Patch "net: fix socket refcounting in skb_complete_tx_timestamp()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

net: fix socket refcounting in skb_complete_tx_timestamp()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
net-fix-socket-refcounting-in-skb_complete_tx_timestamp.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Dumazet <[email protected]>
Date: Fri, 3 Mar 2017 21:01:03 -0800
Subject: net: fix socket refcounting in skb_complete_tx_timestamp()

From: Eric Dumazet <[email protected]>

[ Upstream commit 9ac25fc063751379cb77434fef9f3b088cd3e2f7 ]

TX skbs do not necessarily hold a reference on skb->sk->sk_refcnt By the time TX completion happens, sk_refcnt might be already 0.

sock_hold()/sock_put() would then corrupt critical state, like sk_wmem_alloc and lead to leaks or use after free.

---
 net/core/skbuff.c |   15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

@@ -3678,13 +3678,14 @@ void skb_complete_tx_timestamp(struct sk
 	if (!skb_may_tx_timestamp(sk, false))
 		return;
 
-	/* take a reference to prevent skb_orphan() from freeing the socket */
-	sock_hold(sk);
-
-	*skb_hwtstamps(skb) = *hwtstamps;
-	__skb_complete_tx_timestamp(skb, sk, SCM_TSTAMP_SND);
-
-	sock_put(sk);
+	/* Take a reference to prevent skb_orphan() from freeing the socket,
+	 * but only if the socket refcount is not zero.
+	 */
+	if (likely(atomic_inc_not_zero(&sk->sk_refcnt))) {
+		*skb_hwtstamps(skb) = *hwtstamps;
+		__skb_complete_tx_timestamp(skb, sk, SCM_TSTAMP_SND);
+		sock_put(sk);
+	}
 }
 EXPORT_SYMBOL_GPL(skb_complete_tx_timestamp);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tcp-fix-various-issues-for-sockets-morphing-to-listen-state.patch queue-4.4/dccp-fix-use-after-free-in-dccp_feat_activate_values.patch queue-4.4/net-fix-socket-refcounting-in-skb_complete_tx_timestamp.patch queue-4.4/net-net_enable_timestamp-can-be-called-from-irq-contexts.patch queue-4.4/dccp-unlock-sock-before-calling-sk_free.patch queue-4.4/net-don-t-call-strlen-on-the-user-buffer-in-packet_bind_spkt.patch queue-4.4/net-fix-socket-refcounting-in-skb_complete_wifi_ack.patch