Re: [PATCH] ALSA: rawmidi: Fix potential UAF from sequencer destruction

[John Keeping]

On Wed, 29 Sep 2021 17:28:57 +0200
Takashi Iwai <tiwai@xxxxxxx> wrote:

> On Wed, 29 Sep 2021 17:17:58 +0200,
> John Keeping wrote:
> > 
> > On Wed, 29 Sep 2021 16:51:47 +0200
> > Takashi Iwai <tiwai@xxxxxxx> wrote:
> >   
> > > On Wed, 29 Sep 2021 13:36:20 +0200,
> > > John Keeping wrote:  
> > > > 
> > > > If the sequencer device outlives the rawmidi device, then
> > > > snd_rawmidi_dev_seq_free() will run after release_rawmidi_device() has
> > > > freed the snd_rawmidi structure.
> > > > 
> > > > This can easily be reproduced with CONFIG_DEBUG_KOBJECT_RELEASE.
> > > > 
> > > > Keep a reference to the rawmidi device until the sequencer has been
> > > > destroyed in order to avoid this.
> > > > 
> > > > Signed-off-by: John Keeping <john@xxxxxxxxxxxx>    
> > > 
> > > Thanks for the patch.  I wonder, though, how this could be triggered.
> > > Is this the case where the connected sequencer device is being used
> > > while the sound card gets released?  Or is it something else?  
> > 
> > I'm not sure if it's possible to trigger via the ALSA API; I haven't
> > found a route that can trigger it, but that doesn't mean there isn't
> > one :-)
> > 
> > Mostly this is useful to make CONFIG_DEBUG_KOBJECT_RELEASE cleaner.  
> 
> Hm, then could you check whether the patch below papers over it
> instead?

No, this patch doesn't solve it.  The issue is that the effect of the
final device_put() is delayed from the time it is called and there is no
way to guarantee the ordering without ensuring the sequencer has been
destroyed before the final reference to the rawmidi device is put.

Both of the functions involved are called from the core
device::release() hook.

I'm using the patch below to easily check that the sequencer has been
freed before the rawmidi data.  This can easily be triggered by
unplugging a USB MIDI device (it's not 100% since the kobject release
delays are random).

-- >8 --
--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -1571,7 +1571,10 @@ static int snd_rawmidi_alloc_substreams(struct snd_rawmidi *rmidi,
 
 static void release_rawmidi_device(struct device *dev)
 {
-       kfree(container_of(dev, struct snd_rawmidi, dev));
+       struct snd_rawmidi *rmidi = container_of(dev, struct snd_rawmidi, dev);
+
+       WARN_ON(rmidi->seq_dev);
+       kfree(rmidi);
 }
 
 /**
-- 8< --

> --- a/sound/core/seq/seq_ports.c
> +++ b/sound/core/seq/seq_ports.c
> @@ -415,11 +415,16 @@ static int subscribe_port(struct snd_seq_client *client,
>  			grp->count--;
>  		}
>  	}
> -	if (err >= 0 && send_ack && client->type == USER_CLIENT)
> +	if (err < 0)
> +		return err;
> +
> +	if (send_ack && client->type == USER_CLIENT)
>  		snd_seq_client_notify_subscription(port->addr.client, port->addr.port,
>  						   info, SNDRV_SEQ_EVENT_PORT_SUBSCRIBED);
> +	else if (client->type == KERNEL_CLIENT)
> +		get_device(&client->data.kernel.card->card_dev);
>  
> -	return err;
> +	return 0;
>  }
>  
>  static int unsubscribe_port(struct snd_seq_client *client,
> @@ -439,6 +444,8 @@ static int unsubscribe_port(struct snd_seq_client *client,
>  		snd_seq_client_notify_subscription(port->addr.client, port->addr.port,
>  						   info, SNDRV_SEQ_EVENT_PORT_UNSUBSCRIBED);
>  	module_put(port->owner);
> +	if (client->type == KERNEL_CLIENT)
> +		snd_card_unref(client->data.kernel.card);
>  	return err;
>  }
>