linux-kernel - [ 049/108] USB: Fix race condition when removing host controllers

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Message-Id: <20121007225841.487720345@decadent.org.uk>
Date:	Sun, 07 Oct 2012 23:59:23 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Alan Stern <stern@...land.harvard.edu>,
	Don Zickus <dzickus@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [ 049/108] USB: Fix race condition when removing host controllers

3.2-stable review patch.  If anyone has any objections, please let me know.

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

From: Alan Stern <stern@...land.harvard.edu>

commit 0d00dc2611abbe6ad244d50569c2ee82ce42846c upstream.

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@...land.harvard.edu>
Reported-by: Don Zickus <dzickus@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/usb/core/devices.c |    2 +-
 drivers/usb/core/hcd.c     |    6 ++----
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/core/devices.c b/drivers/usb/core/devices.c
index d956965..3440812 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 bc84106..75ba209 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;
 }


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