netem: fix skb_orphan_partial()

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.

Fixes: f2f872f9272a ("netem: Introduce skb_orphan_partial() helper")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Michael Madsen <mkm@nabto.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 12 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index 79c6aee6af9b..e43e71d7856b 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1803,28 +1803,24 @@ 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);
         }