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https://bugs.launchpad.net/bugs/1673085

Title:
  scheduler hints are unbounded and never deleted

Status in OpenStack Compute (nova):
  New
Status in OpenStack Security Advisory:
  Won't Fix
Status in OpenStack Security Notes:
  New

Bug description:
  I'm initially reporting this as a potential security issue but it
  might not be, I'm just looking for feedback from the VMT.

  The scheduler_hints in the compute API are stored in the
  request_specs.spec column in the nova_api database:

  https://github.com/openstack/nova/blob/15.0.1/nova/db/sqlalchemy/api_models.py#L171

  There is no limit on the size of the keys or values, or number of
  hints, in the API:

  https://github.com/openstack/nova/blob/15.0.1/nova/api/openstack/compute/schemas/scheduler_hints.py#L18

  There are some pre-defined hints, but additionalProperties=True in the
  json schema means that one can provide any hints they want.

  So I could boot a server with a scheduler_hints dict that has a
  million keys which are a million characters long. At best that just
  results in a 500 because the column size limit in the database rejects
  the json blob size. According to the mysql 5.7 docs:

  https://dev.mysql.com/doc/refman/5.7/en/string-type-overview.html

  "TEXT[(M)] [CHARACTER SET charset_name] [COLLATE collation_name]

  A TEXT column with a maximum length of 65,535 (216 − 1) characters.
  The effective maximum length is less if the value contains multibyte
  characters. Each TEXT value is stored using a 2-byte length prefix
  that indicates the number of bytes in the value."

  At worst, I'm able to work backward from a million until I found out
  the limit at which I can fill the request_specs.spec column and then
  just hammer the compute API, filling up the nova_api database.

  So there are two issues:

  1. No key/value size limit in the API json schema for scheduler hints.

  2. No quota limit on the number of hints one can provide (unlike quota
  limits on user-provided metadata key/value pairs which are limited to
  255 for the key/value and 128 for the quota).

  Add to this the fact that we never delete request_specs entries from
  the nova_api database automatically (that's being worked on here:
  https://review.openstack.org/#/c/391060/ ).

  This might not be a security issue, it might just be poor API design
  and we can tighten things up to avoid a 500 error with quota limits
  and json schema validation on the key/value size on each hint, and
  also delete request specs when we delete an instance.

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