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

[Takashi Iwai]

On Wed, 29 Sep 2021 18:56:32 +0200,
John Keeping wrote:
> 
> 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).

Hm, it's strange.  I suppose you're *not* using the MIDI device,
right?

The release path for the USB-audio driver is:
  usb_audio_disconnect() ->
    snd_card_free_when_closed() ->
      release_card_device() (via put_device(&card->card_dev)) ->
        snd_card_do_free()

And here in snd_card_do_free(), the snd_device free-callback chains
are called at the beginning (snd_device_free_all()).
As it's executed in a reverse loop, snd_rawmidi_dev_seq_free() shall
be called before snd_rawmidi_dev_free().  Since the final put_device()
for the rawmidi device is called in the latter function, the device
release must not happen before snd_rawmidi_dev_seq_free()...

So I still wonder how the problem could be triggered at all.  Even if
the device object release itself is delayed, it shouldn't matter in
the scenario above (as the snd_device-free-chains are already called
beforehand).


thanks,

Takashi

> 
> -- >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;
> >  }
> >  
>