firmware: arm_scmi: Validate BASE_DISCOVER_LIST_PROTOCOLS response

Do not blindly trust SCMI platform response about list of implemented
protocols, instead validate the reported length of the list of protocols
against the real payload size of the message reply.

Link: https://lore.kernel.org/r/20220330150551.2573938-5-cristian.marussi@arm.com
Fixes: b6f20ff8 ("firmware: arm_scmi: add common infrastructure and support for base protocol")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
[sudeep.holla: Added early break if loop_num_ret = 0 and simplified calc_list_sz
	calculation]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/base.c

@@ -189,6 +189,9 @@ scmi_base_implementation_list_get(const struct scmi_protocol_handle *ph,
 	list = t->rx.buf + sizeof(*num_ret);
 
 	do {
+		size_t real_list_sz;
+		u32 calc_list_sz;
+
 		/* Set the number of protocols to be skipped/already read */
 		*num_skip = cpu_to_le32(tot_num_ret);
 
@@ -197,11 +200,32 @@ scmi_base_implementation_list_get(const struct scmi_protocol_handle *ph,
 			break;
 
 		loop_num_ret = le32_to_cpu(*num_ret);
+		if (!loop_num_ret)
+			break;
+
 		if (loop_num_ret > MAX_PROTOCOLS_IMP - tot_num_ret) {
 			dev_err(dev, "No. of Protocol > MAX_PROTOCOLS_IMP");
 			break;
 		}
 
+		if (t->rx.len < (sizeof(u32) * 2)) {
+			dev_err(dev, "Truncated reply - rx.len:%zd\n",
+				t->rx.len);
+			ret = -EPROTO;
+			break;
+		}
+
+		real_list_sz = t->rx.len - sizeof(u32);
+		calc_list_sz = (1 + (loop_num_ret - 1) / sizeof(u32)) *
+				sizeof(u32);
+		if (calc_list_sz != real_list_sz) {
+			dev_err(dev,
+				"Malformed reply - real_sz:%zd  calc_sz:%u\n",
+				real_list_sz, calc_list_sz);
+			ret = -EPROTO;
+			break;
+		}
+
 		for (loop = 0; loop < loop_num_ret; loop++)
 			protocols_imp[tot_num_ret + loop] = *(list + loop);