linux-kernel - Re: [PATCH] usb: core: Null deref in kernel with USB webcams.

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Message-ID: <20201113171658.GF322940@rowland.harvard.edu>
Date:   Fri, 13 Nov 2020 12:16:58 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     John Boero <boeroboy@...il.com>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Felipe Balbi <balbi@...nel.org>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: core: Null deref in kernel with USB webcams.

On Fri, Nov 13, 2020 at 04:45:52PM +0000, John Boero wrote:
> Sorry I wanted to include a pastebin or link but was trying to follow maillist
> guidelines and not include links or exceed wrap guidelines.  Full contents:
> https://paste.centos.org/view/3746bc40
> 
> Yes I understand the return dodges the config dereference.
> 
> Original line usb.c:281 is the original error:
> 
> 280| for (i = 0; i < config->desc.bNumInterfaces; i++)
> 281|  if (config->interface[i]->altsetting[0]
> 282|    .desc.bInterfaceNumber == ifnum)
> 283|  return config->interface[i];

Okay.  Without having looked at the code, I would guess that uvcvideo's 
uvc_ioctl_streamon() handler -- or some routine beneath it -- either 
doesn't lock the USB interface while starting I/O, or doesn't check 
(while holding the lock) to see whether the driver has been unbound.

This sort of error (config->interface[i] == NULL) is what you expect to 
see if a driver tries to carry out I/O to a device that has been 
unplugged and that it has been unbound from.

Alan Stern