[PATCH 3.2 098/152] ALSA: rawmidi: Fix possible deadlock with virmidi registration

From: Ben Hutchings
Date: Sun Nov 13 2016 - 21:17:36 EST

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

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

From: Takashi Iwai <tiwai@xxxxxxx>

commit 816f318b2364262a51024096da7ca3b84e78e3b5 upstream.

When a seq-virmidi driver is initialized, it registers a rawmidi
instance with its callback to create an associated seq kernel client.
Currently it's done throughly in rawmidi's register_mutex context.
Recently it was found that this may lead to a deadlock another rawmidi
device that is being attached with the sequencer is accessed, as both
open with the same register_mutex. This was actually triggered by
syzkaller, as Dmitry Vyukov reported:

======================================================
[ INFO: possible circular locking dependency detected ]
4.8.0-rc1+ #11 Not tainted
-------------------------------------------------------
syz-executor/7154 is trying to acquire lock:
(register_mutex#5){+.+.+.}, at: [<ffffffff84fd6d4b>] snd_rawmidi_kernel_open+0x4b/0x260 sound/core/rawmidi.c:341

but task is already holding lock:
(&grp->list_mutex){++++.+}, at: [<ffffffff850138bb>] check_and_subscribe_port+0x5b/0x5c0 sound/core/seq/seq_ports.c:495

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&grp->list_mutex){++++.+}:
[<ffffffff8147a3a8>] lock_acquire+0x208/0x430 kernel/locking/lockdep.c:3746
[<ffffffff863f6199>] down_read+0x49/0xc0 kernel/locking/rwsem.c:22
[< inline >] deliver_to_subscribers sound/core/seq/seq_clientmgr.c:681
[<ffffffff85005c5e>] snd_seq_deliver_event+0x35e/0x890 sound/core/seq/seq_clientmgr.c:822
[<ffffffff85006e96>] > snd_seq_kernel_client_dispatch+0x126/0x170 sound/core/seq/seq_clientmgr.c:2418
[<ffffffff85012c52>] snd_seq_system_broadcast+0xb2/0xf0 sound/core/seq/seq_system.c:101
[<ffffffff84fff70a>] snd_seq_create_kernel_client+0x24a/0x330 sound/core/seq/seq_clientmgr.c:2297
[< inline >] snd_virmidi_dev_attach_seq sound/core/seq/seq_virmidi.c:383
[<ffffffff8502d29f>] snd_virmidi_dev_register+0x29f/0x750 sound/core/seq/seq_virmidi.c:450
[<ffffffff84fd208c>] snd_rawmidi_dev_register+0x30c/0xd40 sound/core/rawmidi.c:1645
[<ffffffff84f816d3>] __snd_device_register.part.0+0x63/0xc0 sound/core/device.c:164
[< inline >] __snd_device_register sound/core/device.c:162
[<ffffffff84f8235d>] snd_device_register_all+0xad/0x110 sound/core/device.c:212
[<ffffffff84f7546f>] snd_card_register+0xef/0x6c0 sound/core/init.c:749
[<ffffffff85040b7f>] snd_virmidi_probe+0x3ef/0x590 sound/drivers/virmidi.c:123
[<ffffffff833ebf7b>] platform_drv_probe+0x8b/0x170 drivers/base/platform.c:564
......

-> #0 (register_mutex#5){+.+.+.}:
[< inline >] check_prev_add kernel/locking/lockdep.c:1829
[< inline >] check_prevs_add kernel/locking/lockdep.c:1939
[< inline >] validate_chain kernel/locking/lockdep.c:2266
[<ffffffff814791f4>] __lock_acquire+0x4d44/0x4d80 kernel/locking/lockdep.c:3335
[<ffffffff8147a3a8>] lock_acquire+0x208/0x430 kernel/locking/lockdep.c:3746
[< inline >] __mutex_lock_common kernel/locking/mutex.c:521
[<ffffffff863f0ef1>] mutex_lock_nested+0xb1/0xa20 kernel/locking/mutex.c:621
[<ffffffff84fd6d4b>] snd_rawmidi_kernel_open+0x4b/0x260 sound/core/rawmidi.c:341
[<ffffffff8502e7c7>] midisynth_subscribe+0xf7/0x350 sound/core/seq/seq_midi.c:188
[< inline >] subscribe_port sound/core/seq/seq_ports.c:427
[<ffffffff85013cc7>] check_and_subscribe_port+0x467/0x5c0 sound/core/seq/seq_ports.c:510
[<ffffffff85015da9>] snd_seq_port_connect+0x2c9/0x500 sound/core/seq/seq_ports.c:579
[<ffffffff850079b8>] snd_seq_ioctl_subscribe_port+0x1d8/0x2b0 sound/core/seq/seq_clientmgr.c:1480
[<ffffffff84ffe9e4>] snd_seq_do_ioctl+0x184/0x1e0 sound/core/seq/seq_clientmgr.c:2225
[<ffffffff84ffeae8>] snd_seq_kernel_client_ctl+0xa8/0x110 sound/core/seq/seq_clientmgr.c:2440
[<ffffffff85027664>] snd_seq_oss_midi_open+0x3b4/0x610 sound/core/seq/oss/seq_oss_midi.c:375
[<ffffffff85023d67>] snd_seq_oss_synth_setup_midi+0x107/0x4c0 sound/core/seq/oss/seq_oss_synth.c:281
[<ffffffff8501b0a8>] snd_seq_oss_open+0x748/0x8d0 sound/core/seq/oss/seq_oss_init.c:274
[<ffffffff85019d8a>] odev_open+0x6a/0x90 sound/core/seq/oss/seq_oss.c:138
[<ffffffff84f7040f>] soundcore_open+0x30f/0x640 sound/sound_core.c:639
......

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&grp->list_mutex);
lock(register_mutex#5);
lock(&grp->list_mutex);
lock(register_mutex#5);

*** DEADLOCK ***
======================================================

The fix is to simply move the registration parts in
snd_rawmidi_dev_register() to the outside of the register_mutex lock.
The lock is needed only to manage the linked list, and it's not
necessarily to cover the whole initialization process.

Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
sound/core/rawmidi.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -1609,11 +1609,13 @@ static int snd_rawmidi_dev_register(stru
return -EBUSY;
}
list_add_tail(&rmidi->list, &snd_rawmidi_devices);
+ mutex_unlock(&register_mutex);
sprintf(name, "midiC%iD%i", rmidi->card->number, rmidi->device);
if ((err = snd_register_device(SNDRV_DEVICE_TYPE_RAWMIDI,
rmidi->card, rmidi->device,
&snd_rawmidi_f_ops, rmidi, name)) < 0) {
snd_printk(KERN_ERR "unable to register rawmidi device %i:%i\n", rmidi->card->number, rmidi->device);
+ mutex_lock(&register_mutex);
list_del(&rmidi->list);
mutex_unlock(&register_mutex);
return err;
@@ -1621,6 +1623,7 @@ static int snd_rawmidi_dev_register(stru
if (rmidi->ops && rmidi->ops->dev_register &&
(err = rmidi->ops->dev_register(rmidi)) < 0) {
snd_unregister_device(SNDRV_DEVICE_TYPE_RAWMIDI, rmidi->card, rmidi->device);
+ mutex_lock(&register_mutex);
list_del(&rmidi->list);
mutex_unlock(&register_mutex);
return err;
@@ -1649,7 +1652,6 @@ static int snd_rawmidi_dev_register(stru
}
}
#endif /* CONFIG_SND_OSSEMUL */
- mutex_unlock(&register_mutex);
sprintf(name, "midi%d", rmidi->device);
entry = snd_info_create_card_entry(rmidi->card, name, rmidi->card->proc_root);
if (entry) {