libnvdimm, pfn: make 'resource' attribute only readable by root

For the same reason that /proc/iomem returns 0's for non-root readers
and acpi tables are root-only, make the 'resource' attribute for pfn
devices only readable by root. Otherwise we disclose physical address
information.

Fixes: f6ed58c7 ("libnvdimm, pfn: 'resource'-address and 'size'...")
Cc: <stable@vger.kernel.org>
Reported-by: NDave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/nvdimm/pfn_devs.c

@@ -282,8 +282,16 @@ static struct attribute *nd_pfn_attributes[] = {
 	NULL,
 };
 
+static umode_t pfn_visible(struct kobject *kobj, struct attribute *a, int n)
+{
+	if (a == &dev_attr_resource.attr)
+		return 0400;
+	return a->mode;
+}
+
 struct attribute_group nd_pfn_attribute_group = {
 	.attrs = nd_pfn_attributes,
+	.is_visible = pfn_visible,
 };
 
 static const struct attribute_group *nd_pfn_attribute_groups[] = {