thunderbolt: xdomain: Avoid potential stack OOB read

tb_xdp_properties_changed_request() was calling tb_xdp_handle_error() with
a struct tb_xdp_properties_changed_response on the stack, which does not
have the "error" field present when cast to struct tb_xdp_error_response.
This was detected when building with -Warray-bounds:

drivers/thunderbolt/xdomain.c: In function 'tb_xdomain_properties_changed':
drivers/thunderbolt/xdomain.c:226:22: error: array subscript 'const struct tb_xdp_error_response[0]' is partly outside array bounds of 'struct tb_xdp_properties_changed_response[1]' [-Werror=array-bounds]
226 | switch (error->error) {
| ~~~~~^~~~~~~
drivers/thunderbolt/xdomain.c:448:51: note: while referencing 'res'
448 | struct tb_xdp_properties_changed_response res;
| ^~~

Add union containing struct tb_xdp_error_response to structures passed
to tb_xdp_handle_error(), so that the "error" field will be present.

Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Mika Westerberg <[e-mail]>

drivers/thunderbolt/tb_msgs.h

···
 	u32 type;
 };
 
+struct tb_xdp_error_response {
+	struct tb_xdp_header hdr;
+	u32 error;
+};
+
 struct tb_xdp_uuid {
 	struct tb_xdp_header hdr;
 };
 
 struct tb_xdp_uuid_response {
-	struct tb_xdp_header hdr;
-	uuid_t src_uuid;
-	u32 src_route_hi;
-	u32 src_route_lo;
+	union {
+		struct tb_xdp_error_response err;
+		struct {
+			struct tb_xdp_header hdr;
+			uuid_t src_uuid;
+			u32 src_route_hi;
+			u32 src_route_lo;
+		};
+	};
 };
 
 struct tb_xdp_properties {
···
 };
 
 struct tb_xdp_properties_response {
-	struct tb_xdp_header hdr;
-	uuid_t src_uuid;
-	uuid_t dst_uuid;
-	u16 offset;
-	u16 data_length;
-	u32 generation;
-	u32 data[0];
+	union {
+		struct tb_xdp_error_response err;
+		struct {
+			struct tb_xdp_header hdr;
+			uuid_t src_uuid;
+			uuid_t dst_uuid;
+			u16 offset;
+			u16 data_length;
+			u32 generation;
+			u32 data[];
+		};
+	};
 };
 
 /*
···
 };
 
 struct tb_xdp_properties_changed_response {
-	struct tb_xdp_header hdr;
+	union {
+		struct tb_xdp_error_response err;
+		struct tb_xdp_header hdr;
+	};
 };
 
 enum tb_xdp_error {
···
 	ERROR_UNKNOWN_DOMAIN,
 	ERROR_NOT_SUPPORTED,
 	ERROR_NOT_READY,
-};
-
-struct tb_xdp_error_response {
-	struct tb_xdp_header hdr;
-	u32 error;
 };
 
 #endif

drivers/thunderbolt/xdomain.c

···
 	memcpy(&hdr->uuid, &tb_xdp_uuid, sizeof(tb_xdp_uuid));
 }
 
-static int tb_xdp_handle_error(const struct tb_xdp_header *hdr)
+static int tb_xdp_handle_error(const struct tb_xdp_error_response *res)
 {
-	const struct tb_xdp_error_response *error;
-
-	if (hdr->type != ERROR_RESPONSE)
+	if (res->hdr.type != ERROR_RESPONSE)
 		return 0;
 
-	error = (const struct tb_xdp_error_response *)hdr;
-
-	switch (error->error) {
+	switch (res->error) {
 	case ERROR_UNKNOWN_PACKET:
 	case ERROR_UNKNOWN_DOMAIN:
 		return -EIO;
···
 	if (ret)
 		return ret;
 
-	ret = tb_xdp_handle_error(&res.hdr);
+	ret = tb_xdp_handle_error(&res.err);
 	if (ret)
 		return ret;
 
···
 		if (ret)
 			goto err;
 
-		ret = tb_xdp_handle_error(&res->hdr);
+		ret = tb_xdp_handle_error(&res->err);
 		if (ret)
 			goto err;
 
···
 	if (ret)
 		return ret;
 
-	return tb_xdp_handle_error(&res.hdr);
+	return tb_xdp_handle_error(&res.err);
 }
 
 static int