[PATCH 3.19 01/27] tcp: prevent fetching dst twice in early demux code

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: =?UTF-8?q?Michal=20Kube=C4=8Dek?= <mkubecek@suse.cz>

[ Upstream commit d0c294c53a771ae7e84506dfbd8c18c30f078735 ]

On s390x, gcc 4.8 compiles this part of tcp_v6_early_demux()

        struct dst_entry *dst = sk->sk_rx_dst;

        if (dst)
                dst = dst_check(dst, inet6_sk(sk)->rx_dst_cookie);

to code reading sk->sk_rx_dst twice, once for the test and once for
the argument of ip6_dst_check() (dst_check() is inline). This allows
ip6_dst_check() to be called with null first argument, causing a crash.

Protect sk->sk_rx_dst access by READ_ONCE() both in IPv4 and IPv6
TCP early demux code.

Fixes: 41063e9dd119 ("ipv4: Early TCP socket demux.")
Fixes: c7109986db3c ("ipv6: Early TCP socket demux")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_ipv4.c |    2 +-
 net/ipv6/tcp_ipv6.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1516,7 +1516,7 @@ void tcp_v4_early_demux(struct sk_buff *
 		skb->sk = sk;
 		skb->destructor = sock_edemux;
 		if (sk->sk_state != TCP_TIME_WAIT) {
-			struct dst_entry *dst = sk->sk_rx_dst;
+			struct dst_entry *dst = READ_ONCE(sk->sk_rx_dst);
 
 			if (dst)
 				dst = dst_check(dst, 0);
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1583,7 +1583,7 @@ static void tcp_v6_early_demux(struct sk
 		skb->sk = sk;
 		skb->destructor = sock_edemux;
 		if (sk->sk_state != TCP_TIME_WAIT) {
-			struct dst_entry *dst = sk->sk_rx_dst;
+			struct dst_entry *dst = READ_ONCE(sk->sk_rx_dst);
 
 			if (dst)
 				dst = dst_check(dst, inet6_sk(sk)->rx_dst_cookie);