Re: [alsa-devel] [RFC PATCH v2] ASoC: dpcm: prevent snd_soc_dpcm use after free

[KaiChieh Chuang <kaichieh.chuang-NuS5LvNUpcJWk0Htik3J/w@public.gmane.org>]

On Wed, 2019-03-06 at 10:19 +0100, Jaroslav Kysela wrote:
> Dne 06. 03. 19 v 9:46 KaiChieh Chuang napsal(a):
> > the dpcm get from fe_clients/be_clients
> > may be free before use
> > 

> > @@ -1294,9 +1294,11 @@ void dpcm_be_disconnect(struct snd_soc_pcm_runtime *fe, int stream)
> >  #ifdef CONFIG_DEBUG_FS
> >  		debugfs_remove(dpcm->debugfs_state);
> >  #endif
> > +		spin_lock(&fe->card->dpcm_lock);
> >  		list_del(&dpcm->list_be);
> >  		list_del(&dpcm->list_fe);
> >  		kfree(dpcm);
> > +		spin_unlock(&fe->card->dpcm_lock);
> 
> The unlock might be moved before kfree(). Also, I don't see the
> list_add() spin lock protection in your patch.
> 
> 				Jaroslav
> 

The dpcm_lock in this patch is to protect the instance of dpcm,
e.g. protect dpcm not to be free while dereference from the
be_clients/fe_clients.

The lock is not meant to protect the list "be_clients" and "fe_client",
e.g. not meant to protect add/remove dpcm from these list.

The lock is added only at the places that may have race with
dpcm_be_disconnect(), e.g. kfree(dpcm). And note that, many places that
call for_each_dpcm_be/fe() cannot use spin_lock, since there are pcm
callbacks which can sleep.