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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 <>
Reported-by: Don Zickus <>
Cc: stable <>
Signed-off-by: Greg Kroah-Hartman <>
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1 parent 01bb650 commit 0d00dc2611abbe6ad244d50569c2ee82ce42846c Alan Stern committed with gregkh Sep 26, 2012
Showing with 3 additions and 5 deletions.
  1. +1 −1 drivers/usb/core/devices.c
  2. +2 −4 drivers/usb/core/hcd.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)
ret = usb_device_dump(&buf, &nbytes, &skip_bytes, ppos,
@@ -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;

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