From e190161f96b88ffae870405fd6c3fdd1d2e7f98d Mon Sep 17 00:00:00 2001 From: Takashi Iwai Date: Fri, 25 Jan 2019 17:11:32 +0100 Subject: [PATCH] ALSA: pcm: Fix tight loop of OSS capture stream When the trigger=off is passed for a PCM OSS stream, it sets the start_threshold of the given substream to the boundary size, so that it won't be automatically started. This can be problematic for a capture stream, unfortunately, as detected by syzkaller. The scenario is like the following: - In __snd_pcm_lib_xfer() that is invoked from snd_pcm_oss_read() loop, we have a check whether the stream was already started or the stream can be auto-started. - The function at this check returns 0 with trigger=off since we explicitly disable the auto-start. - The loop continues and repeats calling __snd_pcm_lib_xfer() tightly, which may lead to an RCU stall. This patch fixes the bug by simply allowing the wait for non-started stream in the case of OSS capture. For native usages, it's supposed to be done by the caller side (which is user-space), hence it returns zero like before. (In theory, __snd_pcm_lib_xfer() could wait even for the native API usage cases, too; but I'd like to stay in a safer side for not breaking the existing stuff for now.) Reported-by: syzbot+fbe0496f92a0ce7b786c@syzkaller.appspotmail.com Cc: Signed-off-by: Takashi Iwai --- sound/core/pcm_lib.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c index 40013b26f671..6c99fa8ac5fa 100644 --- a/sound/core/pcm_lib.c +++ b/sound/core/pcm_lib.c @@ -2112,6 +2112,13 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream, return 0; } +/* allow waiting for a capture stream that hasn't been started */ +#if IS_ENABLED(CONFIG_SND_PCM_OSS) +#define wait_capture_start(substream) ((substream)->oss.oss) +#else +#define wait_capture_start(substream) false +#endif + /* the common loop for read/write data */ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream, void *data, bool interleaved, @@ -2182,7 +2189,7 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream, err = snd_pcm_start(substream); if (err < 0) goto _end_unlock; - } else { + } else if (!wait_capture_start(substream)) { /* nothing to do */ err = 0; goto _end_unlock; -- 2.30.2