Open Stack

Best practices when configuring Glance with COW backends
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### Summary ###
When deploying Glance in a popular configuration where Glance shares a
common storage backend with Nova and/or Cinder, it is possible to open
some known attack vectors by which malicious data modification can
occur.  This note reviews the known issues and suggests a Glance
deployment configuration that can mitigate such attacks.

### Affected Services / Software ###
Glance, all supported releases (Queens through Zed)

### Discussion ###
This note applies to you if you are operating an end-user-facing
glance-api service with the 'show_multiple_locations' option set to
True (the default value is False) or if your end-user-facing
glance-api has the 'show_image_direct_url' option set to True (default
value is False).

Our recommendation is that the image "locations" and "direct_url"
fields [0] *never* be displayed to end users in a cloud.  This can be
accomplished by running two glance-api services:

- A "user-facing" glance-api that is accessible to end users and
   which appears in users' service catalogs.

- An "internal-only-facing" glance-api that is accessible only to
   those services that require access to the 'direct_url' or image
   location fields, and which is protected by firewalls from access by
   end users.  (Nova, Cinder, and Ironic all have configuration options
   to specify the Glance API endpoint each service uses [1].)

This dual glance-api deployment was suggested in "Known Issues" in
Glance release notes in the Rocky [2] through Ussuri releases, but it
seems that the idea has not received sufficient attention.  Hence this
security note.

The attack vector that becomes available when image locations are
exposed to end users was originally outlined in OSSN-0065 [3], though
that note was not clear about the attack surface or mitigation, and
contained some forward-looking statements that were not fulfilled.
The attack vector is:

    [A] malicious user could create an image in Glance, set an
    additional location on that image pointing to an altered image,
    then delete the original location, so that consumers of the
    original image would unwittingly be using the malicious image.
    Note, however, that this attack vector cannot change the original
    image's checksum, and it is limited to images that are owned by the
    attacker.

OSSN-0065 suggests that this is only an issue when users do not
checksum their image data.  It neglects the fact that in some popular
deployment configurations in which Nova creates a root disk snapshot,
data is never uploaded to Glance, but instead a snapshot is created
directly in the backend and Nova creates a Glance image record with
"size" 0 and an empty "os_hash_value" [4], making it impossible to
compare the hash of downloaded image data to the value maintained by
Glance.

Further, when Nova is configured to use the same storage for ephemeral
disks that is used as a Glance image store, Nova efficiently creates a
server root disk directly in the backend without checksumming the
image data.  This is an intentional design choice to optimize storage
space and host resources, but an implication is that even if the image
record has a recorded hash, it is not being checked at the point of
image consumption.

Similarly, when using a shared backend, or a cinder glance_store,
Cinder will efficiently clone a volume created from an image directly
in the backend without checksumming the image data.  Again, this is
done intentionally in order to optimize resources, but it is important
to be aware of the security tradeoff being made by this configuration.
In other words, if the image data is not going to be checked at the
point of image consumption, then extra care needs to be taken to
ensure the integrity of the data path.

OSSN-0065 suggested that the attack vector of substituting image data
by modifying the image locations could be addressed by using policies,
but that has turned out not to be the case.  The only way currently to
mitigate this vector is to deploy glance-api in a dual configuration
as described above, namely with an internal-only-facing glance-api
used by Nova and Cinder (that has show_multiple_locations enabled),
and an end-user-facing glance-api (that has show_multiple_locations
disabled).

So far the focus has been on 'show_multiple_locations'.  When that
setting is disabled in Glance, it is not possible to manipulate the
locations via the OpenStack Images API.  Keep in mind, however, that
in any Glance/Nova/Cinder configuration where Nova and/or Cinder do
copy-on-write directly in the image store, image data transfer takes
place outside Glance's image data download path, and hence the
os_hash_value is *not* checked.  Thus, if the backend store is itself
compromised and image data is replaced directly in the backend, the
substitution will *not* be detected.

This brings us to the 'show_image_direct_url' option, which includes a
"direct_url" field in the image-show response that can be used by
various OpenStack services to consume images directly from the storage
backend.  Exposing the 'direct_url' to end users leaks information
about the storage backend.  What exactly that information consists of
depends upon the backend in use and how it is configured, but in
general, it is not a good idea to provide hints that could be useful
to malicious actors in their attempts to compromise the backend
storage by some type of independent exploit.  The 'direct_url', being
read-only, may appear innocuous, but its use by services is usually to
perform some kind of optimized image data access that most likely does
not include computing a hash of the image data.

We therefore recommend that OpenStack services that require exposure
of the 'direct_url' image property be similarly configured to use an
internal-only-facing glance-api.  It is worth nothing that end users
who wish to download an image do not require access to the
'direct_url' image property because they can simply use the
image-data-download API call [5].

### Recommended Actions ###
A glance-api service with 'show_multiple_locations' enabled should
*never* be exposed directly to end users.  This setting should only be
enabled on an internal-only-facing glance-api that is used by
OpenStack services that require access to image locations.  This could
be done, for example, by running two glance-api services with
different configuration files and using the appropriate configuration
options for each service to specify the Image API endpoint to access,
and making sure the special internal endpoint is firewalled in such a
way that only the appropriate OpenStack services can contact it.

Similarly, enabling 'show_image_direct_url' exposes information about
the storage backend that could be of use to malicious actors in as yet
unknown exploits, so it should likewise only be enabled on an
internal-only-facing glance-api.

### Notes / References ###
[0] https://docs.openstack.org/api-ref/image/v2/index.html#show-image-schema
[1] Nova and Ironic use 'endpoint_override' in the '[glance]' section of
     the configuration file; Cinder uses 'glance_api_servers' in the
     '[DEFAULT]' section.
[2] OSSN-0065: https://wiki.openstack.org/wiki/OSSN/OSSN-0065
[3] The Glance "multihash" metadata pair of 'os_hash_algo' and
     'os_hash_value' were introduced in Rocky to replace the legacy md5
     'checksum' field.  The python-glanceclient has used multihash
     checksumming for download verification since version 2.13.0.
[4] https://docs.openstack.org/releasenotes/glance/rocky.html#known-issues
[5] 
https://docs.openstack.org/api-ref/image/v2/index.html?#download-binary-image-data

### Contacts / References ###
Author: Brian Rosmaita, Red Hat
This OSSN : https://wiki.openstack.org/wiki/OSSN/OSSN-0090
Original LaunchPad Bug : https://bugs.launchpad.net/ossn/+bug/1990157
Mailing List : [Security] tag on openstack-discuss at lists.openstack.org
OpenStack Security Project : https://launchpad.net/~openstack-ossg
CVE: none
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