[PATCH net] ipv6: tcp: fix race in IPV6_2292PKTOPTIONS

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

IPv6 TCP sockets store in np->pktoptions skbs, and use skb_set_owner_r()
to charge the skb to socket.

It means that destructor must be called while socket is locked.

Therefore, we cannot use skb_get() or atomic_inc(&skb->users)
to protect ourselves : kfree_skb() might race with other users
manipulating sk->sk_forward_alloc

Fix this race by holding socket lock for the duration of
ip6_datagram_recv_ctl()

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 Since this bug is very old, feel free to apply on net-next ;)

 net/ipv6/ipv6_sockglue.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 66980d8d98d1f5b3ef7a50dc33cb9b617f25604d..8d766d9100cba408525faf5818b7b0c6b6bc543c 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -996,13 +996,9 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 		lock_sock(sk);
 		skb = np->pktoptions;
 		if (skb)
-			atomic_inc(&skb->users);
-		release_sock(sk);
-
-		if (skb) {
 			ip6_datagram_recv_ctl(sk, &msg, skb);
-			kfree_skb(skb);
-		} else {
+		release_sock(sk);
+		if (!skb) {
 			if (np->rxopt.bits.rxinfo) {
 				struct in6_pktinfo src_info;
 				src_info.ipi6_ifindex = np->mcast_oif ? np->mcast_oif :