Patch "netem: fix skb_orphan_partial()" has been added to the 4.9-stable tree

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

netem: fix skb_orphan_partial()

to the 4.9-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:
netem-fix-skb_orphan_partial.patch
and it can be found in the queue-4.9 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: Thu, 11 May 2017 15:24:41 -0700
Subject: netem: fix skb_orphan_partial()

From: Eric Dumazet <[email protected]>

[ Upstream commit f6ba8d33cfbb46df569972e64dbb5bb7e929bfd9 ]

I should have known that lowering skb->truesize was dangerous :/

In case packets are not leaving the host via a standard Ethernet device, but looped back to local sockets, bad things can happen, as reported by Michael Madsen ( https://bugzilla.kernel.org/show_bug.cgi?id=195713 )

So instead of tweaking skb->truesize, lets change skb->destructor and keep a reference on the owner socket via its sk_refcnt.

---
 net/core/sock.c |   20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

@@ -1687,28 +1687,24 @@ EXPORT_SYMBOL(skb_set_owner_w);
  * delay queue. We want to allow the owner socket to send more
  * packets, as if they were already TX completed by a typical driver.
  * But we also want to keep skb->sk set because some packet schedulers
- * rely on it (sch_fq for example). So we set skb->truesize to a small
- * amount (1) and decrease sk_wmem_alloc accordingly.
+ * rely on it (sch_fq for example).
  */
 void skb_orphan_partial(struct sk_buff *skb)
 {
-	/* If this skb is a TCP pure ACK or already went here,
-	 * we have nothing to do. 2 is already a very small truesize.
-	 */
-	if (skb->truesize <= 2)
+	if (skb_is_tcp_pure_ack(skb))
 		return;
 
-	/* TCP stack sets skb->ooo_okay based on sk_wmem_alloc,
-	 * so we do not completely orphan skb, but transfert all
-	 * accounted bytes but one, to avoid unexpected reorders.
-	 */
 	if (skb->destructor == sock_wfree
 #ifdef CONFIG_INET
 	    || skb->destructor == tcp_wfree
 #endif
 		) {
-		atomic_sub(skb->truesize - 1, &skb->sk->sk_wmem_alloc);
-		skb->truesize = 1;
+		struct sock *sk = skb->sk;
+
+		if (atomic_inc_not_zero(&sk->sk_refcnt)) {
+			atomic_sub(skb->truesize, &sk->sk_wmem_alloc);
+			skb->destructor = sock_efree;
+		}
 	} else {
 		skb_orphan(skb);
 	}

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

queue-4.9/ipv6-fix-out-of-bound-writes-in-__ip6_append_data.patch queue-4.9/tcp-eliminate-negative-reordering-in-tcp_clean_rtx_queue.patch queue-4.9/dccp-tcp-do-not-inherit-mc_list-from-parent.patch queue-4.9/tcp-avoid-fragmenting-peculiar-skbs-in-sack.patch queue-4.9/netem-fix-skb_orphan_partial.patch queue-4.9/net-fix-compile-error-in-skb_orphan_partial.patch queue-4.9/ipv6-dccp-do-not-inherit-ipv6_mc_list-from-parent.patch queue-4.9/sctp-do-not-inherit-ipv6_-mc-ac-fl-_list-from-parent.patch queue-4.9/ipv4-add-reference-counting-to-metrics.patch queue-4.9/tcp-avoid-fastopen-api-to-be-used-on-af_unspec.patch