Commit 0a231403 authored Sep 26, 2012 by Alan Stern Committed by Greg Kroah-Hartman Sep 26, 2012

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>

parent 655db798

drivers/usb/core/devices.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -623,7 +623,7 @@ 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,

drivers/usb/core/hcd.c

+2 −4

Original line number	Diff line number	Diff line
		@@ -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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