Code: static int iucv_sock_recvmsg(struct kiocb *iocb, struct socket *sock,
			     struct msghdr *msg, size_t len, int flags)
{
	int noblock = flags & MSG_DONTWAIT;
	struct sock *sk = sock->sk;
	struct iucv_sock *iucv = iucv_sk(sk);
	unsigned int copied, rlen;
	struct sk_buff *skb, *rskb, *cskb;
 	int err = 0;
 	u32 offset;
 
	msg->msg_namelen = 0;
 	if ((sk->sk_state == IUCV_DISCONN) &&
 	    skb_queue_empty(&iucv->backlog_skb_q) &&
 	    skb_queue_empty(&sk->sk_receive_queue) &&
	    list_empty(&iucv->message_q.list))
		return 0;

	if (flags & (MSG_OOB))
		return -EOPNOTSUPP;

	/* receive/dequeue next skb:
	 * the function understands MSG_PEEK and, thus, does not dequeue skb */
	skb = skb_recv_datagram(sk, flags, noblock, &err);
	if (!skb) {
		if (sk->sk_shutdown & RCV_SHUTDOWN)
			return 0;
		return err;
	}

	offset = IUCV_SKB_CB(skb)->offset;
	rlen   = skb->len - offset;		/* real length of skb */
	copied = min_t(unsigned int, rlen, len);
	if (!rlen)
		sk->sk_shutdown = sk->sk_shutdown | RCV_SHUTDOWN;

	cskb = skb;
	if (skb_copy_datagram_iovec(cskb, offset, msg->msg_iov, copied)) {
		if (!(flags & MSG_PEEK))
			skb_queue_head(&sk->sk_receive_queue, skb);
		return -EFAULT;
	}

	/* SOCK_SEQPACKET: set MSG_TRUNC if recv buf size is too small */
	if (sk->sk_type == SOCK_SEQPACKET) {
		if (copied < rlen)
			msg->msg_flags |= MSG_TRUNC;
		/* each iucv message contains a complete record */
		msg->msg_flags |= MSG_EOR;
	}

	/* create control message to store iucv msg target class:
	 * get the trgcls from the control buffer of the skb due to
	 * fragmentation of original iucv message. */
	err = put_cmsg(msg, SOL_IUCV, SCM_IUCV_TRGCLS,
		       sizeof(IUCV_SKB_CB(skb)->class),
		       (void *)&IUCV_SKB_CB(skb)->class);
	if (err) {
		if (!(flags & MSG_PEEK))
			skb_queue_head(&sk->sk_receive_queue, skb);
		return err;
	}

	/* Mark read part of skb as used */
	if (!(flags & MSG_PEEK)) {

		/* SOCK_STREAM: re-queue skb if it contains unreceived data */
		if (sk->sk_type == SOCK_STREAM) {
			if (copied < rlen) {
				IUCV_SKB_CB(skb)->offset = offset + copied;
				goto done;
			}
		}

		kfree_skb(skb);
		if (iucv->transport == AF_IUCV_TRANS_HIPER) {
			atomic_inc(&iucv->msg_recv);
			if (atomic_read(&iucv->msg_recv) > iucv->msglimit) {
				WARN_ON(1);
				iucv_sock_close(sk);
				return -EFAULT;
			}
		}

		/* Queue backlog skbs */
		spin_lock_bh(&iucv->message_q.lock);
		rskb = skb_dequeue(&iucv->backlog_skb_q);
		while (rskb) {
			IUCV_SKB_CB(rskb)->offset = 0;
			if (sock_queue_rcv_skb(sk, rskb)) {
				skb_queue_head(&iucv->backlog_skb_q,
						rskb);
				break;
			} else {
				rskb = skb_dequeue(&iucv->backlog_skb_q);
			}
		}
		if (skb_queue_empty(&iucv->backlog_skb_q)) {
			if (!list_empty(&iucv->message_q.list))
				iucv_process_message_q(sk);
			if (atomic_read(&iucv->msg_recv) >=
							iucv->msglimit / 2) {
				err = iucv_send_ctrl(sk, AF_IUCV_FLAG_WIN);
				if (err) {
					sk->sk_state = IUCV_DISCONN;
					sk->sk_state_change(sk);
				}
			}
		}
		spin_unlock_bh(&iucv->message_q.lock);
	}

done:
	/* SOCK_SEQPACKET: return real length if MSG_TRUNC is set */
	if (sk->sk_type == SOCK_SEQPACKET && (flags & MSG_TRUNC))
		copied = rlen;

	return copied;
}

Vulnerability Type: +Info 
CWE ID: CWE-20
Summary: The x25_recvmsg function in net/x25/af_x25.c in the Linux kernel before 3.12.4 updates a certain length value without ensuring that an associated data structure has been initialized, which allows local users to obtain sensitive information from kernel memory via a (1) recvfrom, (2) recvmmsg, or (3) recvmsg system call.
Commit Message: net: rework recvmsg handler msg_name and msg_namelen logic

This patch now always passes msg->msg_namelen as 0. recvmsg handlers must
set msg_namelen to the proper size <= sizeof(struct sockaddr_storage)
to return msg_name to the user.

This prevents numerous uninitialized memory leaks we had in the
recvmsg handlers and makes it harder for new code to accidentally leak
uninitialized memory.

Optimize for the case recvfrom is called with NULL as address. We don't
need to copy the address at all, so set it to NULL before invoking the
recvmsg handler. We can do so, because all the recvmsg handlers must
cope with the case a plain read() is called on them. read() also sets
msg_name to NULL.

Also document these changes in include/linux/net.h as suggested by David
Miller.

Changes since RFC:

Set msg->msg_name = NULL if user specified a NULL in msg_name but had a
non-null msg_namelen in verify_iovec/verify_compat_iovec. This doesn't
affect sendto as it would bail out earlier while trying to copy-in the
address. It also more naturally reflects the logic by the callers of
verify_iovec.

With this change in place I could remove "
if (!uaddr || msg_sys->msg_namelen == 0)
	msg->msg_name = NULL
".

This change does not alter the user visible error logic as we ignore
msg_namelen as long as msg_name is NULL.

Also remove two unnecessary curly brackets in ___sys_recvmsg and change
comments to netdev style.

Cc: David Miller <davem@davemloft.net>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>