net: let skb_orphan_partial wake-up waiters.

Currently the mentioned helper can end-up freeing the socket wmem
without waking-up any processes waiting for more write memory.

If the partially orphaned skb is attached to an UDP (or raw) socket,
the lack of wake-up can hang the user-space.

Even for TCP sockets not calling the sk destructor could have bad
effects on TSQ.

Address the issue using skb_orphan to release the sk wmem before
setting the new sock_efree destructor. Additionally bundle the
whole ownership update in a new helper, so that later other
potential users could avoid duplicate code.

v1 -> v2:
 - use skb_orphan() instead of sort of open coding it (Eric)
 - provide an helper for the ownership change (Eric)

Fixes: f6ba8d3 ("netem: fix skb_orphan_partial()")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2221,6 +2221,15 @@ static inline void skb_set_owner_r(struct sk_buff *skb, struct sock *sk)
 	sk_mem_charge(sk, skb->truesize);
 }
 
+static inline void skb_set_owner_sk_safe(struct sk_buff *skb, struct sock *sk)
+{
+	if (sk && refcount_inc_not_zero(&sk->sk_refcnt)) {
+		skb_orphan(skb);
+		skb->destructor = sock_efree;
+		skb->sk = sk;
+	}
+}
+
 void sk_reset_timer(struct sock *sk, struct timer_list *timer,
 		    unsigned long expires);
 

net/core/sock.c

@@ -2132,16 +2132,10 @@ void skb_orphan_partial(struct sk_buff *skb)
 	if (skb_is_tcp_pure_ack(skb))
 		return;
 
-	if (can_skb_orphan_partial(skb)) {
-		struct sock *sk = skb->sk;
-
-		if (refcount_inc_not_zero(&sk->sk_refcnt)) {
-			WARN_ON(refcount_sub_and_test(skb->truesize, &sk->sk_wmem_alloc));
-			skb->destructor = sock_efree;
-		}
-	} else {
+	if (can_skb_orphan_partial(skb))
+		skb_set_owner_sk_safe(skb, skb->sk);
+	else
 		skb_orphan(skb);
-	}
 }
 EXPORT_SYMBOL(skb_orphan_partial);