net/sctp: Validate parameter size for SCTP_GET_ASSOC_STATS

Building sctp may fail with:

In function ‘copy_from_user’,
    inlined from ‘sctp_getsockopt_assoc_stats’ at
    net/sctp/socket.c:5656:20:
arch/x86/include/asm/uaccess_32.h:211:26: error: call to
    ‘copy_from_user_overflow’ declared with attribute error: copy_from_user()
    buffer size is not provably correct

if built with W=1 due to a missing parameter size validation
before the call to copy_from_user.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
SCTP_STATIC int sctp_ioctl(struct sock *sk, int cmd, unsigned long arg)
{
	int rc = -ENOTCONN;

	sctp_lock_sock(sk);

	/*
	 * SEQPACKET-style sockets in LISTENING state are valid, for
	 * SCTP, so only discard TCP-style sockets in LISTENING state.
	 */
	if (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))
		goto out;

	switch (cmd) {
	case SIOCINQ: {
		struct sk_buff *skb;
		unsigned int amount = 0;

		skb = skb_peek(&sk->sk_receive_queue);
		if (skb != NULL) {
			/*
			 * We will only return the amount of this packet since
			 * that is all that will be read.
			 */
			amount = skb->len;
		}
		rc = put_user(amount, (int __user *)arg);
		break;
	}
	default:
		rc = -ENOIOCTLCMD;
		break;
	}
out:
	sctp_release_sock(sk);
	return rc;
}

SCTP_STATIC int sctp_ioctl(struct sock *sk, int cmd, unsigned long arg)
{
	int rc = -ENOTCONN;

	sctp_lock_sock(sk);

	/*
	 * SEQPACKET-style sockets in LISTENING state are valid, for
	 * SCTP, so only discard TCP-style sockets in LISTENING state.
	 */
	if (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))
		goto out;

	switch (cmd) {
	case SIOCINQ: {
		struct sk_buff *skb;
		unsigned int amount = 0;

		skb = skb_peek(&sk->sk_receive_queue);
		if (skb != NULL) {
			/*
			 * We will only return the amount of this packet since
			 * that is all that will be read.
			 */
			amount = skb->len;
		}
		rc = put_user(amount, (int __user *)arg);
		break;
	}
	default:
		rc = -ENOIOCTLCMD;
		break;
	}
out:
	sctp_release_sock(sk);
	return rc;
}
