[PATCH 5.15 027/121] ALSA: pcm: oss: Fix race at SNDCTL_DSP_SYNC

Greg Kroah-Hartman
2022-09-13T14:30:13Z

From: Takashi Iwai <tiwai@suse.de>

commit 8423f0b6d513b259fdab9c9bf4aaa6188d054c2d upstream.

There is a small race window at snd_pcm_oss_sync() that is called from
OSS PCM SNDCTL_DSP_SYNC ioctl; namely the function calls
snd_pcm_oss_make_ready() at first, then takes the params_lock mutex
for the rest. When the stream is set up again by another thread
between them, it leads to inconsistency, and may result in unexpected
results such as NULL dereference of OSS buffer as a fuzzer spotted
recently.

The fix is simply to cover snd_pcm_oss_make_ready() call into the same
params_lock mutex with snd_pcm_oss_make_ready_locked() variant.

Reported-and-tested-by: butt3rflyh4ck <butterflyhuangxx@gmail.com>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAFcO6XN7JDM4xSXGhtusQfS2mSBcx50VJKwQpCq=WeLt57aaZA@mail.gmail.com
Link: https://lore.kernel.org/r/20220905060714.22549-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
sound/core/oss/pcm_oss.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -1664,14 +1664,14 @@ static int snd_pcm_oss_sync(struct snd_p
runtime = substream->runtime;
if (atomic_read(&substream->mmap_count))
goto __direct;
- err = snd_pcm_oss_make_ready(substream);
- if (err < 0)
- return err;
atomic_inc(&runtime->oss.rw_ref);
if (mutex_lock_interruptible(&runtime->oss.params_lock)) {
atomic_dec(&runtime->oss.rw_ref);
return -ERESTARTSYS;
}
+ err = snd_pcm_oss_make_ready_locked(substream);
+ if (err < 0)
+ goto unlock;
format = snd_pcm_oss_format_from(runtime->oss.format);
width = snd_pcm_format_physical_width(format);
if (runtime->oss.buffer_used > 0) {