net/smc: prevent NULL pointer dereference in txopt_get

Since smc_inet6_prot does not initialize ipv6_pinfo_offset, inet6_create()
copies an incorrect address value, sk + 0 (offset), to inet_sk(sk)->pinet6.

In addition, since inet_sk(sk)->pinet6 and smc_sk(sk)->clcsock practically
point to the same address, when smc_create_clcsk() stores the newly
created clcsock in smc_sk(sk)->clcsock, inet_sk(sk)->pinet6 is corrupted
into clcsock. This causes NULL pointer dereference and various other
memory corruptions.

To solve this problem, you need to initialize ipv6_pinfo_offset, add a
smc6_sock structure, and then add ipv6_pinfo as the second member of
the smc_sock structure.

Reported-by: syzkaller <syzkaller@googlegroups.com>
Fixes: d25a92c ("net/smc: Introduce IPPROTO_SMC")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/smc/smc.h

@@ -284,6 +284,9 @@ struct smc_connection {
 
 struct smc_sock {				/* smc sock container */
 	struct sock		sk;
+#if IS_ENABLED(CONFIG_IPV6)
+	struct ipv6_pinfo	*pinet6;
+#endif
 	struct socket		*clcsock;	/* internal tcp socket */
 	void			(*clcsk_state_change)(struct sock *sk);
 						/* original stat_change fct. */

net/smc/smc_inet.c

@@ -60,16 +60,22 @@ static struct inet_protosw smc_inet_protosw = {
 };
 
 #if IS_ENABLED(CONFIG_IPV6)
+struct smc6_sock {
+	struct smc_sock		smc;
+	struct ipv6_pinfo	inet6;
+};
+
 static struct proto smc_inet6_prot = {
 	.name		= "INET6_SMC",
 	.owner		= THIS_MODULE,
 	.init		= smc_inet_init_sock,
 	.hash		= smc_hash_sk,
 	.unhash		= smc_unhash_sk,
 	.release_cb	= smc_release_cb,
-	.obj_size	= sizeof(struct smc_sock),
+	.obj_size	= sizeof(struct smc6_sock),
 	.h.smc_hash	= &smc_v6_hashinfo,
 	.slab_flags	= SLAB_TYPESAFE_BY_RCU,
+	.ipv6_pinfo_offset	= offsetof(struct smc6_sock, inet6),
 };
 
 static const struct proto_ops smc_inet6_stream_ops = {