[RFC] Retire support for config option `config.dynamic.multi_rolespan_enabled` in `config.yml`

Background

I am asking for opinions and comments on the following topic because it influences the work on #3870.

Current situation

The OpenSearch security config file config.yml supports an option called config.dynamic.multi_rolespan_enabled.

This config option is undocumented. Doing a search for it only returns hits which includes dumps of config.yml where the option with default value is just coincidentally contained in the config file:

https://www.google.com/search?q=site%253Aopensearch.org+%22multi_rolespan_enabled%22

The only very brief mention of this option can be found in the docs of a related project:

https://docs.search-guard.com/7.x-53/changelog-searchguard-7-x-35_0_0#breaking-default-changed-for-permissions-evaluation-across-different-roles-multi-rolespan

Functionality

If this option is set to true (which is the default if a config.yml file in version 2/7 is in use), privileges for a single OpenSearch operation can be collected from several roles. If this option is set to false, privileges for a single operation must be located in exactly one role.

This can pertain to actions or indices.

Several indices

If I issue a search request on indices a and b, I need the privilege indices:data/read/search for both indices.

With multi_rolespan_enabled = true, it is possible that the user has two different roles which provide the different privileges:

role_1:
  index_permissions:
    index_patterns:
    - a
    allowed_actions:  
    - indices:data/read/search

role_2:
  index_permissions:
    index_patterns:
    - b
    allowed_actions:  
    - indices:data/read/search

With multi_rolespan_enabled = false, however, these roles would be not sufficient for a single search operation on both indices. Instead, a user would need to have a single role with both privileges:

role_1:
  index_permissions:
    index_patterns:
    - a
    - b
    allowed_actions:  
    - indices:data/read/search

Several actions

For example, in order to issue a bulk index insert request, I need two privileges:

index privilege indices:data/write/bulk
index privilege indices:data/write/index

With multi_rolespan_enabled = true, it is possible that the user has two different roles which provide the different privileges:

role_1:
  index_permissions:
    index_patterns:
    - foo
    allowed_actions:  
    - indices:data/write/bulk

role_2:
  index_permissions:
    index_patterns:
    - foo
    allowed_actions:  
    - indices:data/write/index

With multi_rolespan_enabled = false, however, these roles would be not sufficient for a bulk index operation. Instead, a user would need to have a single role with both privileges:

role_1:
  index_permissions:
    index_patterns:
    - foo
    allowed_actions:  
    - indices:data/write/bulk
    - indices:data/write/index

The set of operations which require two privileges at once is very limited, all can be quickly found by checking the code at:

https://github.com/opensearch-project/security/blob/6dedfb455766601b1fdee8e5334cd0905bfe2ed2/src/main/java/org/opensearch/security/privileges/PrivilegesEvaluator.java#L601

This shows that the following operations need two privileges:

_bulk, indices:data/write/bulk: requires indices:data/write/bulk and depending on the bulk item type indices:data/write/index, indices:data/write/delete or indices:data/write/update
Cross cluster search backing action indices:admin/shards/search_shards requires additionally indices:data/read/search
_aliases, indices:admin/aliases with an item type remove needs additionally indices:admin/delete
Create index, indices:admin/create with a specified alias needs additionally indices:admin/aliases
Restore snapshot, cluster:admin/snapshot/restore needs additionally indices:admin/create, indices:data/write/index -- but only if the config option plugins.security.check_snapshot_restore_write_privileges is set to true

Default value

On OpenSearch clusters where the config.yml configuration with _meta.config_version=2 is in use, this defaults to true.

On OpenSearch clusters where a config.yml configuration from ODFE 0.x in version 1 is in use, this defaults to false.

Reasons for using the non-default value `multi_rolespan_enabled = false`

Seriously, I do not know any reasons for using the non-default value multi_rolespan_enabled = false.

Having the option set to false allows a more granular control over privileges.

However, I would argue that this added granularity does not have any value:

It does not make sense to allow me searching in index a in one request and additionally in index b in another request, but denying my to search a and b at the same time.
It does not make sense to deny me deleting indices using the POST _aliases API while allowing it me using the normal DELETE index API.

Current work

In the context of #3870, we need to re-write significant parts of the privilege evaluation code. Supporting multi_rolespan_enabled would significantly increase the complexity of the code - and thus also increase the necessary work and later maintenance effort.

Proposal: Just drop it

In my opinion, it would be not economical to put any effort into an option that offers hardly any value.

Thus, I am proposing to drop support for that option. We should always behave like its value would be true -- the default value.

As it is an undocumented option, I would argue that such a breaking change would be even possible in a minor version upgrade.

opensearch-project / security