selinux: properly handle multiple messages in selinux_netlink_send()

Fix the SELinux netlink_send hook to properly handle multiple netlink
messages in a single sk_buff; each message is parsed and subject to
SELinux access control.  Prior to this patch, SELinux only inspected
the first message in the sk_buff.

Cc: stable@vger.kernel.org
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -5842,40 +5842,60 @@ static unsigned int selinux_ipv6_postroute(void *priv,
 
 static int selinux_netlink_send(struct sock *sk, struct sk_buff *skb)
 {
-	int err = 0;
-	u32 perm;
+	int rc = 0;
+	unsigned int msg_len;
+	unsigned int data_len = skb->len;
+	unsigned char *data = skb->data;
 	struct nlmsghdr *nlh;
 	struct sk_security_struct *sksec = sk->sk_security;
+	u16 sclass = sksec->sclass;
+	u32 perm;
 
-	if (skb->len < NLMSG_HDRLEN) {
-		err = -EINVAL;
-		goto out;
-	}
-	nlh = nlmsg_hdr(skb);
+	while (data_len >= nlmsg_total_size(0)) {
+		nlh = (struct nlmsghdr *)data;
+
+		/* NOTE: the nlmsg_len field isn't reliably set by some netlink
+		 *       users which means we can't reject skb's with bogus
+		 *       length fields; our solution is to follow what
+		 *       netlink_rcv_skb() does and simply skip processing at
+		 *       messages with length fields that are clearly junk
+		 */
+		if (nlh->nlmsg_len < NLMSG_HDRLEN || nlh->nlmsg_len > data_len)
+			return 0;
 
-	err = selinux_nlmsg_lookup(sksec->sclass, nlh->nlmsg_type, &perm);
-	if (err) {
-		if (err == -EINVAL) {
+		rc = selinux_nlmsg_lookup(sclass, nlh->nlmsg_type, &perm);
+		if (rc == 0) {
+			rc = sock_has_perm(sk, perm);
+			if (rc)
+				return rc;
+		} else if (rc == -EINVAL) {
+			/* -EINVAL is a missing msg/perm mapping */
 			pr_warn_ratelimited("SELinux: unrecognized netlink"
-			       " message: protocol=%hu nlmsg_type=%hu sclass=%s"
-			       " pid=%d comm=%s\n",
-			       sk->sk_protocol, nlh->nlmsg_type,
-			       secclass_map[sksec->sclass - 1].name,
-			       task_pid_nr(current), current->comm);
-			if (!enforcing_enabled(&selinux_state) ||
-			    security_get_allow_unknown(&selinux_state))
-				err = 0;
+				" message: protocol=%hu nlmsg_type=%hu sclass=%s"
+				" pid=%d comm=%s\n",
+				sk->sk_protocol, nlh->nlmsg_type,
+				secclass_map[sclass - 1].name,
+				task_pid_nr(current), current->comm);
+			if (enforcing_enabled(&selinux_state) &&
+			    !security_get_allow_unknown(&selinux_state))
+				return rc;
+			rc = 0;
+		} else if (rc == -ENOENT) {
+			/* -ENOENT is a missing socket/class mapping, ignore */
+			rc = 0;
+		} else {
+			return rc;
 		}
 
-		/* Ignore */
-		if (err == -ENOENT)
-			err = 0;
-		goto out;
+		/* move to the next message after applying netlink padding */
+		msg_len = NLMSG_ALIGN(nlh->nlmsg_len);
+		if (msg_len >= data_len)
+			return 0;
+		data_len -= msg_len;
+		data += msg_len;
 	}
 
-	err = sock_has_perm(sk, perm);
-out:
-	return err;
+	return rc;
 }
 
 static void ipc_init_security(struct ipc_security_struct *isec, u16 sclass)