Re: [PATCH 1/2] ALSA: core: Add snd_card_free_on_error() helper

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

Hi,

On Tue, Apr 12, 2022 at 11:31:40AM +0200, Takashi Iwai wrote:
> This is a small helper function to handle the error path more easily
> when an error happens during the probe for the device with the
> device-managed card.  Since devres releases in the reverser order of
> the creations, usually snd_card_free() gets called at the last in the
> probe error path unless it already reached snd_card_register() calls.
> Due to this nature, when a driver expects the resource releases in
> card->private_free, this might be called too lately.
> 
> As a workaround, one should call the probe like:
> 
>  static int __some_probe(...) { // do real probe.... }
> 
>  static int some_probe(...)
>  {
> 	return snd_card_free_on_error(dev, __some_probe(dev, ...));
>  }
> 
> so that the snd_card_free() is called explicitly at the beginning of
> the error path from the probe.
> 
> This function will be used in the upcoming fixes to address the
> regressions by devres usages.
> 
> Fixes: e8ad415b7a55 ("ALSA: core: Add managed card creation")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> ---
>  include/sound/core.h |  1 +
>  sound/core/init.c    | 28 ++++++++++++++++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/include/sound/core.h b/include/sound/core.h
> index b7e9b58d3c78..6d4cc49584c6 100644
> --- a/include/sound/core.h
> +++ b/include/sound/core.h
> @@ -284,6 +284,7 @@ int snd_card_disconnect(struct snd_card *card);
>  void snd_card_disconnect_sync(struct snd_card *card);
>  int snd_card_free(struct snd_card *card);
>  int snd_card_free_when_closed(struct snd_card *card);
> +int snd_card_free_on_error(struct device *dev, int ret);
>  void snd_card_set_id(struct snd_card *card, const char *id);
>  int snd_card_register(struct snd_card *card);
>  int snd_card_info_init(void);
> diff --git a/sound/core/init.c b/sound/core/init.c
> index 31ba7024e3ad..726a8353201f 100644
> --- a/sound/core/init.c
> +++ b/sound/core/init.c
> @@ -209,6 +209,12 @@ static void __snd_card_release(struct device *dev, void *data)
>   * snd_card_register(), the very first devres action to call snd_card_free()
>   * is added automatically.  In that way, the resource disconnection is assured
>   * at first, then released in the expected order.
> + *
> + * If an error happens at the probe before snd_card_register() is called and
> + * there have been other devres resources, you'd need to free the card manually
> + * via snd_card_free() call in the error; otherwise it may lead to UAF due to
> + * devres call orders.  You can use snd_card_free_on_error() helper for
> + * handling it more easily.
>   */
>  int snd_devm_card_new(struct device *parent, int idx, const char *xid,
>  		      struct module *module, size_t extra_size,
> @@ -235,6 +241,28 @@ int snd_devm_card_new(struct device *parent, int idx, const char *xid,
>  }
>  EXPORT_SYMBOL_GPL(snd_devm_card_new);
>  
> +/**
> + * snd_card_free_on_error - a small helper for handling devm probe errors
> + * @dev: the managed device object
> + * @ret: the return code from the probe callback
> + *
> + * This function handles the explicit snd_card_free() call at the error from
> + * the probe callback.  It's just a small helper for simplifying the error
> + * handling with the managed devices.
> + */
> +int snd_card_free_on_error(struct device *dev, int ret)
> +{
> +	struct snd_card *card;
> +
> +	if (!ret)
> +		return 0;
> +	card = devres_find(dev, __snd_card_release, NULL, NULL);
> +	if (card)
> +		snd_card_free(card);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(snd_card_free_on_error);
> +
>  static int snd_card_init(struct snd_card *card, struct device *parent,
>  			 int idx, const char *xid, struct module *module,
>  			 size_t extra_size)
> -- 
> 2.31.1

The idea looks good itself to me. On the other hand, the name
'snd_card_free_on_error()' is not so suitable since it assumes that
'snd_devm_card_new()' is called in advance, while we have another function,
'snd_card_new()'.

I think it better to use 'snd_devm_card_free_on_error()' instead since
the function doesn't work as expected in the case of 'snd_card_new()'
(the snd_card_free() is not called because nothing found in devres).


Regards

Takashi Sakamoto