ALSA: hda: Hardening for potential Spectre v1

As recently Smatch suggested, one place in HD-audio hwdep ioctl codes
may expand the array directly from the user-space value with
speculation:
  sound/pci/hda/hda_local.h:467 get_wcaps() warn: potential spectre issue 'codec->wcaps'

As get_wcaps() itself is a fairly frequently called inline function,
and there is only one single call with a user-space value, we replace
only the latter one to open-code locally with array_index_nospec()
hardening in this patch.

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

sound/pci/hda/hda_hwdep.c

@@ -21,6 +21,7 @@
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/compat.h>
+#include <linux/nospec.h>
 #include <sound/core.h>
 #include "hda_codec.h"
 #include "hda_local.h"
@@ -51,7 +52,16 @@ static int get_wcap_ioctl(struct hda_codec *codec,
 	
 	if (get_user(verb, &arg->verb))
 		return -EFAULT;
-	res = get_wcaps(codec, verb >> 24);
+	/* open-code get_wcaps(verb>>24) with nospec */
+	verb >>= 24;
+	if (verb < codec->core.start_nid ||
+	    verb >= codec->core.start_nid + codec->core.num_nodes) {
+		res = 0;
+	} else {
+		verb -= codec->core.start_nid;
+		verb = array_index_nospec(verb, codec->core.num_nodes);
+		res = codec->wcaps[verb];
+	}
 	if (put_user(res, &arg->res))
 		return -EFAULT;
 	return 0;