Re: [PATCH] ALSA: hda/cs_dsp_ctl: Actually remove ALSA controls

[Takashi Iwai <tiwai@suse.de>]

On Fri, 03 May 2024 16:49:20 +0200,
Richard Fitzgerald wrote:
> 
> hda_cs_dsp_control_remove() must remove the ALSA control when
> deleting all the infrastructure for handling the control.
> 
> Without this it is possible for ALSA controls to be left in
> the Soundcard after the amp driver module has been unloaded.
> So the get/set callbacks point to code that no longer exists.
> 
> Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
> Fixes: 3233b978af23 ("ALSA: hda: hda_cs_dsp_ctl: Add Library to support CS_DSP ALSA controls")
> ---
> Note: it would be better to use the control private_free to do the
> cleanup, and that is my plan long-term. But that is a larger change
> to the code.
> 
> I like to keep bugfix patches as simple as possible so they are
> low-risk and easy to cherry-pick into older kernels. So this patch
> fixes the bug. Sometime I will send a patch for future kernel
> versions that reworks the cleanup to use private_free.

I also like to keep as simple as possible :)

One slight concern is whether cs_dsp kctls can be deleted at the
snd_card removal (disconnect) before this function gets called.
That is, snd_card_free() of the main card may delete all associated
kctls, and may this function be called afterwards?
If yes, this change would lead to a UAF.

The structure is so complex and I can't follow immediately,
unfortunately...


thanks,

Takashi

> ---
>  sound/pci/hda/hda_cs_dsp_ctl.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/sound/pci/hda/hda_cs_dsp_ctl.c b/sound/pci/hda/hda_cs_dsp_ctl.c
> index 463ca06036bf..a42653d3473d 100644
> --- a/sound/pci/hda/hda_cs_dsp_ctl.c
> +++ b/sound/pci/hda/hda_cs_dsp_ctl.c
> @@ -203,6 +203,10 @@ void hda_cs_dsp_control_remove(struct cs_dsp_coeff_ctl *cs_ctl)
>  {
>  	struct hda_cs_dsp_coeff_ctl *ctl = cs_ctl->priv;
>  
> +	/* Only public firmware controls will have an associated kcontrol */
> +	if (ctl && ctl->kctl)
> +		snd_ctl_remove(ctl->card, ctl->kctl);
> +
>  	kfree(ctl);
>  }
>  EXPORT_SYMBOL_NS_GPL(hda_cs_dsp_control_remove, SND_HDA_CS_DSP_CONTROLS);
> -- 
> 2.39.2
>