[PATCH 3.19.y-ckt 117/196] ALSA: seq: Fix double port list deletion

From: Kamal Mostafa
Date: Tue Mar 08 2016 - 19:36:14 EST

3.19.8-ckt16 -stable review patch. If anyone has any objections, please let me know.


From: Takashi Iwai <tiwai@xxxxxxx>

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@xxxxxxxxxxxxxx
Fixes: 7f0973e973cd ('ALSA: seq: Fix lockdep warnings due to 'double mutex locks)
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Tested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
sound/core/seq/seq_ports.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/sound/core/seq/seq_ports.c b/sound/core/seq/seq_ports.c
index 3c8630f..9c1c8d5 100644
--- a/sound/core/seq/seq_ports.c
+++ b/sound/core/seq/seq_ports.c
@@ -538,19 +538,22 @@ static void delete_and_unsubscribe_port(struct snd_seq_client *client,
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;
- 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;

- unsubscribe_port(client, port, grp, &subs->info, ack);
+ if (!empty)
+ unsubscribe_port(client, port, grp, &subs->info, ack);

/* connect two ports */