[PATCH v2] ALSA: core: Fix potential UAF after asynchronous card release

[Takashi Iwai]

Usually a sound driver releases the resources assigned to the card via
snd_card_free(), and it synchronizes with the whole release procedure.
However, when the card is released asynchronously via
snd_card_free_when_closed() like USB-audio driver, the situation is
slightly different; although the snd_card_disconnect() call at the
disconnection guarantees that any newer accesses will be gated, the
in-flight tasks might be still accessing to the underlying card->dev
device even after the disconnection, which would cause a
use-after-free in the end, as reported by fuzzers.

For addressing the bug above, this patch takes the refcount of
card->dev at initialization of the card object, and releases at its
destructor.   This assures the availability of the card->dev in its
whole lifecycle.

Reported-by: Farhad Alemi <farhad.alemi@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/CA+0ovChexj4TrZL_2iG_P0WBEbZc5+73GfB3DkciQi=R8pZOnA@xxxxxxxxxxxxxx
Closes: https://lore.kernel.org/CA+0ovCgQUQNN=Z1tJTouiCsDaXR5M-3-SQEGk-cpPXQkM5Xh+w@xxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
v1->v2: move put_device() before complete() that may free card object for
	devres-managed case

 sound/core/init.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sound/core/init.c b/sound/core/init.c
index 2f7f83a7611b..d05bea3c87f5 100644
--- a/sound/core/init.c
+++ b/sound/core/init.c
@@ -310,7 +310,7 @@ static int snd_card_init(struct snd_card *card, struct device *parent,
 			kfree(card); /* manually free here, as no destructor called */
 		return err;
 	}
-	card->dev = parent;
+	card->dev = get_device(parent);
 	card->number = idx;
 	WARN_ON(IS_MODULE(CONFIG_SND) && !module);
 	card->module = module;
@@ -603,6 +603,7 @@ static int snd_card_do_free(struct snd_card *card)
 		dev_warn(card->dev, "unable to free card info\n");
 		/* Not fatal error */
 	}
+	put_device(card->dev);
 	if (card->release_completion)
 		complete(card->release_completion);
 	if (!managed)
-- 
2.55.0