[PATCH v2 2/4] ALSA: compress: reject buffer geometry change on gapless next_track

[Peter Ujfalusi <peter.ujfalusi@linux.intel.com>]

snd_compr_set_params() is invoked from the gapless next_track
re-entry path, which calls snd_compr_allocate_buffer() again and lets
buffer_size change while total_bytes_available and
total_bytes_transferred still hold values computed for the previous
buffer. snd_compr_calc_avail() then derives an avail/count value from
those stale counters against the new buffer_size, and
snd_compr_write_data() copy_from_user()s that many bytes into the
new, potentially smaller buffer, overflowing it with user-controlled
length and content.

The buffer is set up once, with the initial SET_PARAMS, and neither the
core nor the DSP expects it to change for a gapless track transition.
Do not call snd_compr_allocate_buffer() on the next_track path and
reject a request that changes the buffer geometry, so that the codec
parameters that the driver receives match the buffer in use.

Fixes: 9727b490e543 ("ALSA: compress: add support for gapless playback")
Cc: stable@vger.kernel.org
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
---
 sound/core/compress_offload.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/sound/core/compress_offload.c b/sound/core/compress_offload.c
index 50301f72579e..d6863336f21c 100644
--- a/sound/core/compress_offload.c
+++ b/sound/core/compress_offload.c
@@ -697,9 +697,19 @@ snd_compr_set_params(struct snd_compr_stream *stream, unsigned long arg)
 		if (retval)
 			return retval;
 
-		retval = snd_compr_allocate_buffer(stream, params);
-		if (retval)
-			return -ENOMEM;
+		if (stream->next_track) {
+			/*
+			 * total_bytes_available/transferred still refer to the
+			 * current buffer; its geometry must not change.
+			 */
+			if (params->buffer.fragment_size != stream->runtime->fragment_size ||
+			    params->buffer.fragments != stream->runtime->fragments)
+				return -EINVAL;
+		} else {
+			retval = snd_compr_allocate_buffer(stream, params);
+			if (retval)
+				return retval;
+		}
 
 		retval = stream->ops->set_params(stream, params);
 		if (retval) {
-- 
2.56.0