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

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

Stefan Bader-2
From: Dmitry Safonov <[hidden email]>

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 <[hidden email]>
Reported-by: Lukas F. Hartmann <[hidden email]>
Tested-by: Lukas F. Hartmann <[hidden email]>
Cc: stable <[hidden email]>
Signed-off-by: Dmitry Safonov <[hidden email]>
Signed-off-by: Greg Kroah-Hartman <[hidden email]>

(cherry picked from commit d3736d82e8169768218ee0ef68718875918091a0)
Signed-off-by: Stefan Bader <[hidden email]>
---

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++;
--
2.17.1


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

Colin Ian King-2
On 30/01/2019 15:43, Stefan Bader wrote:

> From: Dmitry Safonov <[hidden email]>
>
> 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 <[hidden email]>
> Reported-by: Lukas F. Hartmann <[hidden email]>
> Tested-by: Lukas F. Hartmann <[hidden email]>
> Cc: stable <[hidden email]>
> Signed-off-by: Dmitry Safonov <[hidden email]>
> Signed-off-by: Greg Kroah-Hartman <[hidden email]>
>
> (cherry picked from commit d3736d82e8169768218ee0ef68718875918091a0)
> Signed-off-by: Stefan Bader <[hidden email]>
> ---
>
> 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 <[hidden email]>

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

Khaled Elmously
In reply to this post by Stefan Bader-2
On 2019-01-30 16:43:23 , Stefan Bader wrote:

> From: Dmitry Safonov <[hidden email]>
>
> 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 <[hidden email]>
> Reported-by: Lukas F. Hartmann <[hidden email]>
> Tested-by: Lukas F. Hartmann <[hidden email]>
> Cc: stable <[hidden email]>
> Signed-off-by: Dmitry Safonov <[hidden email]>
> Signed-off-by: Greg Kroah-Hartman <[hidden email]>
>
> (cherry picked from commit d3736d82e8169768218ee0ef68718875918091a0)
> Signed-off-by: Stefan Bader <[hidden email]>
> ---
>
> 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++;

Acked-by: Khalid Elmously <[hidden email]>


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

Khaled Elmously
In reply to this post by Stefan Bader-2
On 2019-01-30 16:43:23 , Stefan Bader wrote:

> From: Dmitry Safonov <[hidden email]>
>
> 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 <[hidden email]>
> Reported-by: Lukas F. Hartmann <[hidden email]>
> Tested-by: Lukas F. Hartmann <[hidden email]>
> Cc: stable <[hidden email]>
> Signed-off-by: Dmitry Safonov <[hidden email]>
> Signed-off-by: Greg Kroah-Hartman <[hidden email]>
>
> (cherry picked from commit d3736d82e8169768218ee0ef68718875918091a0)
> Signed-off-by: Stefan Bader <[hidden email]>
> ---
>
> 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++;
> --
> 2.17.1
>
>
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