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From: Greg KH <gregkh@linuxfoundation.org>
To: Raghavendra Rao Ananta <rananta@codeaurora.org>
Cc: andrew@daynix.com, linuxppc-dev@lists.ozlabs.org,
	linux-kernel@vger.kernel.org, jslaby@suse.com
Subject: Re: [PATCH] tty: hvc: Fix data abort due to race in hvc_open
Date: Wed, 6 May 2020 11:48:51 +0200
Message-ID: <20200506094851.GA2787548@kroah.com> (raw)
In-Reply-To: <20200428032601.22127-1-rananta@codeaurora.org>

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@codeaurora.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

  reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-28  3:26 Raghavendra Rao Ananta
2020-05-06  9:48 ` Greg KH [this message]
2020-05-10  1:30   ` rananta
2020-05-10  6:48     ` Greg KH
2020-05-11  7:23       ` rananta
2020-05-11  7:34         ` rananta
2020-05-11  7:41           ` Greg KH
2020-05-11  7:39         ` Greg KH
2020-05-12  7:22           ` Jiri Slaby
2020-05-12  8:25             ` Greg KH
2020-05-12 21:39               ` rananta
2020-05-13  7:04                 ` Greg KH
2020-05-14 23:22                   ` rananta
2020-05-15  7:30                     ` Greg KH
2020-05-15 19:21                       ` rananta
2020-05-20  9:38                     ` Jiri Slaby
2020-05-20 13:49                       ` rananta
2020-04-28 12:48 Markus Elfring

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