USB: devio: Revert "USB: devio: Don't corrupt user memory"

BugLink: http://bugs.launchpad.net/bugs/1729107

commit 845d584f upstream.

Taking the uurb->buffer_length userspace passes in as a maximum for the
actual urbs transfer_buffer_length causes 2 serious issues:

1) It breaks isochronous support for all userspace apps using libusb,
   as existing libusb versions pass in 0 for uurb->buffer_length,
   relying on the kernel using the lenghts of the usbdevfs_iso_packet_desc
   descriptors passed in added together as buffer length.

   This for example causes redirection of USB audio and Webcam's into
   virtual machines using qemu-kvm to no longer work. This is a userspace
   ABI break and as such must be reverted.

   Note that the original commit does not protect other users / the
   kernels memory, it only stops the userspace process making the call
   from shooting itself in the foot.

2) It may cause the kernel to program host controllers to DMA over random
   memory. Just as the devio code used to only look at the iso_packet_desc
   lenghts, the host drivers do the same, relying on the submitter of the
   urbs to make sure the entire buffer is large enough and not checking
   transfer_buffer_length.

   But the "USB: devio: Don't corrupt user memory" commit now takes the
   userspace provided uurb->buffer_length for the buffer-size while copying
   over the user-provided iso_packet_desc lengths 1:1, allowing the user
   to specify a small buffer size while programming the host controller to
   dma a lot more data.

   (Atleast the ohci, uhci, xhci and fhci drivers do not check
    transfer_buffer_length for isoc transfers.)

This reverts commit fa1ed74e ("USB: devio: Don't corrupt user memory")
fixing both these issues.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/devio.c

@@ -1417,11 +1417,7 @@ static int proc_do_submiturb(struct usb_dev_state *ps, struct usbdevfs_urb *uurb
 			totlen += isopkt[u].length;
 		}
 		u *= sizeof(struct usb_iso_packet_descriptor);
-		if (totlen <= uurb->buffer_length)
-			uurb->buffer_length = totlen;
-		else
-			WARN_ONCE(1, "uurb->buffer_length is too short %d vs %d",
-				  totlen, uurb->buffer_length);
+		uurb->buffer_length = totlen;
 		break;
 
 	default: