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ALSA: pcm: set timer->private_data before registering the PCM timer

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snd_pcm_timer_init() calls snd_device_register() to link the new
struct snd_timer into the global timer list while it still carries
hw.c_resolution = snd_pcm_timer_resolution (and hw.start/hw.stop),
and only afterwards sets timer->private_data = substream.

Once the timer is on the list under register_mutex, a concurrent
reader can already reach it through the same mutex and invoke these
callbacks. /proc/asound/timers does this via c_resolution(), and
snd_timer_open()+snd_timer_start() reach start()/stop() the same way.
All three dereference timer->private_data, which for this brief
window is NULL, giving a NULL-pointer dereference:

  substream = timer->private_data;
  return substream->runtime ? ...   // substream is NULL

Move the private_data/private_free assignment before
snd_device_register() so the timer is never visible on the list
without its private_data set. On the snd_device_register() failure
path, private_free() (snd_pcm_timer_free()) can now run, but it only
does substream->timer = NULL, which is already NULL at that point
since substream->timer is set to the new timer just once, after a
successful registration -- so the failure path stays safe.

Reported-by: syzbot+19da64013c46df87f971@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=19da64013c46df87f971
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
Link: https://patch.msgid.link/20260913134446.114724-1-ngocthang2710.1999@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Nguyen Ngoc Thang 2026-09-13 20:44:46 +07:00 • committed by Takashi Iwai
commit 1e713f9bb2
1 changed files with 5 additions and 2 deletions
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