Re: [PATCH] udc: Memory leak on error path and use after free

[Anton Vasilyev]

Sorry for delayed reply.

On 16.08.2017 19:35, Alan Stern wrote:
> On Wed, 16 Aug 2017, Anton Vasilyev wrote:
>
> > On 16.08.2017 18:29, Alan Stern wrote:
> > > On Wed, 16 Aug 2017, Anton Vasilyev wrote:
> > >
> > > > gadget_release() is responsible for cleanup dev memory.
> > > > But if net2280_probe() fails after dev allocation, then
> > > > gadget_release() become unregistered and dev memory leaks.
> > >
> > > This isn't needed if usb_add_gadget_udc_release() is fixed, right?
> >
> > No, this situation could appear before call
> > usb_add_gadget_udc_release().
> >
> > > > Also net2280_remove() calls usb_del_gadget_udc() which
> > > > perform schedule_delayed_work() with gadget_release(), so
> > > > it is possible that dev will be deallocated exactly after
> > > > this call and leads to use after free.
> > >
> > > Where is there a possible use after free?
> >
> > net2280_remove() continue work with struct net2280 *dev after call
> > usb_del_gadget_udc(&dev->gadget), but this net2280 *dev could be
> > deallocated by gadget_release()
> >
> > > > The patch moves deallocation from gadget_release() to
> > > > net2280_remove().
> > >
> > > Alan Stern
>
> Okay, now I understand what you were saying.  Yes, I agree, the
> existing code isn't right.
>
> But a better solution would be to move the usb_del_gadget_udc() call
> from the beginning of net2280_remove() to the end.  And make the call
> conditional, depending on whether usb_add_gadget_udc_release() has
> already been called successfully.

If allow gadget_release() to deallocate net2280 *dev then it will be 
called on fail of usb_add_gadget_udc_release() and it will be unsafe to 
perform clean-up.
My point is that gadget shouldn't deallocate its parent memory at all.

> The point is that the device core does not allow drivers to deallocate
> memory containing a struct device before the ->release callback has
> been invoked.  Your patch might do that, if the release was delayed for
> some reason.

I don't see possibility for parent device to be removed before its child 
was released. Please point if I'm wrong.

Alternative way to move allocation under devm interface.

> Alan Stern

--
Anton Vasilyev
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: vasilyev@xxxxxxxxx