[PATCH 3.12 52/83] tcp: prevent fetching dst twice in early demux code

[Jiri Slaby]

From: Michal KubeÄek <mkubecek@xxxxxxx>

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

===============

[ 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 ACCESS_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@xxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 net/ipv4/tcp_ipv4.c | 2 +-
 net/ipv6/tcp_ipv6.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index aae282839bde..68b409d1afa7 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1875,7 +1875,7 @@ void tcp_v4_early_demux(struct sk_buff *skb)
 		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 = ACCESS_ONCE(sk->sk_rx_dst);
 
 			if (dst)
 				dst = dst_check(dst, 0);
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 3058c4a89b3b..03e3723c8760 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1616,7 +1616,7 @@ static void tcp_v6_early_demux(struct sk_buff *skb)
 		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 = ACCESS_ONCE(sk->sk_rx_dst);
 
 			if (dst)
 				dst = dst_check(dst, inet6_sk(sk)->rx_dst_cookie);
-- 
2.3.5

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