netdev - [PATCH net] ipv6: fix possible use-after-free in ip6

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Message-Id: <20180914190232.184779-1-edumazet@google.com>
Date:   Fri, 14 Sep 2018 12:02:31 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net] ipv6: fix possible use-after-free in ip6_xmit()

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: 1da177e4c3f41 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: syzbot <syzkaller@...glegroups.com>
---
 net/ipv6/ip6_output.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 16f200f06500758c4cae84ea16229d5dbce912cb..f9f8f554d141676a7d342f85088d12d9a6815e9d 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -219,12 +219,10 @@ 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);
-- 
2.19.0.397.gdd90340f6a-goog