[3.16.y-ckt stable] Patch "USB: fix use-after-free bug in usb_hcd_unlink_urb()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    USB: fix use-after-free bug in usb_hcd_unlink_urb()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 1886b1aeddba8ca947e8432f1fedd6e8065e4704 Mon Sep 17 00:00:00 2001
From: Alan Stern <stern at rowland.harvard.edu>
Date: Fri, 30 Jan 2015 12:58:26 -0500
Subject: USB: fix use-after-free bug in usb_hcd_unlink_urb()

commit c99197902da284b4b723451c1471c45b18537cde upstream.

The usb_hcd_unlink_urb() routine in hcd.c contains two possible
use-after-free errors.  The dev_dbg() statement at the end of the
routine dereferences urb and urb->dev even though both structures may
have been deallocated.

This patch fixes the problem by storing urb->dev in a local variable
(avoiding the dereference of urb) and moving the dev_dbg() up before
the usb_put_dev() call.

Signed-off-by: Alan Stern <stern at rowland.harvard.edu>
Reported-by: Joe Lawrence <joe.lawrence at stratus.com>
Tested-by: Joe Lawrence <joe.lawrence at stratus.com>
Signed-off-by: Greg Kroah-Hartman <greg at kroah.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/usb/core/hcd.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 23d44ac95cd6..4195906f94d6 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1621,6 +1621,7 @@ static int unlink1(struct usb_hcd *hcd, struct urb *urb, int status)
 int usb_hcd_unlink_urb (struct urb *urb, int status)
 {
 	struct usb_hcd		*hcd;
+	struct usb_device	*udev = urb->dev;
 	int			retval = -EIDRM;
 	unsigned long		flags;

@@ -1632,20 +1633,19 @@ int usb_hcd_unlink_urb (struct urb *urb, int status)
 	spin_lock_irqsave(&hcd_urb_unlink_lock, flags);
 	if (atomic_read(&urb->use_count) > 0) {
 		retval = 0;
-		usb_get_dev(urb->dev);
+		usb_get_dev(udev);
 	}
 	spin_unlock_irqrestore(&hcd_urb_unlink_lock, flags);
 	if (retval == 0) {
 		hcd = bus_to_hcd(urb->dev->bus);
 		retval = unlink1(hcd, urb, status);
-		usb_put_dev(urb->dev);
+		if (retval == 0)
+			retval = -EINPROGRESS;
+		else if (retval != -EIDRM && retval != -EBUSY)
+			dev_dbg(&udev->dev, "hcd_unlink_urb %p fail %d\n",
+					urb, retval);
+		usb_put_dev(udev);
 	}
-
-	if (retval == 0)
-		retval = -EINPROGRESS;
-	else if (retval != -EIDRM && retval != -EBUSY)
-		dev_dbg(&urb->dev->dev, "hcd_unlink_urb %p fail %d\n",
-				urb, retval);
 	return retval;
 }