > usb_hub_to_struct_hub() can return NULL if the hub without active
> > > configuration. So the result must be checked.
> > >
> > > BUG: unable to handle kernel NULL pointer dereference at 0000015c
> 
> > How did you manage to trigger this BUG?  If hub is NULL then
> > udev->maxchild should be 0.  See the code in hub_disconnect().
> >
> > Alan Stern
> 
> Hello, Alan. The hub also should be null if actconfig is null. You can see it in function
> usb_hub_to_struct_hub().

Yes, I know.  But if actconfig is NULL then udev->maxchild should be 0.

> udev->maxchild will be set to 0 in hub_disconnect(). 

Note that hub_disconnect() runs _before_ actconfig is set to NULL.

>  But before that,
> recursively_mark_NOTATTACHED may be called when calling usb_disconnect().

If this happens before hub_disconnect() has run then actconfig cannot 
be NULL, because hub_disconnect() runs _before_ actconfig is set to 
NULL.

>  So this issue
> will happen when usb_disconnect a hub that not have a configuration yet.

No, it won't.  You can test this easily.  Plug in a hub, write 0 to its
/sys/bus/usb/devices/.../bConfigurationValue, and then unplug the hub.

> It happened once here when unplugging otg cable from DUT(will cause hcd removed) with
> tiers of hub and devices. But it's not easy to reproduce it.
> This is my analysis, how do you think?

There is another reason why usb_hub_to_struct_hub() could return NULL: 
if usb_get_intfdata(hdev->actconfig->interface[0]) is NULL.  This could 
happen if recursively_mark_NOTATTACHED() is called _while_ 
hub_disconnect() is running.

There should be locking to prevent this, but there isn't.  That's what 
we need to fix.  It's not an easy problem.  Can you figure out a 
correct solution?

Alan Stern

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