[PATCH net v2] udp6: set dst cache for a connected sk before udp_v6_send_skb

[Alexey Kodanev <alexey.kodanev@oracle.com>]

After commit 33c162a980fe ("ipv6: datagram: Update dst cache of a
connected datagram sk during pmtu update"), when the error occurs on
sending datagram in udpv6_sendmsg() due to ICMPV6_PKT_TOOBIG type,
error handler can trigger the following path and call ip6_dst_store():

    udpv6_err()
        ip6_sk_update_pmtu()
            ip6_datagram_dst_update()
                ip6_dst_lookup_flow(), can create a RTF_CACHE clone
                ...
                ip6_dst_store()

It can happen before a connected UDP socket invokes ip6_dst_store()
in the end of udpv6_sendmsg(), on destination release, as a result,
the last one changes dst to the old one, preventing getting updated
dst cache on the next udpv6_sendmsg() call.

This patch moves ip6_dst_store() in udpv6_sendmsg(), so that it is
invoked after ip6_sk_dst_lookup_flow() and before udp_v6_send_skb().

Also, increase refcnt for dst, when passing it to ip6_dst_store()
because after that the dst cache can be released by other calls
to ip6_dst_store() with the same socket.

Fixes: 33c162a980fe ("ipv6: datagram: Update dst cache of a connected datagram sk during pmtu update")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
---

v2: * remove 'release_dst:' label

    * move ip6_dst_store() below MSG_CONFIRM check as
      suggested by Eric and add dst_clone()

    * add 'Fixes' commit.


 net/ipv6/udp.c | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 52e3ea0..4508e5a 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1303,6 +1303,16 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		goto do_confirm;
 back_from_confirm:
 
+	if (connected)
+		ip6_dst_store(sk, dst_clone(dst),
+			      ipv6_addr_equal(&fl6.daddr, &sk->sk_v6_daddr) ?
+			      &sk->sk_v6_daddr : NULL,
+#ifdef CONFIG_IPV6_SUBTREES
+			      ipv6_addr_equal(&fl6.saddr, &np->saddr) ?
+			      &np->saddr :
+#endif
+			      NULL);
+
 	/* Lockless fast path for the non-corking case */
 	if (!corkreq) {
 		struct sk_buff *skb;
@@ -1314,7 +1324,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		err = PTR_ERR(skb);
 		if (!IS_ERR_OR_NULL(skb))
 			err = udp_v6_send_skb(skb, &fl6);
-		goto release_dst;
+		goto out;
 	}
 
 	lock_sock(sk);
@@ -1348,23 +1358,6 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		err = np->recverr ? net_xmit_errno(err) : 0;
 	release_sock(sk);
 
-release_dst:
-	if (dst) {
-		if (connected) {
-			ip6_dst_store(sk, dst,
-				      ipv6_addr_equal(&fl6.daddr, &sk->sk_v6_daddr) ?
-				      &sk->sk_v6_daddr : NULL,
-#ifdef CONFIG_IPV6_SUBTREES
-				      ipv6_addr_equal(&fl6.saddr, &np->saddr) ?
-				      &np->saddr :
-#endif
-				      NULL);
-		} else {
-			dst_release(dst);
-		}
-		dst = NULL;
-	}
-
 out:
 	dst_release(dst);
 	fl6_sock_release(flowlabel);
-- 
1.8.3.1