ALSA: pcm: Fix potential data race at PCM memory allocation helpers

The PCM memory allocation helpers have a sanity check against too many
buffer allocations.  However, the check is performed without a proper
lock and the allocation isn't serialized; this allows user to allocate
more memories than predefined max size.

Practically seen, this isn't really a big problem, as it's more or
less some "soft limit" as a sanity check, and it's not possible to
allocate unlimitedly.  But it's still better to address this for more
consistent behavior.

The patch covers the size check in do_alloc_pages() with the
card->memory_mutex, and increases the allocated size there for
preventing the further overflow.  When the actual allocation fails,
the size is decreased accordingly.

Reported-by: BassCheck <bass@buaa.edu.cn>
Reported-by: Tuo Li <islituo@gmail.com>
Link: https://lore.kernel.org/r/CADm8Tek6t0WedK+3Y6rbE5YEt19tML8BUL45N2ji4ZAz1KcN_A@mail.gmail.com
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20230703112430.30634-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_memory.c

@@ -31,15 +31,41 @@ static unsigned long max_alloc_per_card = 32UL * 1024UL * 1024UL;
 module_param(max_alloc_per_card, ulong, 0644);
 MODULE_PARM_DESC(max_alloc_per_card, "Max total allocation bytes per card.");
 
+static void __update_allocated_size(struct snd_card *card, ssize_t bytes)
+{
+	card->total_pcm_alloc_bytes += bytes;
+}
+
+static void update_allocated_size(struct snd_card *card, ssize_t bytes)
+{
+	mutex_lock(&card->memory_mutex);
+	__update_allocated_size(card, bytes);
+	mutex_unlock(&card->memory_mutex);
+}
+
+static void decrease_allocated_size(struct snd_card *card, size_t bytes)
+{
+	mutex_lock(&card->memory_mutex);
+	WARN_ON(card->total_pcm_alloc_bytes < bytes);
+	__update_allocated_size(card, -(ssize_t)bytes);
+	mutex_unlock(&card->memory_mutex);
+}
+
 static int do_alloc_pages(struct snd_card *card, int type, struct device *dev,
 			  int str, size_t size, struct snd_dma_buffer *dmab)
 {
 	enum dma_data_direction dir;
 	int err;
 
+	/* check and reserve the requested size */
+	mutex_lock(&card->memory_mutex);
 	if (max_alloc_per_card &&
-	    card->total_pcm_alloc_bytes + size > max_alloc_per_card)
+	    card->total_pcm_alloc_bytes + size > max_alloc_per_card) {
+		mutex_unlock(&card->memory_mutex);
 		return -ENOMEM;
+	}
+	__update_allocated_size(card, size);
+	mutex_unlock(&card->memory_mutex);
 
 	if (str == SNDRV_PCM_STREAM_PLAYBACK)
 		dir = DMA_TO_DEVICE;
@@ -47,9 +73,14 @@ static int do_alloc_pages(struct snd_card *card, int type, struct device *dev,
 		dir = DMA_FROM_DEVICE;
 	err = snd_dma_alloc_dir_pages(type, dev, dir, size, dmab);
 	if (!err) {
-		mutex_lock(&card->memory_mutex);
-		card->total_pcm_alloc_bytes += dmab->bytes;
-		mutex_unlock(&card->memory_mutex);
+		/* the actual allocation size might be bigger than requested,
+		 * and we need to correct the account
+		 */
+		if (dmab->bytes != size)
+			update_allocated_size(card, dmab->bytes - size);
+	} else {
+		/* take back on allocation failure */
+		decrease_allocated_size(card, size);
 	}
 	return err;
 }
@@ -58,10 +89,7 @@ static void do_free_pages(struct snd_card *card, struct snd_dma_buffer *dmab)
 {
 	if (!dmab->area)
 		return;
-	mutex_lock(&card->memory_mutex);
-	WARN_ON(card->total_pcm_alloc_bytes < dmab->bytes);
-	card->total_pcm_alloc_bytes -= dmab->bytes;
-	mutex_unlock(&card->memory_mutex);
+	decrease_allocated_size(card, dmab->bytes);
 	snd_dma_free_pages(dmab);
 	dmab->area = NULL;
 }