Subject: [PATCH 30/48] net: fix socket refcounting in skb_complete_wifi_ack()

From: Eric Dumazet <[email-masked]>
Date: Fri, 3 Mar 2017 21:01:02 -0800

[ Upstream commit dd4f10722aeb10f4f582948839f066bebe44e5fb ]

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.

Fixes: bf7fa551e0ce ("mac80211: Resolve sk_refcnt/sk_wmem_alloc issue in wifi ack path")
Signed-off-by: Eric Dumazet <[email-masked]>
Cc: Alexander Duyck <[email-masked]>
Cc: Johannes Berg <[email-masked]>
Cc: Soheil Hassas Yeganeh <[email-masked]>
Cc: Willem de Bruijn <[email-masked]>
Acked-by: Soheil Hassas Yeganeh <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/core/skbuff.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 734c714..cebb961 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3889,7 +3889,7 @@ void skb_complete_wifi_ack(struct sk_buff *skb, bool acked)
 {
 	struct sock *sk = skb->sk;
 	struct sock_exterr_skb *serr;
-	int err;
+	int err = 1;
 
 	skb->wifi_acked_valid = 1;
 	skb->wifi_acked = acked;
@@ -3899,14 +3899,15 @@ void skb_complete_wifi_ack(struct sk_buff *skb, bool acked)
 	serr->ee.ee_errno = ENOMSG;
 	serr->ee.ee_origin = SO_EE_ORIGIN_TXSTATUS;
 
-	/* take a reference to prevent skb_orphan() from freeing the socket */
-	sock_hold(sk);
-
-	err = sock_queue_err_skb(sk, skb);
+	/* 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))) {
+		err = sock_queue_err_skb(sk, skb);
+		sock_put(sk);
+	}
 	if (err)
 		kfree_skb(skb);
-
-	sock_put(sk);
 }
 EXPORT_SYMBOL_GPL(skb_complete_wifi_ack);
 
-- 
2.1.0