ALSA: rme9652: Fix potential Spectre v1 vulnerability

info->channel 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/pci/rme9652/hdsp.c:4100 snd_hdsp_channel_info() warn: potential spectre issue 'hdsp->channel_map' [r] (local cap)

Fix this by sanitizing info->channel before using it to index hdsp->channel_map

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].

Also, notice that I refactored the code a bit in order to get rid of the
following checkpatch warning:

ERROR: do not use assignment in if condition
FILE: sound/pci/rme9652/hdsp.c:4103:
	if ((mapped_channel = hdsp->channel_map[info->channel]) < 0)

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

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/rme9652/hdsp.c

@@ -30,6 +30,7 @@
 #include <linux/math64.h>
 #include <linux/vmalloc.h>
 #include <linux/io.h>
+#include <linux/nospec.h>
 
 #include <sound/core.h>
 #include <sound/control.h>
@@ -4092,15 +4093,16 @@ static int snd_hdsp_channel_info(struct snd_pcm_substream *substream,
 				    struct snd_pcm_channel_info *info)
 {
 	struct hdsp *hdsp = snd_pcm_substream_chip(substream);
-	int mapped_channel;
+	unsigned int channel = info->channel;
 
-	if (snd_BUG_ON(info->channel >= hdsp->max_channels))
+	if (snd_BUG_ON(channel >= hdsp->max_channels))
+		return -EINVAL;
+	channel = array_index_nospec(channel, hdsp->max_channels);
+
+	if (hdsp->channel_map[channel] < 0)
 		return -EINVAL;
 
-	if ((mapped_channel = hdsp->channel_map[info->channel]) < 0)
-		return -EINVAL;
-
-	info->offset = mapped_channel * HDSP_CHANNEL_BUFFER_BYTES;
+	info->offset = hdsp->channel_map[channel] * HDSP_CHANNEL_BUFFER_BYTES;
 	info->first = 0;
 	info->step = 32;
 	return 0;