n_tty: Fix stall at n_tty_receive_char_special().

syzbot is reporting stalls at n_tty_receive_char_special() [1]. This is
because comparison is not working as expected since ldata->read_head can
change at any moment. Mitigate this by explicitly masking with buffer size
when checking condition for "while" loops.

[1] https://syzkaller.appspot.com/bug?id=3d7481a346958d9469bebbeb0537d5f056bdd6e8

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+18df353d7540aa6b5467@syzkaller.appspotmail.com>
Fixes: bc5a5e3f45d04784 ("n_tty: Don't wrap input buffer indices at buffer size")
Cc: stable <stable@vger.kernel.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -124,6 +124,8 @@ struct n_tty_data {
 	struct mutex output_lock;
 };
 
+#define MASK(x) ((x) & (N_TTY_BUF_SIZE - 1))
+
 static inline size_t read_cnt(struct n_tty_data *ldata)
 {
 	return ldata->read_head - ldata->read_tail;
@@ -978,14 +980,15 @@ static void eraser(unsigned char c, struct tty_struct *tty)
 	}
 
 	seen_alnums = 0;
-	while (ldata->read_head != ldata->canon_head) {
+	while (MASK(ldata->read_head) != MASK(ldata->canon_head)) {
 		head = ldata->read_head;
 
 		/* erase a single possibly multibyte character */
 		do {
 			head--;
 			c = read_buf(ldata, head);
-		} while (is_continuation(c, tty) && head != ldata->canon_head);
+		} while (is_continuation(c, tty) &&
+			 MASK(head) != MASK(ldata->canon_head));
 
 		/* do not partially erase */
 		if (is_continuation(c, tty))
@@ -1027,7 +1030,7 @@ static void eraser(unsigned char c, struct tty_struct *tty)
 				 * This info is used to go back the correct
 				 * number of columns.
 				 */
-				while (tail != ldata->canon_head) {
+				while (MASK(tail) != MASK(ldata->canon_head)) {
 					tail--;
 					c = read_buf(ldata, tail);
 					if (c == '\t') {
@@ -1302,7 +1305,7 @@ n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
 			finish_erasing(ldata);
 			echo_char(c, tty);
 			echo_char_raw('\n', ldata);
-			while (tail != ldata->read_head) {
+			while (MASK(tail) != MASK(ldata->read_head)) {
 				echo_char(read_buf(ldata, tail), tty);
 				tail++;
 			}
@@ -2411,7 +2414,7 @@ static unsigned long inq_canon(struct n_tty_data *ldata)
 	tail = ldata->read_tail;
 	nr = head - tail;
 	/* Skip EOF-chars.. */
-	while (head != tail) {
+	while (MASK(head) != MASK(tail)) {
 		if (test_bit(tail & (N_TTY_BUF_SIZE - 1), ldata->read_flags) &&
 		    read_buf(ldata, tail) == __DISABLED_CHAR)
 			nr--;