Re: [PATCH] usb: misc: legousbtower: Fix NULL pointer deference

[Greg KH]

On Mon, Sep 19, 2016 at 07:09:51PM +0100, James wrote:
> This patch fixes a NULL pointer dereference caused by a race codition in the
> probe function of the legousbtower driver. It re-structures the probe
> function to only register the interface after successfully reading the
> board's firmware ID.
> 
> The probe function does not deregister the usb interface after an error
> receiving the devices firmware ID. The device file registered
> (/dev/usb/legousbtower%d) may be read/written globally before the probe
> function returns. When tower_delete is called in the probe function
> (after an r/w has been initiated), core dev structures are deleted while
> the file operation functions are still running. If the 0 address is mappable
> on the machine, this vulnerability can be used to create a Local Priviege
> Escalation exploit via a write-what-where condition by remapping
> dev->interrupt_out_buffer in tower_write. A forged USB device and local
> program execution would be required for LPE. The USB
> device would have to delay the control message in tower_probe and accept
> the control urb in tower_open whilst guest code initiated a write to the
> device file as tower_delete is called from the error in tower_probe.
> 
> This bug has existed since 2003. Patch tested by emulated device.
> 
> Signed-off-by: James Patrick-Evans <james@xxxxxxxxx>
> ---
> drivers/usb/misc/legousbtower.c | 35 +++++++++++++++++------------------
> 1 file changed, 17 insertions(+), 18 deletions(-)

When you sent this the last time (back on Sept 2), I sent you a
"simpler" patch, the same one below, and asked you if that worked
instead.  I never heard back, could you test this patch and see if it
resolves the issue for you?

thanks,

greg k-h

---------------

How about this simpler patch, that moves the registering for the minor
number as the last thing that happens in the probe function (which is
what it should be).

Can you test this to verify it prevents the above problem?

And this driver really needs an overhaul one of these days...

thanks,

greg k-h

diff --git a/drivers/usb/misc/legousbtower.c b/drivers/usb/misc/legousbtower.c
index 7771be3ac178..4dd531ac5a7f 100644
--- a/drivers/usb/misc/legousbtower.c
+++ b/drivers/usb/misc/legousbtower.c
@@ -898,24 +898,6 @@ static int tower_probe (struct usb_interface *interface, const struct usb_device
 	dev->interrupt_in_interval = interrupt_in_interval ? interrupt_in_interval : dev->interrupt_in_endpoint->bInterval;
 	dev->interrupt_out_interval = interrupt_out_interval ? interrupt_out_interval : dev->interrupt_out_endpoint->bInterval;
 
-	/* we can register the device now, as it is ready */
-	usb_set_intfdata (interface, dev);
-
-	retval = usb_register_dev (interface, &tower_class);
-
-	if (retval) {
-		/* something prevented us from registering this driver */
-		dev_err(idev, "Not able to get a minor for this device.\n");
-		usb_set_intfdata (interface, NULL);
-		goto error;
-	}
-	dev->minor = interface->minor;
-
-	/* let the user know what node this device is now attached to */
-	dev_info(&interface->dev, "LEGO USB Tower #%d now attached to major "
-		 "%d minor %d\n", (dev->minor - LEGO_USB_TOWER_MINOR_BASE),
-		 USB_MAJOR, dev->minor);
-
 	/* get the firmware version and log it */
 	result = usb_control_msg (udev,
 				  usb_rcvctrlpipe(udev, 0),
@@ -936,6 +918,23 @@ static int tower_probe (struct usb_interface *interface, const struct usb_device
 		 get_version_reply.minor,
 		 le16_to_cpu(get_version_reply.build_no));
 
+	/* we can register the device now, as it is ready */
+	usb_set_intfdata (interface, dev);
+
+	retval = usb_register_dev (interface, &tower_class);
+
+	if (retval) {
+		/* something prevented us from registering this driver */
+		dev_err(idev, "Not able to get a minor for this device.\n");
+		usb_set_intfdata (interface, NULL);
+		goto error;
+	}
+	dev->minor = interface->minor;
+
+	/* let the user know what node this device is now attached to */
+	dev_info(&interface->dev, "LEGO USB Tower #%d now attached to major "
+		 "%d minor %d\n", (dev->minor - LEGO_USB_TOWER_MINOR_BASE),
+		 USB_MAJOR, dev->minor);
 
 exit:
 	return retval;