Re: [PATCH v2] iio: proximity: as3935: fix use-after-free on device remove

[Tomasz Duszynski <tduszyns@gmail.com>]

On Fri, Mar 08, 2019 at 12:59:35PM -0500, Sven Van Asbroeck wrote:
> This driver's probe() uses a mix of devm_ and non-devm_ functions. This
> means that the remove order will not be the exact opposite of the probe
> order.
>
> Remove order:
> 1. remove() executes:
> 	iio_device_unregister
> 	iio_triggered_buffer_cleanup
> 	iio_trigger_unregister
> 	(A)
> 2. core frees devm resources in reverse order:
> 	free_irq
> 	iio_trigger_free
> 	iio_device_free
>
> In (A) the trigger has been unregistered, but the irq handler is still
> registered and active, so the trigger may still be touched via
> interrupt -> as3935_event_work. This is a potential use-after-unregister.
>
> Given that the delayed work is never canceled explicitly, it may run even
> after iio_device_free. This is a potential use-after-free.
>
> Solution: convert all probe functions to their devm_ equivalents.
> Add a devm callback, called by the core on remove right after irq_free,
> which explicitly cancels the delayed work. This will guarantee that all
> resources are freed in the correct order.
>
> As an added bonus, some boilerplate code can be removed.
>
> Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
> ---
>  drivers/iio/proximity/as3935.c | 49 ++++++++++++++--------------------
>  1 file changed, 20 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/iio/proximity/as3935.c b/drivers/iio/proximity/as3935.c
> index f130388a16a0..6e366e772164 100644
> --- a/drivers/iio/proximity/as3935.c
> +++ b/drivers/iio/proximity/as3935.c
> @@ -345,6 +345,14 @@ static SIMPLE_DEV_PM_OPS(as3935_pm_ops, as3935_suspend, as3935_resume);
>  #define AS3935_PM_OPS NULL
>  #endif
>
> +static void as3935_stop_work(void *data)
> +{
> +	struct iio_dev *indio_dev = data;
> +	struct as3935_state *st = iio_priv(indio_dev);
> +
> +	cancel_delayed_work_sync(&st->work);
> +}
> +
>  static int as3935_probe(struct spi_device *spi)
>  {
>  	struct iio_dev *indio_dev;
> @@ -368,7 +376,6 @@ static int as3935_probe(struct spi_device *spi)
>
>  	spi_set_drvdata(spi, indio_dev);
>  	mutex_init(&st->lock);
> -	INIT_DELAYED_WORK(&st->work, as3935_event_work);

Any specific reason for moving this elsewhere?

>
>  	ret = of_property_read_u32(np,
>  			"ams,tuning-capacitor-pf", &st->tune_cap);
> @@ -414,22 +421,27 @@ static int as3935_probe(struct spi_device *spi)
>  	iio_trigger_set_drvdata(trig, indio_dev);
>  	trig->ops = &iio_interrupt_trigger_ops;
>
> -	ret = iio_trigger_register(trig);
> +	ret = devm_iio_trigger_register(&spi->dev, trig);
>  	if (ret) {
>  		dev_err(&spi->dev, "failed to register trigger\n");
>  		return ret;
>  	}
>
> -	ret = iio_triggered_buffer_setup(indio_dev, iio_pollfunc_store_time,
> -		&as3935_trigger_handler, NULL);
> +	ret = devm_iio_triggered_buffer_setup(&spi->dev, indio_dev,
> +		iio_pollfunc_store_time, as3935_trigger_handler, NULL);

You can fix arguments alignment while you are at it.

>
>  	if (ret) {
>  		dev_err(&spi->dev, "cannot setup iio trigger\n");
> -		goto unregister_trigger;
> +		return ret;
>  	}
>
>  	calibrate_as3935(st);
>
> +	INIT_DELAYED_WORK(&st->work, as3935_event_work);
> +	ret = devm_add_action(&spi->dev, as3935_stop_work, indio_dev);
> +	if (ret)
> +		return ret;
> +
>  	ret = devm_request_irq(&spi->dev, spi->irq,
>  				&as3935_interrupt_handler,
>  				IRQF_TRIGGER_RISING,
> @@ -438,35 +450,15 @@ static int as3935_probe(struct spi_device *spi)
>
>  	if (ret) {
>  		dev_err(&spi->dev, "unable to request irq\n");
> -		goto unregister_buffer;
> +		return ret;
>  	}
>
> -	ret = iio_device_register(indio_dev);
> +	ret = devm_iio_device_register(&spi->dev, indio_dev);
>  	if (ret < 0) {
>  		dev_err(&spi->dev, "unable to register device\n");
> -		goto unregister_buffer;
> +		return ret;
>  	}
>  	return 0;
> -
> -unregister_buffer:
> -	iio_triggered_buffer_cleanup(indio_dev);
> -
> -unregister_trigger:
> -	iio_trigger_unregister(st->trig);
> -
> -	return ret;
> -}
> -
> -static int as3935_remove(struct spi_device *spi)
> -{
> -	struct iio_dev *indio_dev = spi_get_drvdata(spi);
> -	struct as3935_state *st = iio_priv(indio_dev);
> -
> -	iio_device_unregister(indio_dev);
> -	iio_triggered_buffer_cleanup(indio_dev);
> -	iio_trigger_unregister(st->trig);
> -
> -	return 0;
>  }
>
>  static const struct of_device_id as3935_of_match[] = {
> @@ -488,7 +480,6 @@ static struct spi_driver as3935_driver = {
>  		.pm	= AS3935_PM_OPS,
>  	},
>  	.probe		= as3935_probe,
> -	.remove		= as3935_remove,
>  	.id_table	= as3935_id,
>  };
>  module_spi_driver(as3935_driver);
> --
> 2.17.1
>