netfilter: xt_owner: Fix for unsafe access of sk->sk_socket

A concurrently running sock_orphan() may NULL the sk_socket pointer in
between check and deref. Follow other users (like nft_meta.c for
instance) and acquire sk_callback_lock before dereferencing sk_socket.

Fixes: 0265ab44 ("[NETFILTER]: merge ipt_owner/ip6t_owner in xt_owner")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_owner.c

@@ -76,19 +76,24 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 		 */
 		return false;
 
-	filp = sk->sk_socket->file;
-	if (filp == NULL)
+	read_lock_bh(&sk->sk_callback_lock);
+	filp = sk->sk_socket ? sk->sk_socket->file : NULL;
+	if (filp == NULL) {
+		read_unlock_bh(&sk->sk_callback_lock);
 		return ((info->match ^ info->invert) &
 		       (XT_OWNER_UID | XT_OWNER_GID)) == 0;
+	}
 
 	if (info->match & XT_OWNER_UID) {
 		kuid_t uid_min = make_kuid(net->user_ns, info->uid_min);
 		kuid_t uid_max = make_kuid(net->user_ns, info->uid_max);
 		if ((uid_gte(filp->f_cred->fsuid, uid_min) &&
 		     uid_lte(filp->f_cred->fsuid, uid_max)) ^
-		    !(info->invert & XT_OWNER_UID))
+		    !(info->invert & XT_OWNER_UID)) {
+			read_unlock_bh(&sk->sk_callback_lock);
 			return false;
 		}
+	}
 
 	if (info->match & XT_OWNER_GID) {
 		unsigned int i, match = false;
@@ -112,10 +117,13 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 			}
 		}
 
-		if (match ^ !(info->invert & XT_OWNER_GID))
+		if (match ^ !(info->invert & XT_OWNER_GID)) {
+			read_unlock_bh(&sk->sk_callback_lock);
 			return false;
 		}
+	}
 
+	read_unlock_bh(&sk->sk_callback_lock);
 	return true;
 }