netdev - Re: use-after-free in usbnet

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Message-ID: <CACVXFVODFvtF1DRyWJmnYCByw7sr9QDv_+Taxo5maVaUauDNtg@mail.gmail.com>
Date:	Thu, 22 Mar 2012 17:30:36 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Alan Stern <stern@...land.harvard.edu>, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org,
	Fedora Kernel Team <kernel-team@...oraproject.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: use-after-free in usbnet

On Thu, Mar 22, 2012 at 5:08 PM, Oliver Neukum <oneukum@...e.de> wrote:
>
> this looks good, but could you add a comment explaining the reason for
> taking a reference?

OK, I will post a formal one if you have no objection on the below.

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 4b8b52c..febfdce 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -589,6 +589,14 @@ static int unlink_urbs (struct usbnet *dev,
struct sk_buff_head *q)
 		entry = (struct skb_data *) skb->cb;
 		urb = entry->urb;

+		/*
+		 * Get a reference count of the URB to avoid it to be
+		 * freed during usb_unlink_urb, which may trigger
+		 * use-after-free problem inside usb_unlink_urb since
+		 * usb_unlink_urb is always racing with .complete
+		 * handler(include defer_bh).
+		 */
+		usb_get_urb(urb);
 		spin_unlock_irqrestore(&q->lock, flags);
 		// during some PM-driven resume scenarios,
 		// these (async) unlinks complete immediately
@@ -597,6 +605,7 @@ static int unlink_urbs (struct usbnet *dev, struct
sk_buff_head *q)
 			netdev_dbg(dev->net, "unlink urb err, %d\n", retval);
 		else
 			count++;
+		usb_put_urb(urb);
 		spin_lock_irqsave(&q->lock, flags);
 	}
 	spin_unlock_irqrestore (&q->lock, flags);


Thanks,
-- 
Ming Lei
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