s390/vmcp: fix uaccess check and avoid undefined behavior

The vmcp device driver should return -EFAULT if get_user() fails, due
to an invalid user space address. In addition the buffer size value
from user space is passed unchecked to get_order(). The return value
of get_order(0) undefined.

Therefore explicitly test for zero before calling get_order() and also
return -EFAULT if get_user() fails.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/vmcp.c

@@ -150,7 +150,9 @@ static long vmcp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 				get_order(session->bufsize));
 		session->response=NULL;
 		temp = get_user(session->bufsize, argp);
-		if (get_order(session->bufsize) > 8) {
+		if (temp)
+			session->bufsize = PAGE_SIZE;
+		if (!session->bufsize || get_order(session->bufsize) > 8) {
 			session->bufsize = PAGE_SIZE;
 			temp = -EINVAL;
 		}