ALSA: pcm: Fix race of buffer access at PCM OSS layer

The PCM OSS layer tries to clear the buffer with the silence data at
initialization (or reconfiguration) of a stream with the explicit call
of snd_pcm_format_set_silence() with runtime->dma_area.  But this may
lead to a UAF because the accessed runtime->dma_area might be freed
concurrently, as it's performed outside the PCM ops.

For avoiding it, move the code into the PCM core and perform it inside
the buffer access lock, so that it won't be changed during the
operation.

Reported-by: syzbot+32d4647f551007595173@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/68164d8e.050a0220.11da1b.0019.GAE@google.com
Cc: <stable@vger.kernel.org>
Link: https://patch.msgid.link/20250516080817.20068-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: tiwai

include/sound/pcm.h

@@ -1404,6 +1404,8 @@ int snd_pcm_lib_mmap_iomem(struct snd_pcm_substream *substream, struct vm_area_s
 #define snd_pcm_lib_mmap_iomem	NULL
 #endif
 
+void snd_pcm_runtime_buffer_set_silence(struct snd_pcm_runtime *runtime);
+
 /**
  * snd_pcm_limit_isa_dma_size - Get the max size fitting with ISA DMA transfer
  * @dma: DMA number

sound/core/oss/pcm_oss.c

@@ -1074,8 +1074,7 @@ static int snd_pcm_oss_change_params_locked(struct snd_pcm_substream *substream)
 	runtime->oss.params = 0;
 	runtime->oss.prepare = 1;
 	runtime->oss.buffer_used = 0;
-	if (runtime->dma_area)
-		snd_pcm_format_set_silence(runtime->format, runtime->dma_area, bytes_to_samples(runtime, runtime->dma_bytes));
+	snd_pcm_runtime_buffer_set_silence(runtime);
 
 	runtime->oss.period_frames = snd_pcm_alsa_frames(substream, oss_period_size);
 

sound/core/pcm_native.c

@@ -723,6 +723,17 @@ static void snd_pcm_buffer_access_unlock(struct snd_pcm_runtime *runtime)
 	atomic_inc(&runtime->buffer_accessing);
 }
 
+/* fill the PCM buffer with the current silence format; called from pcm_oss.c */
+void snd_pcm_runtime_buffer_set_silence(struct snd_pcm_runtime *runtime)
+{
+	snd_pcm_buffer_access_lock(runtime);
+	if (runtime->dma_area)
+		snd_pcm_format_set_silence(runtime->format, runtime->dma_area,
+					   bytes_to_samples(runtime, runtime->dma_bytes));
+	snd_pcm_buffer_access_unlock(runtime);
+}
+EXPORT_SYMBOL_GPL(snd_pcm_runtime_buffer_set_silence);
+
 #if IS_ENABLED(CONFIG_SND_PCM_OSS)
 #define is_oss_stream(substream)	((substream)->oss.oss)
 #else