Re: [PATCH v2] tty: hvc: Fix data abort due to race in hvc_open

[rananta]

On 2020-05-20 01:59, Jiri Slaby wrote:
> On 20. 05. 20, 8:47, Raghavendra Rao Ananta wrote:
> > Potentially, hvc_open() can be called in parallel when two tasks calls
> > open() on /dev/hvcX. In such a scenario, if the 
> > hp->ops->notifier_add()
> > callback in the function fails, where it sets the tty->driver_data to
> > NULL, the parallel hvc_open() can see this NULL and cause a memory 
> > abort.
> > Hence, do a NULL check at the beginning, before proceeding ahead.
> >
> > The issue can be easily reproduced by launching two tasks 
> > simultaneously
> > that does an open() call on /dev/hvcX.
> > For example:
> > $ cat /dev/hvc0 & cat /dev/hvc0 &
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxxxxxx>
> > ---
> >  drivers/tty/hvc/hvc_console.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/tty/hvc/hvc_console.c 
> > b/drivers/tty/hvc/hvc_console.c
> > index 436cc51c92c3..80709f754cc8 100644
> > --- a/drivers/tty/hvc/hvc_console.c
> > +++ b/drivers/tty/hvc/hvc_console.c
> > @@ -350,6 +350,9 @@ static int hvc_open(struct tty_struct *tty, struct 
> > file * filp)
> >  	unsigned long flags;
> >  	int rc = 0;
> >
> > +	if (!hp)
> > +		return -ENODEV;
> > +
>
> This is still not fixing the bug properly. See:
> https://lore.kernel.org/linuxppc-dev/0f7791f5-0a53-59f6-7277-247a789f30c2@xxxxxxx/
>
> In particular, the paragraph starting "IOW".
You are right. This doesn't fix the problem entirely. There are other 
parts to it which is
not handled in a clean way by the driver. Apart from the things you've 
mentioned, it doesn't
seem to handle the hp->port.count correctly as well.

hvc_open:
  hp->port.count++
  hp->ops->notifier_add(hp, hp->data) fails
  tty->driver_data = NULL

hvc_close:
  returns immediately as tty->driver_data == NULL, without 
hp->port.count--

This would leave the port in a stale state, and the second caller of 
hvc_open doesn't get
a chance to call/retry hp->ops->notifier_add(hp, hp->data);

However, the patch is not trying to address the logical issues with 
hvc_open and hvc_close.
It's only trying to eliminate the potential NULL pointer dereference, 
leading to a panic.
From what I see, the hvc_open is serialized by tty_lock, and adding a 
NULL check here is
preventing the second caller.
> thanks,

Thank you.
Raghavendra