tty: tty_open can return to userspace holding tty_mutex

tty: tty_open can return to userspace holding tty_mutex

[Eric Paris]

__tty_open could return (to userspace) holding the tty_mutex thanks to a
regression introduced by 4a2b5fddd53b80efcb3266ee36e23b8de28e761a.  This was
found by bisecting an fsfuzzer problem.  Admittedly I have no idea how it
managed to tickle this 100% reliably, but it is clearly a regression and
when hit leaves the box in a completely unusable state.  This patch lets
the fsfuzzer test complete every time.
    
Signed-off-by: Eric Paris <eparis@redhat.com>

---
 drivers/char/tty_io.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index d33e5ab..bc84e12 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1817,8 +1817,10 @@ got_driver:
 		/* check whether we're reopening an existing tty */
 		tty = tty_driver_lookup_tty(driver, inode, index);
 
-		if (IS_ERR(tty))
+		if (IS_ERR(tty)) {
+			mutex_unlock(&tty_mutex);
 			return PTR_ERR(tty);
+		}
 	}
 
 	if (tty) {

Re: tty: tty_open can return to userspace holding tty_mutex

[Alan Cox]

On Mon, 26 Jan 2009 16:02:40 -0500
Eric Paris <eparis@redhat.com> wrote:

> __tty_open could return (to userspace) holding the tty_mutex thanks to a
> regression introduced by 4a2b5fddd53b80efcb3266ee36e23b8de28e761a.  This was
> found by bisecting an fsfuzzer problem.  Admittedly I have no idea how it
> managed to tickle this 100% reliably, but it is clearly a regression and
> when hit leaves the box in a completely unusable state.  This patch lets
> the fsfuzzer test complete every time.

Well spotted, and will go fix now. I think your fsfuzzer created a device
node for an out of range tty subdevice

Re: tty: tty_open can return to userspace holding tty_mutex

[Michael Tokarev]

Eric Paris wrote:
> __tty_open could return (to userspace) holding the tty_mutex thanks to a
> regression introduced by 4a2b5fddd53b80efcb3266ee36e23b8de28e761a.  This was
> found by bisecting an fsfuzzer problem.  Admittedly I have no idea how it
> managed to tickle this 100% reliably, but it is clearly a regression and
> when hit leaves the box in a completely unusable state.  This patch lets
> the fsfuzzer test complete every time.

This should go to -stable too, right?
It's present in 2.6.28 (but not 2.6.27).

/mjt

> Signed-off-by: Eric Paris <eparis@redhat.com>
> 
> ---
>  drivers/char/tty_io.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
> index d33e5ab..bc84e12 100644
> --- a/drivers/char/tty_io.c
> +++ b/drivers/char/tty_io.c
> @@ -1817,8 +1817,10 @@ got_driver:
>  		/* check whether we're reopening an existing tty */
>  		tty = tty_driver_lookup_tty(driver, inode, index);
>  
> -		if (IS_ERR(tty))
> +		if (IS_ERR(tty)) {
> +			mutex_unlock(&tty_mutex);
>  			return PTR_ERR(tty);
> +		}
>  	}
>  
>  	if (tty) {

Re: tty: tty_open can return to userspace holding tty_mutex

[Sukadev Bhattiprolu]

Eric Paris [eparis@redhat.com] wrote:
| __tty_open could return (to userspace) holding the tty_mutex thanks to a
| regression introduced by 4a2b5fddd53b80efcb3266ee36e23b8de28e761a.  This was
| found by bisecting an fsfuzzer problem.  Admittedly I have no idea how it
| managed to tickle this 100% reliably, but it is clearly a regression and
| when hit leaves the box in a completely unusable state.  This patch lets
| the fsfuzzer test complete every time.

Good catch.


|     
| Signed-off-by: Eric Paris <eparis@redhat.com>

Acked-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>

| 
| ---
|  drivers/char/tty_io.c |    4 +++-
|  1 files changed, 3 insertions(+), 1 deletions(-)
| 
| diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
| index d33e5ab..bc84e12 100644
| --- a/drivers/char/tty_io.c
| +++ b/drivers/char/tty_io.c
| @@ -1817,8 +1817,10 @@ got_driver:
|  		/* check whether we're reopening an existing tty */
|  		tty = tty_driver_lookup_tty(driver, inode, index);
| 
| -		if (IS_ERR(tty))
| +		if (IS_ERR(tty)) {
| +			mutex_unlock(&tty_mutex);
|  			return PTR_ERR(tty);
| +		}
|  	}
| 
|  	if (tty) {
|