nbd-server: do not check request length except for reads and writes

Only reads and writes need to allocate memory correspondent to the
request length.  Other requests can be sent to the storage without
allocating any memory, and thus any request length is acceptable.

Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Cc: qemu-block@nongnu.org
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd/server.c

@@ -818,13 +818,6 @@ static ssize_t nbd_co_receive_request(NBDRequest *req, struct nbd_request *reque
         goto out;
     }
 
-    if (request->len > NBD_MAX_BUFFER_SIZE) {
-        LOG("len (%u) is larger than max len (%u)",
-            request->len, NBD_MAX_BUFFER_SIZE);
-        rc = -EINVAL;
-        goto out;
-    }
-
     if ((request->from + request->len) < request->from) {
         LOG("integer overflow detected! "
             "you're probably being attacked");
@@ -836,6 +829,13 @@ static ssize_t nbd_co_receive_request(NBDRequest *req, struct nbd_request *reque
 
     command = request->type & NBD_CMD_MASK_COMMAND;
     if (command == NBD_CMD_READ || command == NBD_CMD_WRITE) {
+        if (request->len > NBD_MAX_BUFFER_SIZE) {
+            LOG("len (%u) is larger than max len (%u)",
+                request->len, NBD_MAX_BUFFER_SIZE);
+            rc = -EINVAL;
+            goto out;
+        }
+
         req->data = blk_blockalign(client->exp->blk, request->len);
     }
     if (command == NBD_CMD_WRITE) {