[PATCH 3.16 214/366] ALSA: control: Hardening for potential Spectre v1

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@xxxxxxx>

commit 088e861edffb84879cf0c0d1b02eda078c3a0ffe upstream.

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@xxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/sound/control.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/include/sound/control.h
+++ b/include/sound/control.h
@@ -22,6 +22,7 @@
  *
  */
 
+#include <linux/nospec.h>
 #include <sound/asound.h>
 
 #define snd_kcontrol_chip(kcontrol) ((kcontrol)->private_data)
@@ -135,12 +136,14 @@ int snd_ctl_unregister_ioctl_compat(snd_
 
 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)