USB: Fix race condition when removing host controllers

This patch (as1607) fixes a race that can occur if a USB host
controller is removed while a process is reading the
/sys/kernel/debug/usb/devices file.

The usb_device_read() routine uses the bus->root_hub pointer to
determine whether or not the root hub is registered.  The is not a
valid test, because the pointer is set before the root hub gets
registered and remains set even after the root hub is unregistered and
deallocated.  As a result, usb_device_read() or usb_device_dump() can
access freed memory, causing an oops.

The patch changes the test to use the hcd->rh_registered flag, which
does get set and cleared at the appropriate times.  It also makes sure
to hold the usb_bus_list_lock mutex while setting the flag, so that
usb_device_read() will become aware of new root hubs as soon as they
are registered.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Don Zickus <dzickus@redhat.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 3 insertions, 5 deletions

diff --git a/drivers/usb/core/devices.c b/drivers/usb/core/devices.c
index d95696584762..3440812b4a84 100644
--- a/drivers/usb/core/devices.c
+++ b/drivers/usb/core/devices.c
@@ -624,7 +624,7 @@ static ssize_t usb_device_read(struct file *file, char __user *buf,
         /* print devices for all busses */
         list_for_each_entry(bus, &usb_bus_list, bus_list) {
                 /* recurse through all children of the root hub */
-                if (!bus->root_hub)
+                if (!bus_to_hcd(bus)->rh_registered)
                         continue;
                 usb_lock_device(bus->root_hub);
                 ret = usb_device_dump(&buf, &nbytes, &skip_bytes, ppos,
diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index bc84106ac057..75ba2091f9b4 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1011,10 +1011,7 @@ static int register_root_hub(struct usb_hcd *hcd)
         if (retval) {
                 dev_err (parent_dev, "can't register root hub for %s, %d\n",
                                 dev_name(&usb_dev->dev), retval);
-        }
-        mutex_unlock(&usb_bus_list_lock);
-
-        if (retval == 0) {
+        } else {
                 spin_lock_irq (&hcd_root_hub_lock);
                 hcd->rh_registered = 1;
                 spin_unlock_irq (&hcd_root_hub_lock);
@@ -1023,6 +1020,7 @@ static int register_root_hub(struct usb_hcd *hcd)
                 if (HCD_DEAD(hcd))
                         usb_hc_died (hcd);      /* This time clean up */
         }
+        mutex_unlock(&usb_bus_list_lock);
 
         return retval;
 }