[PATCH 4.19 04/99] tty: Dont hold ldisc lock in tty_reopen() if ldisc present

[Greg Kroah-Hartman]

4.19-stable review patch.  If anyone has any objections, please let me know.

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

From: Dmitry Safonov <d...@arista.com>

commit d3736d82e8169768218ee0ef68718875918091a0 upstream.

Try to get reference for ldisc during tty_reopen().
If ldisc present, we don't need to do tty_ldisc_reinit() and lock the
write side for line discipline semaphore.
Effectively, it optimizes fast-path for tty_reopen(), but more
importantly it won't interrupt ongoing IO on the tty as no ldisc change
is needed.
Fixes user-visible issue when tty_reopen() interrupted login process for
user with a long password, observed and reported by Lukas.

Fixes: c96cf923a98d ("tty: Don't block on IO when ldisc change is pending")
Fixes: 83d817f41070 ("tty: Hold tty_ldisc_lock() during tty_reopen()")
Cc: Jiri Slaby <jsl...@suse.com>
Reported-by: Lukas F. Hartmann <lu...@mntmn.com>
Tested-by: Lukas F. Hartmann <lu...@mntmn.com>
Cc: stable <sta...@vger.kernel.org>
Signed-off-by: Dmitry Safonov <d...@arista.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/tty/tty_io.c |   22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1255,7 +1255,8 @@ static void tty_driver_remove_tty(struct
 static int tty_reopen(struct tty_struct *tty)
 {
        struct tty_driver *driver = tty->driver;
-       int retval;
+       struct tty_ldisc *ld;
+       int retval = 0;
 
        if (driver->type == TTY_DRIVER_TYPE_PTY &&
            driver->subtype == PTY_TYPE_MASTER)
@@ -1267,13 +1268,18 @@ static int tty_reopen(struct tty_struct
        if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
                return -EBUSY;
 
-       retval = tty_ldisc_lock(tty, 5 * HZ);
-       if (retval)
-               return retval;
-
-       if (!tty->ldisc)
-               retval = tty_ldisc_reinit(tty, tty->termios.c_line);
-       tty_ldisc_unlock(tty);
+       ld = tty_ldisc_ref_wait(tty);
+       if (ld) {
+               tty_ldisc_deref(ld);
+       } else {
+               retval = tty_ldisc_lock(tty, 5 * HZ);
+               if (retval)
+                       return retval;
+
+               if (!tty->ldisc)
+                       retval = tty_ldisc_reinit(tty, tty->termios.c_line);
+               tty_ldisc_unlock(tty);
+       }
 
        if (retval == 0)
                tty->count++;