ALSA: seq: Fix nested rwsem annotation for lockdep splat

BugLink: http://bugs.launchpad.net/bugs/1731915
commit 1f20f9ff57ca23b9f5502fca85ce3977e8496cb1 upstream.

syzkaller reported the lockdep splat due to the possible deadlock of
grp->list_mutex of each sequencer client object.  Actually this is
rather a false-positive report due to the missing nested lock
annotations.  The sequencer client may deliver the event directly to
another client which takes another own lock.

For addressing this issue, this patch replaces the simple down_read()
with down_read_nested().  As a lock subclass, the already existing
"hop" can be re-used, which indicates the depth of the call.

Reference: http://lkml.kernel.org/r/089e082686ac9b482e055c832617@google.com
Reported-by: syzbot <bot+7feb8de6b4d6bf810cf098bef942cc387e79d0ad@syzkaller.appspotmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index e847b9923c1994c94d7ea1c16278c88bf0304258..b36de76f24e26a50dce5e2c78226a2bebe4b3a6c 100644 (file)
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -676,7 +676,7 @@ static int deliver_to_subscribers(struct snd_seq_client *client,
        if (atomic)
                read_lock(&grp->list_lock);
        else
-               down_read(&grp->list_mutex);
+               down_read_nested(&grp->list_mutex, hop);
        list_for_each_entry(subs, &grp->list_head, src_list) {
                /* both ports ready? */
                if (atomic_read(&subs->ref_count) != 2)