ipv6: fix possible use-after-free in ip6_xmit()

BugLink: https://bugs.launchpad.net/bugs/1798617

[ Upstream commit bbd6528d ]

In the unlikely case ip6_xmit() has to call skb_realloc_headroom(),
we need to call skb_set_owner_w() before consuming original skb,
otherwise we risk a use-after-free.

Bring IPv6 in line with what we do in IPv4 to fix this.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv6/ip6_output.c

@@ -194,12 +194,10 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 				kfree_skb(skb);
 				return -ENOBUFS;
 			}
+			if (skb->sk)
+				skb_set_owner_w(skb2, skb->sk);
 			consume_skb(skb);
 			skb = skb2;
-			/* skb_set_owner_w() changes sk->sk_wmem_alloc atomically,
-			 * it is safe to call in our context (socket lock not held)
-			 */
-			skb_set_owner_w(skb, (struct sock *)sk);
 		}
 		if (opt->opt_flen)
 			ipv6_push_frag_opts(skb, opt, &proto);