ALSA: hdspm: Hardening for potential Spectre v1

As recently Smatch suggested, a couple of places in HDSP MADI driver
may expand the array directly from the user-space value with
speculation:
  sound/pci/rme9652/hdspm.c:5717 snd_hdspm_channel_info() warn: potential spectre issue 'hdspm->channel_map_out' (local cap)
  sound/pci/rme9652/hdspm.c:5734 snd_hdspm_channel_info() warn: potential spectre issue 'hdspm->channel_map_in' (local cap)

This patch puts array_index_nospec() for hardening against them.

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 10513142)
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/hdspm.c

@@ -137,6 +137,7 @@
 #include <linux/pci.h>
 #include <linux/math64.h>
 #include <linux/io.h>
+#include <linux/nospec.h>
 
 #include <sound/core.h>
 #include <sound/control.h>
@@ -5692,40 +5693,43 @@ static int snd_hdspm_channel_info(struct snd_pcm_substream *substream,
 		struct snd_pcm_channel_info *info)
 {
 	struct hdspm *hdspm = snd_pcm_substream_chip(substream);
+	unsigned int channel = info->channel;
 
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
-		if (snd_BUG_ON(info->channel >= hdspm->max_channels_out)) {
+		if (snd_BUG_ON(channel >= hdspm->max_channels_out)) {
 			dev_info(hdspm->card->dev,
 				 "snd_hdspm_channel_info: output channel out of range (%d)\n",
-				 info->channel);
+				 channel);
 			return -EINVAL;
 		}
 
-		if (hdspm->channel_map_out[info->channel] < 0) {
+		channel = array_index_nospec(channel, hdspm->max_channels_out);
+		if (hdspm->channel_map_out[channel] < 0) {
 			dev_info(hdspm->card->dev,
 				 "snd_hdspm_channel_info: output channel %d mapped out\n",
-				 info->channel);
+				 channel);
 			return -EINVAL;
 		}
 
-		info->offset = hdspm->channel_map_out[info->channel] *
+		info->offset = hdspm->channel_map_out[channel] *
 			HDSPM_CHANNEL_BUFFER_BYTES;
 	} else {
-		if (snd_BUG_ON(info->channel >= hdspm->max_channels_in)) {
+		if (snd_BUG_ON(channel >= hdspm->max_channels_in)) {
 			dev_info(hdspm->card->dev,
 				 "snd_hdspm_channel_info: input channel out of range (%d)\n",
-				 info->channel);
+				 channel);
 			return -EINVAL;
 		}
 
-		if (hdspm->channel_map_in[info->channel] < 0) {
+		channel = array_index_nospec(channel, hdspm->max_channels_in);
+		if (hdspm->channel_map_in[channel] < 0) {
 			dev_info(hdspm->card->dev,
 				 "snd_hdspm_channel_info: input channel %d mapped out\n",
-				 info->channel);
+				 channel);
 			return -EINVAL;
 		}
 
-		info->offset = hdspm->channel_map_in[info->channel] *
+		info->offset = hdspm->channel_map_in[channel] *
 			HDSPM_CHANNEL_BUFFER_BYTES;
 	}