linux-kernel - [PATCH 3.2 19/62] ALSA: seq: Fix double port list deletion

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Message-ID: <lsq.1459279102.557259836@decadent.org.uk>
Date:	Tue, 29 Mar 2016 20:18:22 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Dmitry Vyukov" <dvyukov@...gle.com>,
	"Takashi Iwai" <tiwai@...e.de>
Subject: [PATCH 3.2 19/62] ALSA: seq: Fix double port list deletion

3.2.79-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@...e.de>

commit 13d5e5d4725c64ec06040d636832e78453f477b7 upstream.

The commit [7f0973e973cd: ALSA: seq: Fix lockdep warnings due to
double mutex locks] split the management of two linked lists (source
and destination) into two individual calls for avoiding the AB/BA
deadlock.  However, this may leave the possible double deletion of one
of two lists when the counterpart is being deleted concurrently.
It ends up with a list corruption, as revealed by syzkaller fuzzer.

This patch fixes it by checking the list emptiness and skipping the
deletion and the following process.

BugLink: http://lkml.kernel.org/r/CACT4Y+bay9qsrz6dQu31EcGaH9XwfW7o3oBzSQUG9fMszoh=Sg@mail.gmail.com
Fixes: 7f0973e973cd ('ALSA: seq: Fix lockdep warnings due to 'double mutex locks)
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Tested-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 sound/core/seq/seq_ports.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

--- a/sound/core/seq/seq_ports.c
+++ b/sound/core/seq/seq_ports.c
@@ -540,19 +540,22 @@ static void delete_and_unsubscribe_port(
 					bool is_src, bool ack)
 {
 	struct snd_seq_port_subs_info *grp;
+	struct list_head *list;
+	bool empty;
 
 	grp = is_src ? &port->c_src : &port->c_dest;
+	list = is_src ? &subs->src_list : &subs->dest_list;
 	down_write(&grp->list_mutex);
 	write_lock_irq(&grp->list_lock);
-	if (is_src)
-		list_del(&subs->src_list);
-	else
-		list_del(&subs->dest_list);
+	empty = list_empty(list);
+	if (!empty)
+		list_del_init(list);
 	grp->exclusive = 0;
 	write_unlock_irq(&grp->list_lock);
 	up_write(&grp->list_mutex);
 
-	unsubscribe_port(client, port, grp, &subs->info, ack);
+	if (!empty)
+		unsubscribe_port(client, port, grp, &subs->info, ack);
 }
 
 /* connect two ports */