powerpc/powernv: Check sysparam size before creation

The size of the sysparam sysfs files is determined from the device tree
at boot. However the buffer is hard coded to 64 bytes. If we encounter a
parameter that is larger than 64, or miss-parse the device tree, the
buffer will overflow when reading or writing to the parameter.

Check it at discovery time, and if the parameter is too large, do not
create a sysfs entry for it.
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powernv/opal-sysparam.c

@@ -241,6 +241,12 @@ void __init opal_sys_param_init(void)
 
 	/* For each of the parameters, populate the parameter attributes */
 	for (i = 0; i < count; i++) {
+		if (size[i] > MAX_PARAM_DATA_LEN) {
+			pr_warn("SYSPARAM: Not creating parameter %d as size "
+				"exceeds buffer length\n", i);
+			continue;
+		}
+
 		sysfs_attr_init(&attr[i].kobj_attr.attr);
 		attr[i].param_id = id[i];
 		attr[i].param_size = size[i];