linux-kernel - [PATCH 4.4.y] ALSA: pcm: Add missing copy ops check before clearing buffer

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Date:   Wed, 29 Jan 2020 10:40:41 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andreas Schneider <asn@...ptomilk.org>, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 4.4.y] ALSA: pcm: Add missing copy ops check before clearing buffer

[ this is a fix specific to 4.4.y and 4.9.y stable trees;
  4.14.y and older already contain the right fix ]

The stable 4.4.y and 4.9.y backports of the upstream commit
add9d56d7b37 ("ALSA: pcm: Avoid possible info leaks from PCM stream
buffers") dropped the check of substream->ops->copy_user as copy_user
is a new member that isn't present in the older kernels.
Although upstream drivers should work without this NULL check, it may
cause a regression with a downstream driver that sets some
inaccessible address to runtime->dma_area, leading to a crash at
worst.

Since such drivers must have ops->copy member on older kernels instead
of ops->copy_user, this patch adds the missing check of ops->copy for
fixing the regression.

Reported-and-tested-by: Andreas Schneider <asn@...ptomilk.org>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
---
 sound/core/pcm_native.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index b9bfbf394959..59423576b1cc 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -588,7 +588,7 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 		runtime->boundary *= 2;

 	/* clear the buffer for avoiding possible kernel info leaks */
-	if (runtime->dma_area)
+	if (runtime->dma_area && !substream->ops->copy)
 		memset(runtime->dma_area, 0, runtime->dma_bytes);

 	snd_pcm_timer_resolution_change(substream);
-- 
2.16.4