linux-kernel - Re: [PATCH] usb: usbip: fix null pointer dereference

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Date:	Fri, 3 Jun 2016 10:21:31 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Krzysztof Opasiak <k.opasiak@...sung.com>
cc:	Sudip Mukherjee <sudipm.mukherjee@...il.com>,
	Valentina Manea <valentina.manea.m@...il.com>,
	Shuah Khan <shuah.kh@...sung.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] usb: usbip: fix null pointer dereference

On Fri, 3 Jun 2016, Krzysztof Opasiak wrote:

> On 06/02/2016 03:22 PM, Sudip Mukherjee wrote:
> > We have been dereferencing udc before checking it. Lets use it after it
> > has been checked.
> > 
> 
> To be honest I have mixed feelings about this patch.
> 
> On one hand it prevents us from dereferencing potential NULL ptr what is
> generally good. But on the other hand it seems to be a little bit
> pointless overhead. This function is called only in one place, it's
> internal function of vudc driver and in addition generally it is
> currently impossible that this function will get NULL ptr as parameter
> as it's value is taken from container_of(). Not to mention that if this
> is NULL or garbage we will end up in NULL ptr dereference much earlier
> before calling this function.
> 
> So if there is something that you would like to fix with this patch and
> you have a real problem with this function could you please provide us
> some more details (for example stack trace)? If this patch is just to
> prevent us from something that will never happen then I would rather to
> not submit this. In my opinion if we get a NULL in this function this
> means that we have some serious problem in UDC core and this check will
> just mask this error.

Normally in this situation, somebody would write a patch that removes 
these lines from get_gadget_descs():

	if (!udc || !udc->driver || !udc->pullup)
		return -EINVAL;

Or maybe (I'm not familiar with the driver so I don't know the best 
approach) changes the test to:

	if (!udc->driver || !udc->pullup)

After all, there's no reason to check !udc if it is known beforehand 
that udc will never be NULL.

Alan Stern