ALSA: pcm: Fix potential Spectre v1 vulnerability

stream is indirectly controlled by user-space, hence leading to a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

sound/core/pcm.c:140 snd_pcm_control_ioctl() warn: potential spectre issue 'pcm->streams' [r] (local cap)

Fix this by sanitizing stream before using it to index pcm->streams

Notice that given that speculation windows are large, the policy is to kill the speculation on the first load and not worry if it can be completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

---
 sound/core/pcm.c | 3 +++
 1 file changed, 3 insertions(+)

index fdb9b92fc8d6..23141003a90d 100644
@@ -32,6 +32,8 @@
 #include <sound/control.h>
 #include <sound/info.h>
 
+#include <linux/nospec.h>
+
 #include "pcm_local.h"
 
 MODULE_AUTHOR("Jaroslav Kysela <[email protected]>, Abramo Bagnara <[email protected]>");

@@ -129,6 +131,7 @@ static int snd_pcm_control_ioctl(struct snd_card *card,
 				return -EFAULT;
 			if (stream < 0 || stream > 1)
 				return -EINVAL;
+			stream = array_index_nospec(stream, 2);
 			if (get_user(subdevice, &info->subdevice))
 				return -EFAULT;
 			mutex_lock(&register_mutex);

2.17.1

Hi,

[This is an automated email]

This commit has been processed because it contains a -stable tag. The stable tag indicates that it's relevant for the following trees: all

The bot has tested the following trees: v4.19.8, v4.14.87, v4.9.144, v4.4.166, v3.18.128,

v4.19.8: Build OK!
v4.14.87: Build OK!
v4.9.144: Failed to apply! Possible dependencies:
2c4842d3b6b3 ("ALSA: pcm: add local header file for snd-pcm module")

v4.4.166: Failed to apply! Possible dependencies:
2c4842d3b6b3 ("ALSA: pcm: add local header file for snd-pcm module")

v3.18.128: Failed to apply! Possible dependencies:
2c4842d3b6b3 ("ALSA: pcm: add local header file for snd-pcm module") 90bbaf66ee7b ("ALSA: timer: add config item to export PCM timer disabling for expert") 9bc889b4ba88 ("ALSA: pcm: Update the state properly before notification") f5914908a5b7 ("ALSA: pcm: Replace PCM hwptr tracking with tracepoints")

How should we proceed with this patch?

--
Thanks,
Sasha

Applied now (again after moving the include line into a more appropriate place).

thanks,

Takashi