net: validate the range we feed to iov_iter_init() in sys_sendto/sys_recvfrom

Cc: stable@vger.kernel.org # v3.19
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
int kernel_recvmsg(struct socket *sock, struct msghdr *msg,
		   struct kvec *vec, size_t num, size_t size, int flags)
{
	mm_segment_t oldfs = get_fs();
	int result;

	set_fs(KERNEL_DS);
	/*
	 * the following is safe, since for compiler definitions of kvec and
	 * iovec are identical, yielding the same in-core layout and alignment
	 */
	iov_iter_init(&msg->msg_iter, READ, (struct iovec *)vec, num, size);
	result = sock_recvmsg(sock, msg, size, flags);
	set_fs(oldfs);
	return result;
}

int kernel_recvmsg(struct socket *sock, struct msghdr *msg,
		   struct kvec *vec, size_t num, size_t size, int flags)
{
	mm_segment_t oldfs = get_fs();
	int result;

	set_fs(KERNEL_DS);
	/*
	 * the following is safe, since for compiler definitions of kvec and
	 * iovec are identical, yielding the same in-core layout and alignment
	 */
	iov_iter_init(&msg->msg_iter, READ, (struct iovec *)vec, num, size);
	result = sock_recvmsg(sock, msg, size, flags);
	set_fs(oldfs);
	return result;
}
