[PATCH 3.16 238/305] USB: don't free bandwidth_mutex too early

[Ben Hutchings]

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

commit ab2a4bf83902c170d29ba130a8abb5f9d90559e1 upstream.

The USB core contains a bug that can show up when a USB-3 host
controller is removed.  If the primary (USB-2) hcd structure is
released before the shared (USB-3) hcd, the core will try to do a
double-free of the common bandwidth_mutex.

The problem was described in graphical form by Chung-Geol Kim, who
first reported it:

=================================================
     At *remove USB(3.0) Storage
     sequence <1> --> <5> ((Problem Case))
=================================================
                                  VOLD
------------------------------------|------------
                                 (uevent)
                            ________|_________
                           |<1>               |
                           |dwc3_otg_sm_work  |
                           |usb_put_hcd       |
                           |peer_hcd(kref=2)|
                           |__________________|
                            ________|_________
                           |<2>               |
                           |New USB BUS #2    |
                           |                  |
                           |peer_hcd(kref=1)  |
                           |                  |
                         --(Link)-bandXX_mutex|
                         | |__________________|
                         |
    ___________________  |
   |<3>                | |
   |dwc3_otg_sm_work   | |
   |usb_put_hcd        | |
   |primary_hcd(kref=1)| |
   |___________________| |
    _________|_________  |
   |<4>                | |
   |New USB BUS #1     | |
   |hcd_release        | |
   |primary_hcd(kref=0)| |
   |                   | |
   |bandXX_mutex(free) |<-
   |___________________|
                               (( VOLD ))
                            ______|___________
                           |<5>               |
                           |      SCSI        |
                           |usb_put_hcd       |
                           |peer_hcd(kref=0)  |
                           |*hcd_release      |
                           |bandXX_mutex(free*)|<- double free
                           |__________________|

=================================================

This happens because hcd_release() frees the bandwidth_mutex whenever
it sees a primary hcd being released (which is not a very good idea
in any case), but in the course of releasing the primary hcd, it
changes the pointers in the shared hcd in such a way that the shared
hcd will appear to be primary when it gets released.

This patch fixes the problem by changing hcd_release() so that it
deallocates the bandwidth_mutex only when the _last_ hcd structure
referencing it is released.  The patch also removes an unnecessary
test, so that when an hcd is released, both the shared_hcd and
primary_hcd pointers in the hcd's peer will be cleared.

Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Reported-by: Chung-Geol Kim <chunggeol.kim@xxxxxxxxxxx>
Tested-by: Chung-Geol Kim <chunggeol.kim@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: free only usb_hcd::bandwidth_mutex, not
 usb_hcd::address0_mutex too]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -2518,24 +2518,22 @@ EXPORT_SYMBOL_GPL(usb_create_hcd);
  * Don't deallocate the bandwidth_mutex until the last shared usb_hcd is
  * deallocated.
  *
- * Make sure to only deallocate the bandwidth_mutex when the primary HCD is
- * freed.  When hcd_release() is called for either hcd in a peer set
- * invalidate the peer's ->shared_hcd and ->primary_hcd pointers to
- * block new peering attempts
+ * Make sure to deallocate the bandwidth_mutex only when the last HCD is
+ * freed.  When hcd_release() is called for either hcd in a peer set,
+ * invalidate the peer's ->shared_hcd and ->primary_hcd pointers.
  */
 static void hcd_release(struct kref *kref)
 {
 	struct usb_hcd *hcd = container_of (kref, struct usb_hcd, kref);
 
 	mutex_lock(&usb_port_peer_mutex);
-	if (usb_hcd_is_primary_hcd(hcd))
-		kfree(hcd->bandwidth_mutex);
 	if (hcd->shared_hcd) {
 		struct usb_hcd *peer = hcd->shared_hcd;
 
 		peer->shared_hcd = NULL;
-		if (peer->primary_hcd == hcd)
-			peer->primary_hcd = NULL;
+		peer->primary_hcd = NULL;
+	} else {
+		kfree(hcd->bandwidth_mutex);
 	}
 	mutex_unlock(&usb_port_peer_mutex);
 	kfree(hcd);