diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index b3b8f2f3ad106aea4b4f33c9f61bf7eb01919563..6f0121e3be69f84e294af73c9ccb4b6df16a8f6c 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -810,6 +810,12 @@ static int process_msg(void)
 		goto out;
 	}
 
+	if (msg->hdr.len > XENSTORE_PAYLOAD_MAX) {
+		kfree(msg);
+		err = -EINVAL;
+		goto out;
+	}
+
 	body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);
 	if (body == NULL) {
 		kfree(msg);
diff --git a/include/xen/interface/io/xs_wire.h b/include/xen/interface/io/xs_wire.h
index f0b6890370be053e25feadfd7bf0eeeacb78f800..3c1877caaefcdeba09e951604712599af78f17ed 100644
--- a/include/xen/interface/io/xs_wire.h
+++ b/include/xen/interface/io/xs_wire.h
@@ -88,4 +88,7 @@ struct xenstore_domain_interface {
     XENSTORE_RING_IDX rsp_cons, rsp_prod;
 };
 
+/* Violating this is very bad.  See docs/misc/xenstore.txt. */
+#define XENSTORE_PAYLOAD_MAX 4096
+
 #endif /* _XS_WIRE_H */