[PATCH 6.18 206/396] ALSA: timer: Clear SNDRV_TIMER_IFLG_DEAD once the close completes

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

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

From: Norbert Szetei <[email protected]>

commit c2744d5f3aea474513fd2298daecb94a952ce441 upstream.

snd_timer_close_locked() marks an instance with SNDRV_TIMER_IFLG_DEAD
and returns early when the flag is already set, but the flag is never
cleared again.  A completed close ends in remove_slave_links(), which
leaves timeri->timer NULL, so a second close is already harmless through
the timer == NULL path; the early return can only be reached by an
instance that was opened again in between.  For such an instance the
close unlinks nothing, so snd_timer_instance_free() frees an object that
is still on timer->open_list_head, still on snd_timer_master_list if it
was opened with a slave key, still owns any adopted slaves, and still
holds its timer and module references.

snd_seq_timer_open() reopens an instance exactly like that: it retries
its fallback open on the same object after a failure that has already
run snd_timer_close_locked() internally.  An unprivileged user with
access to /dev/snd/timer and /dev/snd/seq can force that failure, since
snd_timer_check_master() returns -EBUSY when a pending slave matches the
new master's (slave_class, slave_id) key and the target timer has
reached max_instances, and SNDRV_TIMER_IOCTL_SELECT with dev_class =
SNDRV_TIMER_CLASS_SLAVE keeps the caller-supplied dev_sclass, so a
sequencer queue's key can be forged.  The freed instance is afterwards
dereferenced by any further snd_timer_open() on that timer, by
snd_timer_check_slave(), and by /proc/asound/timers, which faults on the
stale ti->owner pointer.

The flag only has to be visible while the close is in progress, which is
all its other users need.  Clear it in remove_slave_links(), under the
same timer->lock that sets it, once the instance is off every list.

Fixes: da3039e91d1f ("ALSA: timer: Forcibly close timer instances at closing")
Cc: [email protected]
Assisted-by: Claude:claude-opus-5
Signed-off-by: Norbert Szetei <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 sound/core/timer.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -409,6 +409,8 @@ static void remove_slave_links(struct sn
 		list_del_init(&slave->ack_list);
 		list_del_init(&slave->active_list);
 	}
+	/* the close is done; a reopen must not see the mark */
+	timeri->flags &= ~SNDRV_TIMER_IFLG_DEAD;
 }
 
 /*