[PATCH 4.14 12/14] ALSA: pcm: Fix races among concurrent prealloc proc writes

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Takashi Iwai <tiwai@suse.de>

commit 69534c48ba8ce552ce383b3dfdb271ffe51820c3 upstream.

We have no protection against concurrent PCM buffer preallocation
changes via proc files, and it may potentially lead to UAF or some
weird problem.  This patch applies the PCM open_mutex to the proc
write operation for avoiding the racy proc writes and the PCM stream
open (and further operations).

Cc: <stable@vger.kernel.org>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20220322170720.3529-5-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[OP: backport to 4.14: adjusted context]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 sound/core/pcm_memory.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/sound/core/pcm_memory.c
+++ b/sound/core/pcm_memory.c
@@ -160,19 +160,20 @@ static void snd_pcm_lib_preallocate_proc
 	size_t size;
 	struct snd_dma_buffer new_dmab;
 
+	mutex_lock(&substream->pcm->open_mutex);
 	if (substream->runtime) {
 		buffer->error = -EBUSY;
-		return;
+		goto unlock;
 	}
 	if (!snd_info_get_line(buffer, line, sizeof(line))) {
 		snd_info_get_str(str, line, sizeof(str));
 		size = simple_strtoul(str, NULL, 10) * 1024;
 		if ((size != 0 && size < 8192) || size > substream->dma_max) {
 			buffer->error = -EINVAL;
-			return;
+			goto unlock;
 		}
 		if (substream->dma_buffer.bytes == size)
-			return;
+			goto unlock;
 		memset(&new_dmab, 0, sizeof(new_dmab));
 		new_dmab.dev = substream->dma_buffer.dev;
 		if (size > 0) {
@@ -180,7 +181,7 @@ static void snd_pcm_lib_preallocate_proc
 						substream->dma_buffer.dev.dev,
 						size, &new_dmab) < 0) {
 				buffer->error = -ENOMEM;
-				return;
+				goto unlock;
 			}
 			substream->buffer_bytes_max = size;
 		} else {
@@ -192,6 +193,8 @@ static void snd_pcm_lib_preallocate_proc
 	} else {
 		buffer->error = -EINVAL;
 	}
+ unlock:
+	mutex_unlock(&substream->pcm->open_mutex);
 }
 
 static inline void preallocate_info_init(struct snd_pcm_substream *substream)