n_tty: check for negative and zero space return from tty_write_room

mainline inclusion
from mainline-5.1
commit 9ef8927f45f2
category: bugfix
bugzilla: 15274
CVE: NA

-------------------------------------------------

The return from tty_write_room could potentially be negative if
a tty write_room driver returns an error number (not that any seem
to do). Rather than just check for a zero return, also check for
a -ve return. This avoids the unsigned nr being set to a large unsigned
value on the assignment from variable space and can lead to overflowing
the buffer buf.  Better to be safe than assume all write_room
implementations in tty drivers are going to do the right thing.
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NYao Hongbo <yaohongbo@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/tty/n_tty.c

@@ -548,9 +548,9 @@ static ssize_t process_output_block(struct tty_struct *tty,
 	mutex_lock(&ldata->output_lock);
 
 	space = tty_write_room(tty);
-	if (!space) {
+	if (space <= 0) {
 		mutex_unlock(&ldata->output_lock);
-		return 0;
+		return space;
 	}
 	if (nr > space)
 		nr = space;