Re: [PATCH] ALSA: usb-audio: Fix double-free in snd_usb_add_audio_stream()

[Takashi Iwai]

On Wed, 30 Mar 2016 22:31:15 +0200,
Takashi Iwai wrote:
> 
> On Wed, 30 Mar 2016 21:03:22 +0200,
> Vladis Dronov wrote:
> > 
> > There is a double-free bug in [snd-usb-audio] module due to alloc/free logic
> > flaw in snd_usb_add_audio_stream() function. This leads to kernel structures
> > corruption and panic. Fix the code flow and alloc/free logic so there is no
> > double-free.
> > 
> > The detailed analysis: https://bugzilla.redhat.com/show_bug.cgi?id=1283358
> > 
> > Reported-by: Ralf Spenneberg <ralf@xxxxxxxxxxxxxx>
> > Signed-off-by: Vladis Dronov <vdronov@xxxxxxxxxx>
> 
> Thanks for the report.  But how about a simpler fix like below?

Maybe the one below is more straightforward (and even simpler).
Let me know if this works enough for you.


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@xxxxxxx>
Subject: [PATCH] ALSA: usb-audio: Fix double free in create_fixed_stream_quirk() error paths

create_fixed_stream_quirk() function allocates the audioformat object
by itself and frees it upon error before returning.  However, once
when the object is linked to a stream, it's freed again in
snd_usb_audio_pcm_free(), thus it'll be double-freed, eventually
resulting in a memory corruption.

This patch fixes this failure in the error path by unlinking the
audioformat object before freeing it.

[Note for stable backports:
 this patch requires the commit 902eb7fd1e4a ('ALSA: usb-audio: Minor
 code cleanup in create_fixed_stream_quirk()')]

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1283358
Reported-by: Ralf Spenneberg <ralf@xxxxxxxxxxxxxx>
Reported-by: Vladis Dronov <vdronov@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # see the note above
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/usb/quirks.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index fb62bce2435c..a5a9ecaafb37 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -150,6 +150,7 @@ static int create_fixed_stream_quirk(struct snd_usb_audio *chip,
 		usb_audio_err(chip, "cannot memdup\n");
 		return -ENOMEM;
 	}
+	INIT_LIST_HEAD(&fp->list);
 	if (fp->nr_rates > MAX_NR_RATES) {
 		kfree(fp);
 		return -EINVAL;
@@ -193,6 +194,7 @@ static int create_fixed_stream_quirk(struct snd_usb_audio *chip,
 	return 0;
 
  error:
+	list_del(&fp->list); /* unlink for avoiding double-free */
 	kfree(fp);
 	kfree(rate_table);
 	return err;
-- 
2.7.4