ALSA: control: Hardening for potential Spectre v1

As recently Smatch suggested, a few places in ALSA control core codes
may expand the array directly from the user-space value with
speculation:

  sound/core/control.c:1003 snd_ctl_elem_lock() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:1031 snd_ctl_elem_unlock() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:844 snd_ctl_elem_info() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:891 snd_ctl_elem_read() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:939 snd_ctl_elem_write() warn: potential spectre issue 'kctl->vd'

Although all these seem doing only the first load without further
reference, we may want to stay in a safer side, so hardening with
array_index_nospec() would still make sense.

In this patch, we put array_index_nospec() to the common
snd_ctl_get_ioff*() helpers instead of each caller.  These helpers are
also referred from some drivers, too, and basically all usages are to
calculate the array index from the user-space value, hence it's better
to cover there.

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>

include/sound/control.h

@@ -23,6 +23,7 @@
  */
 
 #include <linux/wait.h>
+#include <linux/nospec.h>
 #include <sound/asound.h>
 
 #define snd_kcontrol_chip(kcontrol) ((kcontrol)->private_data)
@@ -148,12 +149,14 @@ int snd_ctl_get_preferred_subdevice(struct snd_card *card, int type);
 
 static inline unsigned int snd_ctl_get_ioffnum(struct snd_kcontrol *kctl, struct snd_ctl_elem_id *id)
 {
-	return id->numid - kctl->id.numid;
+	unsigned int ioff = id->numid - kctl->id.numid;
+	return array_index_nospec(ioff, kctl->count);
 }
 
 static inline unsigned int snd_ctl_get_ioffidx(struct snd_kcontrol *kctl, struct snd_ctl_elem_id *id)
 {
-	return id->index - kctl->id.index;
+	unsigned int ioff = id->index - kctl->id.index;
+	return array_index_nospec(ioff, kctl->count);
 }
 
 static inline unsigned int snd_ctl_get_ioff(struct snd_kcontrol *kctl, struct snd_ctl_elem_id *id)