uref->usage_index can be indirectly controlled by userspace, hence leading
to a potential exploitation of the Spectre variant 1 vulnerability.

This field is used as an array index by the hiddev_ioctl_usage() function,
when 'cmd' is either HIDIOCGCOLLECTIONINDEX, HIDIOCGUSAGES or
HIDIOCSUSAGES.

For cmd == HIDIOCGCOLLECTIONINDEX case, uref->usage_index is compared to
field->maxusage and then used as an index to dereference field->usage
array. The same thing happens to the cmd == HIDIOC{G,S}USAGES cases, where
uref->usage_index is checked against an array maximum value and then it is
used as an index in an array.

This is a summary of the HIDIOCGCOLLECTIONINDEX case, which matches the
traditional Spectre V1 first load:

	copy_from_user(uref, user_arg, sizeof(*uref))
	if (uref->usage_index >= field->maxusage)
		goto inval;
	i = field->usage[uref->usage_index].collection_index;
	return i;

This patch fixes this by sanitizing field uref->usage_index before using it
to index field->usage (HIDIOCGCOLLECTIONINDEX) or field->value in
HIDIOC{G,S}USAGES arrays, thus, avoiding speculation in the first load.

Cc: <stable@vger.kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
--

v2: Contemplate cmd == HIDIOC{G,S}USAGES case
Signed-off-by: Jiri Kosina <jkosina@suse.cz>