[PATCH] ALSA: pcm: oss: Avoid plugin buffer overflow

[PATCH] ALSA: pcm: oss: Avoid plugin buffer overflow

[Takashi Iwai]

Each OSS PCM plugins allocate its internal buffer per pre-calculation
of the max buffer size through the chain of plugins (calling
src_frames and dst_frames callbacks).  This works for most plugins,
but the rate plugin might behave incorrectly.  The calculation in the
rate plugin involves with the fractional position, i.e. it may vary
depending on the input position.  Since the buffer size
pre-calculation is always done with the offset zero, it may return a
shorter size than it might be; this may result in the out-of-bound
access as spotted by fuzzer.

This patch addresses those possible buffer overflow accesses by simply
setting the upper limit per the given buffer size for each plugin
before src_frames() and after dst_frames() calls.

Reported-by: syzbot+e1fe9f44fb8ecf4fb5dd@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000b25ea005a02bcf21@google.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/core/oss/pcm_plugin.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sound/core/oss/pcm_plugin.c b/sound/core/oss/pcm_plugin.c
index 240e4702c098..c9401832967c 100644
--- a/sound/core/oss/pcm_plugin.c
+++ b/sound/core/oss/pcm_plugin.c
@@ -209,6 +209,8 @@ snd_pcm_sframes_t snd_pcm_plug_client_size(struct snd_pcm_substream *plug, snd_p
 	if (stream == SNDRV_PCM_STREAM_PLAYBACK) {
 		plugin = snd_pcm_plug_last(plug);
 		while (plugin && drv_frames > 0) {
+			if (drv_frames > plugin->buf_frames)
+				drv_frames = plugin->buf_frames;
 			plugin_prev = plugin->prev;
 			if (plugin->src_frames)
 				drv_frames = plugin->src_frames(plugin, drv_frames);
@@ -220,6 +222,8 @@ snd_pcm_sframes_t snd_pcm_plug_client_size(struct snd_pcm_substream *plug, snd_p
 			plugin_next = plugin->next;
 			if (plugin->dst_frames)
 				drv_frames = plugin->dst_frames(plugin, drv_frames);
+			if (drv_frames > plugin->buf_frames)
+				drv_frames = plugin->buf_frames;
 			plugin = plugin_next;
 		}
 	} else
@@ -248,11 +252,15 @@ snd_pcm_sframes_t snd_pcm_plug_slave_size(struct snd_pcm_substream *plug, snd_pc
 				if (frames < 0)
 					return frames;
 			}
+			if (frames > plugin->buf_frames)
+				frames = plugin->buf_frames;
 			plugin = plugin_next;
 		}
 	} else if (stream == SNDRV_PCM_STREAM_CAPTURE) {
 		plugin = snd_pcm_plug_last(plug);
 		while (plugin) {
+			if (frames > plugin->buf_frames)
+				frames = plugin->buf_frames;
 			plugin_prev = plugin->prev;
 			if (plugin->src_frames) {
 				frames = plugin->src_frames(plugin, frames);
-- 
2.16.4

Re: [PATCH] ALSA: pcm: oss: Avoid plugin buffer overflow

[Serge Belyshev]

> ...
>
> This patch addresses those possible buffer overflow accesses by simply
> setting the upper limit per the given buffer size for each plugin
> before src_frames() and after dst_frames() calls.

Hi!

This patch breaks any output via oss interface, as evident by "mpv
--ao=oss somefile.mp3" or "mpg123 -o oss somefile.mp3" or just "cat
/dev/urandom > /dev/dsp", which worked previously in kernel version 5.5
but not any longer starting with 5.6.

It appears here that plugin->buf_frames is zero which results in ENXIO
returned to userspace.

> ...
> --- a/sound/core/oss/pcm_plugin.c
> +++ b/sound/core/oss/pcm_plugin.c
> @@ -209,6 +209,8 @@ snd_pcm_sframes_t snd_pcm_plug_client_size(struct snd_pcm_substream *plug, snd_p
>  	if (stream == SNDRV_PCM_STREAM_PLAYBACK) {
>  		plugin = snd_pcm_plug_last(plug);
>  		while (plugin && drv_frames > 0) {
> +			if (drv_frames > plugin->buf_frames)
> +				drv_frames = plugin->buf_frames;
>  			plugin_prev = plugin->prev;
>  			if (plugin->src_frames)
>  				drv_frames = plugin->src_frames(plugin, drv_frames);
> ...

Re: [PATCH] ALSA: pcm: oss: Avoid plugin buffer overflow

[Takashi Iwai]

On Thu, 30 Apr 2020 18:34:02 +0200,
Serge Belyshev wrote:
> 
> > ...
> >
> > This patch addresses those possible buffer overflow accesses by simply
> > setting the upper limit per the given buffer size for each plugin
> > before src_frames() and after dst_frames() calls.
> 
> Hi!
> 
> This patch breaks any output via oss interface, as evident by "mpv
> --ao=oss somefile.mp3" or "mpg123 -o oss somefile.mp3" or just "cat
> /dev/urandom > /dev/dsp", which worked previously in kernel version 5.5
> but not any longer starting with 5.6.
> 
> It appears here that plugin->buf_frames is zero which results in ENXIO
> returned to userspace.

The fix is already in sound git tree, and will be in the next pull
request to 5.7-rc4.


thanks,

Takashi

Re: [PATCH] ALSA: pcm: oss: Avoid plugin buffer overflow

[Serge Belyshev]

> The fix is already in sound git tree, and will be in the next pull
> request to 5.7-rc4.
>

I have tested sound-5.7-rc4 tag in sound git tree and it worked for me,
thanks!