Commit cb7f9a46 authored Jan 09, 2019 by Dmitry Safonov Committed by Greg Kroah-Hartman Jan 23, 2019

tty: Don't hold ldisc lock in tty_reopen() if ldisc present



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 <jslaby@suse.com>
Reported-by: Lukas F. Hartmann <lukas@mntmn.com>
Tested-by: Lukas F. Hartmann <lukas@mntmn.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 4086e287

drivers/tty/tty_io.c

+13 −7

Original line number	Diff line number	Diff line
		@@ -1254,7 +1254,8 @@ static void tty_driver_remove_tty(struct tty_driver driver, struct 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)
		@@ -1266,6 +1267,10 @@ static int tty_reopen(struct tty_struct *tty)
		if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
		return -EBUSY;

		ld = tty_ldisc_ref_wait(tty);
		if (ld) {
		tty_ldisc_deref(ld);
		} else {
		retval = tty_ldisc_lock(tty, 5 * HZ);
		if (retval)
		return retval;
		@@ -1273,6 +1278,7 @@ static int tty_reopen(struct tty_struct *tty)
		if (!tty->ldisc)
		retval = tty_ldisc_reinit(tty, tty->termios.c_line);
		tty_ldisc_unlock(tty);
		}

		if (retval == 0)
		tty->count++;