ALSA: rme9652: Hardening for potential Spectre v1

As recently Smatch suggested, one place in RME9652 driver may expand
the array directly from the user-space value with speculation:
  sound/pci/rme9652/rme9652.c:2074 snd_rme9652_channel_info() warn: potential spectre issue 'rme9652->channel_map' (local cap)

This patch puts array_index_nospec() for hardening against it.

BugLink: https://marc.info/?l=linux-kernel&m=152411496503418&w=2Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

CVE-2017-5753

(cherry picked from commit f526afcd)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

sound/pci/rme9652/rme9652.c

@@ -26,6 +26,7 @@
 #include <linux/pci.h>
 #include <linux/module.h>
 #include <linux/io.h>
+#include <linux/nospec.h>
 
 #include <sound/core.h>
 #include <sound/control.h>
@@ -2036,9 +2037,10 @@ static int snd_rme9652_channel_info(struct snd_pcm_substream *substream,
 	if (snd_BUG_ON(info->channel >= RME9652_NCHANNELS))
 		return -EINVAL;
 
-	if ((chn = rme9652->channel_map[info->channel]) < 0) {
+	chn = rme9652->channel_map[array_index_nospec(info->channel,
+						      RME9652_NCHANNELS)];
+	if (chn < 0)
 		return -EINVAL;
-	}
 
 	info->offset = chn * RME9652_CHANNEL_BUFFER_BYTES;
 	info->first = 0;