ALSA: hda: Fix potential deadlock at codec unbinding

When a codec is unbound dynamically via sysfs while its stream is in
use, we may face a potential deadlock at the proc remove or a UAF.
This happens since the hda_pcm is managed by a linked list, as it
handles the hda_pcm object release via kref.

When a PCM is opened at the unbinding time, the release of hda_pcm
gets delayed and it ends up with the close of the PCM stream releasing
the associated hda_pcm object of its own. The hda_pcm destructor
contains the PCM device release that includes the removal of procfs
entries. And, this removal has the sync of the close of all in-use
files -- which would never finish because it's called from the PCM
file descriptor itself, i.e. it's trying to shoot its foot.

For addressing the deadlock above, this patch changes the way to
manage and release the hda_pcm object. The kref of hda_pcm is
dropped, and instead a simple refcount is introduced in hda_codec for
keeping the track of the active PCM streams, and at each PCM open and
close, this refcount is adjusted accordingly. At unbinding, the
driver calls snd_device_disconnect() for each PCM stream, then
synchronizes with the refcount finish, and finally releases the object
resources.

Fixes: bbbc7e8502c9 ("ALSA: hda - Allocate hda_pcm objects dynamically")
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

include/sound/hda_codec.h

···
 #ifndef __SOUND_HDA_CODEC_H
 #define __SOUND_HDA_CODEC_H
 
-#include <linux/kref.h>
+#include <linux/refcount.h>
 #include <linux/mod_devicetable.h>
 #include <sound/info.h>
 #include <sound/control.h>
···
 	bool own_chmap;		/* codec driver provides own channel maps */
 	/* private: */
 	struct hda_codec *codec;
-	struct kref kref;
 	struct list_head list;
+	unsigned int disconnected:1;
 };
 
 /* codec information */
···
 
 	/* PCM to create, set by patch_ops.build_pcms callback */
 	struct list_head pcm_list_head;
+	refcount_t pcm_ref;
+	wait_queue_head_t remove_sleep;
 
 	/* codec specific info */
 	void *spec;
···
 
 static inline void snd_hda_codec_pcm_get(struct hda_pcm *pcm)
 {
-	kref_get(&pcm->kref);
+	refcount_inc(&pcm->codec->pcm_ref);
 }
 void snd_hda_codec_pcm_put(struct hda_pcm *pcm);
 

sound/pci/hda/hda_bind.c

···
 		return codec->bus->core.ext_ops->hdev_detach(&codec->core);
 	}
 
+	refcount_dec(&codec->pcm_ref);
+	snd_hda_codec_disconnect_pcms(codec);
+	wait_event(codec->remove_sleep, !refcount_read(&codec->pcm_ref));
+	snd_power_sync_ref(codec->bus->card);
+
 	if (codec->patch_ops.free)
 		codec->patch_ops.free(codec);
 	snd_hda_codec_cleanup_for_unbind(codec);

sound/pci/hda/hda_codec.c

···
 /*
  * PCM device
  */
-static void release_pcm(struct kref *kref)
-{
-	struct hda_pcm *pcm = container_of(kref, struct hda_pcm, kref);
-
-	if (pcm->pcm)
-		snd_device_free(pcm->codec->card, pcm->pcm);
-	clear_bit(pcm->device, pcm->codec->bus->pcm_dev_bits);
-	kfree(pcm->name);
-	kfree(pcm);
-}
-
 void snd_hda_codec_pcm_put(struct hda_pcm *pcm)
 {
-	kref_put(&pcm->kref, release_pcm);
+	if (refcount_dec_and_test(&pcm->codec->pcm_ref))
+		wake_up(&pcm->codec->remove_sleep);
 }
 EXPORT_SYMBOL_GPL(snd_hda_codec_pcm_put);
 
···
 		return NULL;
 
 	pcm->codec = codec;
-	kref_init(&pcm->kref);
 	va_start(args, fmt);
 	pcm->name = kvasprintf(GFP_KERNEL, fmt, args);
 	va_end(args);
···
 	}
 
 	list_add_tail(&pcm->list, &codec->pcm_list_head);
+	refcount_inc(&codec->pcm_ref);
 	return pcm;
 }
 EXPORT_SYMBOL_GPL(snd_hda_codec_pcm_new);
···
 /*
  * codec destructor
  */
+void snd_hda_codec_disconnect_pcms(struct hda_codec *codec)
+{
+	struct hda_pcm *pcm;
+
+	list_for_each_entry(pcm, &codec->pcm_list_head, list) {
+		if (pcm->disconnected)
+			continue;
+		if (pcm->pcm)
+			snd_device_disconnect(codec->card, pcm->pcm);
+		snd_hda_codec_pcm_put(pcm);
+		pcm->disconnected = 1;
+	}
+}
+
 static void codec_release_pcms(struct hda_codec *codec)
 {
 	struct hda_pcm *pcm, *n;
 
 	list_for_each_entry_safe(pcm, n, &codec->pcm_list_head, list) {
-		list_del_init(&pcm->list);
+		list_del(&pcm->list);
 		if (pcm->pcm)
-			snd_device_disconnect(codec->card, pcm->pcm);
-		snd_hda_codec_pcm_put(pcm);
+			snd_device_free(pcm->codec->card, pcm->pcm);
+		clear_bit(pcm->device, pcm->codec->bus->pcm_dev_bits);
+		kfree(pcm->name);
+		kfree(pcm);
 	}
 }
 
···
 		codec->registered = 0;
 	}
 
+	snd_hda_codec_disconnect_pcms(codec);
 	cancel_delayed_work_sync(&codec->jackpoll_work);
 	if (!codec->in_freeing)
 		snd_hda_ctls_clear(codec);
···
 	remove_conn_list(codec);
 	snd_hdac_regmap_exit(&codec->core);
 	codec->configured = 0;
+	refcount_set(&codec->pcm_ref, 1); /* reset refcount */
 }
 EXPORT_SYMBOL_GPL(snd_hda_codec_cleanup_for_unbind);
 
···
 	snd_array_init(&codec->verbs, sizeof(struct hda_verb *), 8);
 	INIT_LIST_HEAD(&codec->conn_list);
 	INIT_LIST_HEAD(&codec->pcm_list_head);
+	refcount_set(&codec->pcm_ref, 1);
+	init_waitqueue_head(&codec->remove_sleep);
 
 	INIT_DELAYED_WORK(&codec->jackpoll_work, hda_jackpoll_work);
 	codec->depop_delay = -1;

sound/pci/hda/hda_local.h

···
 int snd_hda_codec_reset(struct hda_codec *codec);
 void snd_hda_codec_register(struct hda_codec *codec);
 void snd_hda_codec_cleanup_for_unbind(struct hda_codec *codec);
+void snd_hda_codec_disconnect_pcms(struct hda_codec *codec);
 
 #define snd_hda_regmap_sync(codec)	snd_hdac_regmap_sync(&(codec)->core)