Subject: [PATCH] ipv4: dst_entry leak in ip_append_data()

From: Vasily Averin <[email-masked]>
Date: Mon, 13 Oct 2014 14:17:34 +0400

Fixes: 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")

If sk_write_queue is empty ip_append_data() executes ip_setup_cork()
that "steals" dst entry from rt to cork. Later it calls __ip_append_data()
that creates skb and adds it to sk_write_queue.

If skb was added successfully following ip_push_pending_frames() call
reassign dst entry from cork to skb, and kfree_skb frees dst_entry.

However nobody frees stolen dst_entry if skb was not added into sk_write_queue.

Signed-off-by: Vasily Averin <[email-masked]>
---
 net/ipv4/ip_output.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e35b712..cc7b579 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1120,6 +1120,15 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork,
 	return 0;
 }
 
+static void ip_cork_release(struct inet_cork *cork)
+{
+	cork->flags &= ~IPCORK_OPT;
+	kfree(cork->opt);
+	cork->opt = NULL;
+	dst_release(cork->dst);
+	cork->dst = NULL;
+}
+
 /*
  *	ip_append_data() and ip_append_page() can make one large IP datagram
  *	from many pieces of data. Each pieces will be holded on the socket
@@ -1152,9 +1161,14 @@ int ip_append_data(struct sock *sk, struct flowi4 *fl4,
 		transhdrlen = 0;
 	}
 
-	return __ip_append_data(sk, fl4, &sk->sk_write_queue, &inet->cork.base,
+	err = __ip_append_data(sk, fl4, &sk->sk_write_queue, &inet->cork.base,
 				sk_page_frag(sk), getfrag,
 				from, length, transhdrlen, flags);
+
+	if (skb_queue_empty(&sk->sk_write_queue))
+		ip_cork_release(&inet->cork.base);
+
+	return err;
 }
 
 ssize_t	ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
@@ -1304,15 +1318,6 @@ error:
 	return err;
 }
 
-static void ip_cork_release(struct inet_cork *cork)
-{
-	cork->flags &= ~IPCORK_OPT;
-	kfree(cork->opt);
-	cork->opt = NULL;
-	dst_release(cork->dst);
-	cork->dst = NULL;
-}
-
 /*
  *	Combined all pending IP fragments on the socket as one IP datagram
  *	and push them out.
-- 
1.9.1