udp6: Fix race condition in udp6_sendmsg & connect

[ Upstream commit 448a5ce112 ]

Syzkaller got the following report:
BUG: KASAN: use-after-free in sk_setup_caps+0x621/0x690 net/core/sock.c:2018
Read of size 8 at addr ffff888027f82780 by task syz-executor276/3255

The function sk_setup_caps (called by ip6_sk_dst_store_flow->
ip6_dst_store) referenced already freed memory as this memory was
freed by parallel task in udpv6_sendmsg->ip6_sk_dst_lookup_flow->
sk_dst_check.

          task1 (connect)              task2 (udp6_sendmsg)
        sk_setup_caps->sk_dst_set |
                                  |  sk_dst_check->
                                  |      sk_dst_set
                                  |      dst_release
        sk_setup_caps references  |
        to already freed dst_entry|

The reason for this race condition is: sk_setup_caps() keeps using
the dst after transferring the ownership to the dst cache.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Vladislav Efanov <VEfanov@ispras.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/core/sock.c

@@ -2165,7 +2165,6 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
 {
 	u32 max_segs = 1;
 
-	sk_dst_set(sk, dst);
 	sk->sk_route_caps = dst->dev->features | sk->sk_route_forced_caps;
 	if (sk->sk_route_caps & NETIF_F_GSO)
 		sk->sk_route_caps |= NETIF_F_GSO_SOFTWARE;
@@ -2180,6 +2179,7 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
 		}
 	}
 	sk->sk_gso_max_segs = max_segs;
+	sk_dst_set(sk, dst);
 }
 EXPORT_SYMBOL_GPL(sk_setup_caps);