RE: [PATCH] usb/core: fix NULL pointer dereference inrecursively_mark_NOTATTACHED

From: Alan Stern
Date: Tue Dec 24 2013 - 16:53:47 EST

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

> 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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