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

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Date:   Thu, 30 Sep 2021 08:55:52 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     John Keeping <john@...anate.com>
Cc:     linux-kernel@...r.kernel.org, alsa-devel@...a-project.org,
        Takashi Iwai <tiwai@...e.com>
Subject: Re: [PATCH] ALSA: rawmidi: Fix potential UAF from sequencer destruction

On Thu, 30 Sep 2021 08:31:56 +0200,
Takashi Iwai wrote:
> 
> On Wed, 29 Sep 2021 18:56:32 +0200,
> John Keeping wrote:
> > 
> > On Wed, 29 Sep 2021 17:28:57 +0200
> > Takashi Iwai <tiwai@...e.de> 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@...e.de> 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@...anate.com>    
> > > > > 
> > > > > 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()...

Correction: now I finally understood what I misunderstood.
Although the snd_device call chain mentioned above itself is correct,
the snd_rawmidi_dev_seq_free() function isn't called directly from the
snd_device chain, but it's rater the own private_free of
snd_seq_device object.  That is, the call of snd_seq_device
private_free is done in a wrong place; it should be called in the
snd_device call chain instead of the device release.

A fix patch is something like below.  Could you check whether this
fixes the problem?


thanks,

Takashi

--- a/sound/core/seq_device.c
+++ b/sound/core/seq_device.c
@@ -156,6 +156,8 @@ static int snd_seq_device_dev_free(struct snd_device *device)
 	struct snd_seq_device *dev = device->device_data;
 
 	cancel_autoload_drivers();
+	if (dev->private_free)
+		dev->private_free(dev);
 	put_device(&dev->dev);
 	return 0;
 }
@@ -183,11 +185,7 @@ static int snd_seq_device_dev_disconnect(struct snd_device *device)
 
 static void snd_seq_dev_release(struct device *dev)
 {
-	struct snd_seq_device *sdev = to_seq_dev(dev);
-
-	if (sdev->private_free)
-		sdev->private_free(sdev);
-	kfree(sdev);
+	kfree(to_seq_dev(dev));
 }
 
 /*