Mitigate OSSN-0075

Mitigate OSSN-0075

https://blueprints.launchpad.net/glance/+spec/mitigate-ossn-0075

OpenStack Security Note OSSN-0075, “Deleted Glance image IDs may be reassigned”, was made public on 13 September 2016. The current situation is that due to a lack of agreement of how to fix it, we’ve left operators in a bad state: our advice is that soft-deleted rows in the ‘images’ table in the Glance database should not be purged from the database, yet at the same time, the glance-manage tool deletes such rows without warning.

Problem description

Briefly, the problem is that Glance has always allowed a user with permission to make the image-create call the option of specifying an image_id. If the specified image_id clashed with an existing image_id, the image-create operation would fail; otherwise, the specified image_id would be applied to the new image. Consistency is enforced by a uniqueness constraint on the ‘id’ column in the ‘images’ table in the database. Since Glance database entries are soft-deleted, a proposed image_id will be checked against all image_ids that were assigned since the last purge of the ‘images’ table.

As described in OSSN-0075, this problem becomes a security exploit when (a) a popular public or community image is deleted, (b) the database is purged, and (c) a user creates a new image with that same image_id. Users consuming an image by image_id, which is the way Nova and Cinder consume images, may then wind up booting virtual machines using an image different from the one they intend to use.

Note that the new image would have its own data and checksum that would be different from the original data and checksum, but there would be no way for Nova, for instance, to know that these had changed. Were someone to boot a server using the image_id, Nova would receive image data and then verify the checksum against whatever checksum Glance has recorded as associated with the image, which would be the new checksum.

The idea that once an image goes to ‘active’ status, the (image_id, image data, checksum) will not change is called image immutability. It’s important to note that image immutability is required for Glance or else it cannot function as an image catalog. If each consumer had to keep track of the image_id and checksum and other essential properties in order to verify the downloaded data, then there’d be no point in having Glance maintain this information.

Note

The primary use case for allowing end-users to specify an image_id at the time of image creation is to make it easy to find the “same” image data (that is, the data is bit-for-bit identical although it’s stored in different locations) in different regions of a cloud. It’s important to note that the “sameness” of images in different regions is not guaranteed by Glance. (A Glance installation can guarantee the immutability of images within its own region, but it has no way of knowing what’s happening in other regions.) Thus, under the current situation, when an end user relies on the image_id as the guarantor that they’re getting the “same” data in different cloud regions, the end user is actually relying upon the trustworthiness of the image owner.

This is a separate issue from OSSN-0075 and is independent of whether or not the Glance database is ever purged. We point it out as something for operators to keep in mind. To be clear about the issue, here’s an example. Suppose that a cloud operator puts an image with image_id A in regions R, S, T, though for some reason the operator does not put that image in region U. Any cloud user in region U could create an image with image_id A in region U. The image could then be made available to some target user by image sharing, or with the entire cloud by giving it ‘community’ visibility.

An operator can avoid this scenario by creating an image record with image_id A in region U and not uploading any data to it. The image will remain in ‘queued’ status, and if the visibility is not changed to ‘public’ or ‘community’, the image will not appear in any end user’s image-list response.

There is also room for end user education here, namely, that image consumers should not rely solely upon image_id to guarantee that they are receiving the same image data in cross-region scenarios.

Through discussions with operators, it’s clear that the ability to set the image_id on image creation is being used out in the field, so we can’t simply block this ability. At the same time, we must allow the database to be occasionally purged, as there is evidence that for large deployments, having a large number of soft-deleted rows in the ‘images’ table affects the response time of the image-list API call.

Proposed change

Modify the current glance-manage db purge command so that it will not purge the images table.

Introduce a new command, glance-manage db purge-images-table to purge the images table. The new command will take the same options as the current purge, namely, --age-in-days and --max-rows. The rationale for this being a new command (rather than a --force option to the current command) is twofold: (1) it’s likely that the age-in-days used will be different for the images table, and (2) given that purging the images table has a security impact, having it as a completely separate command emphasizes this.

Alternatives

  1. Introduce a policy governing whether or not a user is allowed to specify the image_id at the time of image creation. The downside of this proposal is twofold:

    • it breaks backward compatibility given that this ability has been allowed up to now in both the v1 and v2 versions of the Image API
    • it breaks interoperability in that end uses will have the ability in some clouds but not in others

    A further problem with this proposal is that if the cross-region use of a particular image_id is denied to end users, they will have to use some other piece of image metadata for this purpose. Since cinder and nova both use the image_id when services are requested, user workflows will have to change to introduce an extra call to the image service to find the image record before the image_id to pass to cinder or nova is determined.

  2. Instead of introducing a new column in the images table, introduce a new single-column table with a uniqueness constraint to record “used” UUIDs. The image-create operation would try to insert a proposed UUID into this table instead of the ‘images’ table and fail as it currently does if the uniqueness constraint were violated. This “used” UUID table would never be purged, but the glance-manage tool could continue to purge all other tables.

    This alternative has the advantage of not impacting the image-list call. It would eventually introduce a small delay into the image-create operation, but that’s probably acceptable.

    The downside is that this proposal introduces an unpurgable table that is unbounded in size.

  3. A variation on alternative #2: instead of a single-column table, have at least a deleted_at column in addition to the image_id. This table would not be touched by the “normal” glance-manage database purge operation. Rather, an additional purge operation could be introduced for this table that would purge rows that were, say, 5 years old from the table.

    A problem with this suggestion is that a determined attacker could nonetheless flood the “used” image_ids table. This is possible because while it might make sense to limit the number of existing images a user owns, it doesn’t make sense to limit the number of deleted images a user owns. For example, an end user who creates an image of some important server every day, but only keeps around a week’s worth, will accumulate many deleted images (multiplied by the number of servers this is being done for), but this is perfectly legitimate behavior. So I’m not sure how flooding the “used” image_id table could be prevented, except by something like rate-limiting, though that would have to be set in such a way as not to impact legitimate use cases.

  4. Introduce a new field, preserve_id, for use in the images table. This field will be for internal Glance use only and will not be exposed through the API. This field will be null by default and will be set true whenever the ‘visibility’ field of an image is set to ‘public’ or ‘community’. There will be no way to unset the value of the field. In addition to this, modify the glance-manage tool so that it will never delete an entry from the images table that has preserve_id == True.

    As with alternatives 2 and 3, the database table will continue to grow, but this growth is constrained by keeping only rows relevant to the OSSN-0075 exploit. On the other hand, all an attacker has to do is read this spec to realize that by creating image records with community visibilty, the images table can still be flooded with spurious image records. Thus this strategy is too easily defeated to be worth implementing, especially as it might give operators a false sense of security.

Data model impact

None

REST API impact

None

Security impact

This change will enhance security by providing operators with a means of mitigating the exploit described in OSSN-0075.

Notifications impact

None

Other end user impact

None

Performance Impact

The images table will grow indefinitely, though the associated tables (image_properties, image_tags, image_members, image_locations) can be purged by the glance-manage tool.

The images table can be partially purged at appropriate intervals.

Other deployer impact

Operators will have to monitor Glance for abnormal usage patterns and take appropriate action.

Additionally, operators should be made aware of the cross-region version of the OSSN-0075 exploit (as discussed in the Note in the Problem Description section).

Developer impact

None

Implementation

Assignee(s)

Primary assignee:

  • brian-rosmaita

Other contributors:

  • undetermined

Work Items

  1. Modify the glance-manage tool:
    • The current behavior is that it purges all tables of soft-deleted rows. Change the behavior so that the images table is not purged by default.
    • Add a new command to purge the images table. It should take the --age-in-days and --max-rows options just like the current purge command.
  2. update operator documentation
  3. release note

Dependencies

No new dependencies.

Testing

Appropriate unit tests to ensure the changes to glance and the glance-manage tool function correctly.

Documentation Impact

The Glance Administrator Guide will need to be updated.

References

OSSN-0075: Deleted Glance image IDs may be reassigned.

Creative Commons Attribution 3.0 License

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.