From: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com> stable inclusion from stable-v6.6.158 commit 686c7a6af1eea8d2a919303e4c6bbec3de79dbdc category: bugfix bugzilla: https://atomgit.com/src-openeuler/kernel/issues/20047 CVE: CVE-2026-98314 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=... -------------------------------- ALSA: pcm: set timer->private_data before registering the PCM timer [ Upstream commit 1e713f9bb2ac583521f06b0eb4e22440b1e3d078 ] 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: 1da177e4c3f4 ("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.co... Signed-off-by: Takashi Iwai <tiwai@suse.de> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Pan Taixi <pantaixi1@huawei.com> --- sound/core/pcm_timer.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/sound/core/pcm_timer.c b/sound/core/pcm_timer.c index c43484b22b34..725a7272cd07 100644 --- a/sound/core/pcm_timer.c +++ b/sound/core/pcm_timer.c @@ -110,16 +110,19 @@ void snd_pcm_timer_init(struct snd_pcm_substream *substream) sprintf(timer->name, "PCM %s %i-%i-%i", substream->stream == SNDRV_PCM_STREAM_CAPTURE ? "capture" : "playback", tid.card, tid.device, tid.subdevice); timer->hw = snd_pcm_timer; + /* Set before registering: a concurrent reader can invoke our hw + * callbacks as soon as the timer is on the global list. + */ + timer->private_data = substream; + timer->private_free = snd_pcm_timer_free; if (snd_device_register(timer->card, timer) < 0) { snd_device_free(timer->card, timer); return; } - timer->private_data = substream; - timer->private_free = snd_pcm_timer_free; substream->timer = timer; } void snd_pcm_timer_done(struct snd_pcm_substream *substream) { -- 2.34.1