security: smack: Fix possible null-pointer dereferences in smack_socket_sock_rcv_skb()

In smack_socket_sock_rcv_skb(), there is an if statement
on line 3920 to check whether skb is NULL:
    if (skb && skb->secmark != 0)

This check indicates skb can be NULL in some cases.

But on lines 3931 and 3932, skb is used:
    ad.a.u.net->netif = skb->skb_iif;
    ipv6_skb_to_auditdata(skb, &ad.a, NULL);

Thus, possible null-pointer dereferences may occur when skb is NULL.

To fix these possible bugs, an if statement is added to check skb.

These bugs are found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -3923,6 +3923,8 @@ static int smack_socket_sock_rcv_skb(struct sock *sk, struct sk_buff *skb)
 			skp = smack_ipv6host_label(&sadd);
 		if (skp == NULL)
 			skp = smack_net_ambient;
+		if (skb == NULL)
+			break;
 #ifdef CONFIG_AUDIT
 		smk_ad_init_net(&ad, __func__, LSM_AUDIT_DATA_NET, &net);
 		ad.a.u.net->family = family;