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

From: [email-masked]
Date: Mon, 11 May 2020 00:23:58 -0700

On 2020-05-09 23:48, Greg KH wrote:
> On Sat, May 09, 2020 at 06:30:56PM -0700, [email-masked] wrote:
>> On 2020-05-06 02:48, Greg KH wrote:
>> > On Mon, Apr 27, 2020 at 08:26:01PM -0700, 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, serialize hvc_open and check if tty->private_data is NULL
>> > > before
>> > > proceeding ahead.
>> > >
>> > > The issue can be easily reproduced by launching two tasks
>> > > simultaneously
>> > > that does nothing but open() and close() on /dev/hvcX.
>> > > For example:
>> > > $ ./simple_open_close /dev/hvc0 & ./simple_open_close /dev/hvc0 &
>> > >
>> > > Signed-off-by: Raghavendra Rao Ananta <[email-masked]>
>> > > ---
>> > >  drivers/tty/hvc/hvc_console.c | 16 ++++++++++++++--
>> > >  1 file changed, 14 insertions(+), 2 deletions(-)
>> > >
>> > > diff --git a/drivers/tty/hvc/hvc_console.c
>> > > b/drivers/tty/hvc/hvc_console.c
>> > > index 436cc51c92c3..ebe26fe5ac09 100644
>> > > --- a/drivers/tty/hvc/hvc_console.c
>> > > +++ b/drivers/tty/hvc/hvc_console.c
>> > > @@ -75,6 +75,8 @@ static LIST_HEAD(hvc_structs);
>> > >   */
>> > >  static DEFINE_MUTEX(hvc_structs_mutex);
>> > >
>> > > +/* Mutex to serialize hvc_open */
>> > > +static DEFINE_MUTEX(hvc_open_mutex);
>> > >  /*
>> > >   * This value is used to assign a tty->index value to a hvc_struct
>> > > based
>> > >   * upon order of exposure via hvc_probe(), when we can not match it
>> > > to
>> > > @@ -346,16 +348,24 @@ static int hvc_install(struct tty_driver
>> > > *driver, struct tty_struct *tty)
>> > >   */
>> > >  static int hvc_open(struct tty_struct *tty, struct file * filp)
>> > >  {
>> > > -	struct hvc_struct *hp = tty->driver_data;
>> > > +	struct hvc_struct *hp;
>> > >  	unsigned long flags;
>> > >  	int rc = 0;
>> > >
>> > > +	mutex_lock(&hvc_open_mutex);
>> > > +
>> > > +	hp = tty->driver_data;
>> > > +	if (!hp) {
>> > > +		rc = -EIO;
>> > > +		goto out;
>> > > +	}
>> > > +
>> > >  	spin_lock_irqsave(&hp->port.lock, flags);
>> > >  	/* Check and then increment for fast path open. */
>> > >  	if (hp->port.count++ > 0) {
>> > >  		spin_unlock_irqrestore(&hp->port.lock, flags);
>> > >  		hvc_kick();
>> > > -		return 0;
>> > > +		goto out;
>> > >  	} /* else count == 0 */
>> > >  	spin_unlock_irqrestore(&hp->port.lock, flags);
>> >
>> > Wait, why isn't this driver just calling tty_port_open() instead of
>> > trying to open-code all of this?
>> >
>> > Keeping a single mutext for open will not protect it from close, it will
>> > just slow things down a bit.  There should already be a tty lock held by
>> > the tty core for open() to keep it from racing things, right?
>> The tty lock should have been held, but not likely across ->install() 
>> and
>> ->open() callbacks, thus resulting in a race between hvc_install() and
>> hvc_open(),
> 
> How?  The tty lock is held in install, and should not conflict with
> open(), otherwise, we would be seeing this happen in all tty drivers,
> right?
> 
Well, I was expecting the same, but IIRC, I see that the open() was 
being
called in parallel for the same device node.

Is it expected that the tty core would allow only one thread to
access the dev-node, while blocking the other, or is it the client
driver's responsibility to handle the exclusiveness?
>> where hvc_install() sets a data and the hvc_open() clears it. 
>> hvc_open()
>> doesn't
>> check if the data was set to NULL and proceeds.
> 
> What data is being set that hvc_open is checking?
hvc_install sets tty->private_data to hp, while hvc_open sets it to NULL 
(in one of the paths).
> 
> And you are not grabbing a lock in your install callback, you are only
> serializing your open call here, I don't see how this is fixing 
> anything
> other than perhaps slowing down your codepaths.
Basically, my intention was to add a NULL check before accessing *hp in 
open().
The intention of the lock was to protect against this check.
If the tty layer would have taken care of this, then perhaps there won't 
be a
need to check for NULL.
> 
> As an arument why this isn't correct, can you answer why this same type
> of change wouldn't be required for all tty drivers in the tree?
> 
I agree, that if it's already taken care by the tty-core, we don't need 
it here.
Correct me if I'm wrong, but looks like the tty layer is allowing 
parallel accesses
to open(),
> thanks,
> greg k-h