ACK: [SRU X/B/C] tty: Don't hold ldisc lock in tty_reopen() if ldisc present

[Colin Ian King]

On 30/01/2019 15:43, Stefan Bader wrote:
> From: Dmitry Safonov <dima at arista.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1813873
> 
> 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 at suse.com>
> Reported-by: Lukas F. Hartmann <lukas at mntmn.com>
> Tested-by: Lukas F. Hartmann <lukas at mntmn.com>
> Cc: stable <stable at vger.kernel.org>
> Signed-off-by: Dmitry Safonov <dima at arista.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> 
> (cherry picked from commit d3736d82e8169768218ee0ef68718875918091a0)
> Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
> ---
> 
> The commits being fixed got requested via LP: #1791758 but
> were missing this follow-up.  Compile-tested on Xenial.
> 
> -Stefan
> 
>  drivers/tty/tty_io.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index bfe9ad85b362..23c6fd238422 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -1256,7 +1256,8 @@ 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)
> @@ -1268,13 +1269,18 @@ static int tty_reopen(struct tty_struct *tty)
>  	if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
>  		return -EBUSY;
>  
> -	retval = tty_ldisc_lock(tty, 5 * HZ);
> -	if (retval)
> -		return retval;
> +	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 (!tty->ldisc)
> +			retval = tty_ldisc_reinit(tty, tty->termios.c_line);
> +		tty_ldisc_unlock(tty);
> +	}
>  
>  	if (retval == 0)
>  		tty->count++;
> 

Cherry pick and a stable fix, so I'm good with this.

Acked-by: Colin Ian King <colin.king at canonical.com>