[PATCH] USB: EHCI: fix conflation of buf == 0 with len == 0

When the ehci-hcd driver prepares a control URB, it tests for a
zero-length data stage by looking at the transfer_dma value instead of
the transfer_buffer_length.  (In fact it does this even for non-control
URBs, which is an additional aspect of the same bug.)

However, under certain circumstances it's possible for transfer_dma to
be 0 while transfer_buffer_length is non-zero.  This can happen when a
freshly allocated page (mapped to address 0 and marked Copy-On-Write,
but never written to) is used as the source buffer for an OUT transfer.
This patch (as598) fixes the problem.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/ehci-q.c

@@ -514,18 +514,18 @@ qh_urb_transaction (
 		qtd->urb = urb;
 		qtd_prev->hw_next = QTD_NEXT (qtd->qtd_dma);
 		list_add_tail (&qtd->qtd_list, head);
+
+		/* for zero length DATA stages, STATUS is always IN */
+		if (len == 0)
+			token |= (1 /* "in" */ << 8);
 	} 
 
 	/*
 	 * data transfer stage:  buffer setup
 	 */
-	if (likely (len > 0))
-		buf = urb->transfer_dma;
-	else
-		buf = 0;
+	buf = urb->transfer_dma;
 
-	/* for zero length DATA stages, STATUS is always IN */
-	if (!buf || is_input)
+	if (is_input)
 		token |= (1 /* "in" */ << 8);
 	/* else it's already initted to "out" pid (0 << 8) */
 
@@ -572,7 +572,7 @@ qh_urb_transaction (
 	 * control requests may need a terminating data "status" ack;
 	 * bulk ones may need a terminating short packet (zero length).
 	 */
-	if (likely (buf != 0)) {
+	if (likely (urb->transfer_buffer_length != 0)) {
 		int	one_more = 0;
 
 		if (usb_pipecontrol (urb->pipe)) {