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Date:   Wed, 6 May 2020 11:48:51 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Raghavendra Rao Ananta <rananta@...eaurora.org>
Cc:     jslaby@...e.com, andrew@...nix.com, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: hvc: Fix data abort due to race in hvc_open

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 <rananta@...eaurora.org>
> ---
>  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?

Try just removing all of this logic and replacing it with a call to
tty_port_open() and see if that fixes this issue.

As "proof" of this, I don't see other serial drivers needing a single
mutex for their open calls, do you?

thanks,

greg k-h

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