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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Takashi Iwai <tiwai@suse.de>
Subject: [PATCH 4.14 050/210] ALSA: oss: Fix potential deadlock at unregistration
Date:   Mon, 24 Oct 2022 13:29:27 +0200
Message-Id: <20221024112958.610709500@linuxfoundation.org>
In-Reply-To: <20221024112956.797777597@linuxfoundation.org>
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From: Takashi Iwai <tiwai@suse.de>

commit 97d917879d7f92df09c3f21fd54609a8bcd654b2 upstream.

We took sound_oss_mutex around the calls of unregister_sound_special()
at unregistering OSS devices.  This may, however, lead to a deadlock,
because we manage the card release via the card's device object, and
the release may happen at unregister_sound_special() call -- which
will take sound_oss_mutex again in turn.

Although the deadlock might be fixed by relaxing the rawmidi mutex in
the previous commit, it's safer to move unregister_sound_special()
calls themselves out of the sound_oss_mutex, too.  The call is
race-safe as the function has a spinlock protection by itself.

Link: https://lore.kernel.org/r/CAB7eexJP7w1B0mVgDF0dQ+gWor7UdkiwPczmL7pn91xx8xpzOA@mail.gmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20221011070147.7611-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 sound/core/sound_oss.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

--- a/sound/core/sound_oss.c
+++ b/sound/core/sound_oss.c
@@ -177,7 +177,6 @@ int snd_unregister_oss_device(int type,
 		mutex_unlock(&sound_oss_mutex);
 		return -ENOENT;
 	}
-	unregister_sound_special(minor);
 	switch (SNDRV_MINOR_OSS_DEVICE(minor)) {
 	case SNDRV_MINOR_OSS_PCM:
 		track2 = SNDRV_MINOR_OSS(cidx, SNDRV_MINOR_OSS_AUDIO);
@@ -189,12 +188,18 @@ int snd_unregister_oss_device(int type,
 		track2 = SNDRV_MINOR_OSS(cidx, SNDRV_MINOR_OSS_DMMIDI1);
 		break;
 	}
-	if (track2 >= 0) {
-		unregister_sound_special(track2);
+	if (track2 >= 0)
 		snd_oss_minors[track2] = NULL;
-	}
 	snd_oss_minors[minor] = NULL;
 	mutex_unlock(&sound_oss_mutex);
+
+	/* call unregister_sound_special() outside sound_oss_mutex;
+	 * otherwise may deadlock, as it can trigger the release of a card
+	 */
+	unregister_sound_special(minor);
+	if (track2 >= 0)
+		unregister_sound_special(track2);
+
 	kfree(mptr);
 	return 0;
 }