> From : Colleen Murphy <[email protected]>
> To : <[email protected]>
> Date : Tue, 25 Sep 2018 18:33:30 +0900
> Subject : Re: [openstack-dev] [keystone] Domain-namespaced user attributes in
> SAML assertions from Keystone IdPs
> ============ Forwarded message ============
> > On Mon, Sep 24, 2018, at 8:40 PM, John Dennis wrote:
> > > On 9/24/18 8:00 AM, Colleen Murphy wrote:
> > > > This is in regard to https://launchpad.net/bugs/1641625 and the
> proposed patch https://review.openstack.org/588211 for it. Thanks Vishakha
> for getting the ball rolling.
> > > >
> > > > tl;dr: Keystone as an IdP should support sending
> non-strings/lists-of-strings as user attribute values, specifically lists of
> keystone groups, here's how that might happen.
> > > >
> > > > Problem statement:
> > > >
> > > > When keystone is set up as a service provider with an external
> non-keystone identity provider, it is common to configure the mapping rules
> to accept a list of group names from the IdP and map them to some property of
> a local keystone user, usually also a keystone group name. When keystone acts
> as the IdP, it's not currently possible to send a group name as a user
> property in the assertion. There are a few problems:
> > > >
> > > > 1. We haven't added any openstack_groups key in the creation of
> the SAML assertion
> (http://git.openstack.org/cgit/openstack/keystone/tree/keystone/federation/idp.py?h=14.0.0#n164).
> > > > 2. If we did, this would not be enough. Unlike other IdPs, in
> keystone there can be multiple groups with the same name, namespaced by
> domain. So it's not enough for the SAML AttributeStatement to contain a
> semi-colon-separated list of group names, since a user could theoretically be
> a member of two or more groups with the same name.
> > > > * Why can't we just send group IDs, which are unique? Because two
> different keystones are not going to have independent groups with the same
> UUID, so we cannot possibly map an ID of a group from keystone A to the ID of
> a different group in keystone B. We could map the ID of the group in in A to
> the name of a group in B but then operators need to create groups with UUIDs
> as names which is a little awkward for both the operator and the user who now
> is a member of groups with nondescriptive names.
> > > > 3. If we then were able to encode a complex type like a group
> dict in a SAML assertion, we'd have to deal with it on the service provider
> side by being able to parse such an environment variable from the Apache
> headers.
> > > > 4. The current mapping rules engine uses basic python string
> formatting to translate remote key-value pairs to local rules. We would need
> to change the mapping API to work with values more complex than strings and
> lists of strings.
> > > >
> > > > Possible solution:
> > > >
> > > > Vishakha's patch (https://review.openstack.org/588211) starts to solve
> (1) but it doesn't go far enough to solve (2-4). What we talked about at the
> PTG was:
> > > >
> > > > 2. Encode the group+domain as a string, for example by using the
> dict string repr or a string representation of some custom XML and maybe
> base64 encoding it.
> > > > * It's not totally clear whether the AttributeValue class of
> the pysaml2 library supports any data types outside of the xmlns:xs namespace
> or whether nested XML is an option, so encoding the whole thing as an
> xs:string seems like the simplest solution.
> > > > 3. The SP will have to be aware that openstack_groups is a
> special key that needs the encoding reversed.
> > > > * I wrote down "MultiDict" in my notes but I don't recall
> exactly what format the environment variable would take that would make a
> MultiDict make sense here, in any case I think encoding the whole thing as a
> string eliminates the need for this.
> > > > 4. We didn't talk about the mapping API, but here's what I think.
> If we were just talking about group names, the mapping API today would work
> like this (slight oversimplification for brevity):
> > > >
> > > > Given a list of openstack_groups like ["A", "B", "C"], it would work
> like this:
> > > >
> > > > [
> > > > {
> > > > "local":
> > > > [
> > > > {
> > > > "group":
> > > > {
> > > > "name": "{0}",
> > > > "domain":
> > > > {
> > > > "name": "federated_domain"
> > > > }
> > > > }
> > > > }
> > > > ], "remote":
> > > > [
> > > > {
> > > > "type": "openstack_groups"
> > > > }
> > > > ]
> > > > }
> > > > ]
> > > > (paste in case the spacing makes this unreadable:
> http://paste.openstack.org/show/730623/ )
> > > >
> > > > But now, we no longer have a list of strings but something more like
> [{"name": "A", "domain_name": "Default"} {"name": "B", "domain_name":
> "Default", "name": "A", "domain_name": "domainB"}]. Since {0} isn't a string,
> this example doesn't really work. Instead, let's assume that in step (3) we
> converted the decoded AttributeValue text to an object. Then the mapping
> could look more like this:
> > > >
> > > > [
> > > > {
> > > > "local":
> > > > [
> > > > {
> > > > "group":
> > > > {
> > > > "name": "{0.name}",
> > > > "domain":
> > > > {
> > > > "name": "{0.domain_name}"
> > > > }
> > > > }
> > > > }
> > > > ], "remote":
> > > > [
> > > > {
> > > > "type": "openstack_groups"
> > > > }
> > > > ]
> > > > }
> > > > ]
> > > > (paste: http://paste.openstack.org/show/730622/ )
> > > >
> > > > Alternatively, we could forget about the namespacing problem and
> simply say we only pass group names in the assertion, and if you have
> ambiguous group names you're on your own. We could also try to support both,
> e.g. have an openstack_groups mean a list of group names for simpler use
> cases, and openstack_groups_unique mean the list of encoded group+domain
> strings for advanced use cases.
> > > >
> > > > Finally, whatever we decide for groups we should also apply to
> openstack_roles which currently only supports global roles and not
> domain-specific roles.
> > > >
> > > > (It's also worth noting, for clarity, that the samlize function does
> handle namespaced projects, but this is because it's retrieving the project
> from the token and therefore there is only ever one project and one project
> domain so there is no ambiguity.)
> > > >
> > >
> > > A few thoughts to help focus the discussion:
> > >
> > > * Namespacing is critical, no design should be permitted which allows
> > > for ambiguous names. Ambiguous names are a security issue and can be
> > > used by an attacker. The SAML designers recognized the importance to
> > > disambiguate names. In SAML names are conveyed inside a NameIdentifier
> > > element which (optionally) includes "name qualifier" attributes which in
> > > SAML lingo is a namespace name.
> > >
> > > * SAML does not define the format of an attribute value. You can use
> > > anything you want as long as it can be expressed in valid XML as long as
> > > the cooperating parties know how to interpret the XML content. But
> > > herein lies the problem. Very few SAML implementations know how to
> > > consume an attribute value other than a string. In the real world,
> > > despite what the SAML spec says is permitted is the constraint attribute
> > > values is a string.
> > >
> > > * I haven't looked at the pysaml implementation but I'd be surprised if
> > > it treated attribute values as anything other than a string. In theory
> > > it could take any Python object (or JSON) and serialize it into XML but
> > > you would still be stuck with the receiver being unable to parse the
> > > attribute value (see above point).
> > >
> > > * You can encode complex data in an attribute value while only using a
> > > simple string. The only requirement is the relying party knowing how to
> > > interpret the string value. Note, this is distinctly different than
> > > using non-string attribute values because of who is responsible for
> > > parsing the value. If you use a non-string attribute value the SAML
> > > library need to know how to parse it, none or very few will know how to
> > > process that element. But if it's a string value the SAML library will
> > > happily pass that string back up to the application who can then
> > > interpret it. The easiest way to embed complex data in a string is with
> > > JSON, we do it all the time, all over the place in OpenStack. [1][2]
> > >
> > > So my suggestion would be to give the attribute a meaningful name.
> > > Define a JSON schema for the data and then let the upper layers decode
> > > the JSON and operate on it. This is no different than any other SAML
> > > attribute passed as a string, the receive MUST know how to interpret the
> > > string value.
> > >
> > > [1] We already pass complex data in a SAML attribute string value. We
> > > permit a comma separated list of group names to appear in the 'groups'
> > > mapping rule (although I don't think this feature is documented in our
> > > mapping rules documentation). The receiver (our mapping engine) has
> > > hard-coded logic to look for a list of names.
> > >
> > > [2] We might want to prepend a format specifier to string containing
> > > complex data, e.g. "JSON:{json object}". Our parser could then look for
> > > a leading format tag and if if finds one strip it off and pass the rest
> > > of the string into the proper parser.
> > >
> > > --
> > > John Dennis
> > >
> >
> > Thanks for this response, John. I think serializing to JSON and prepending
> a format specifier makes sense.
> >
> > Colleen
Thanks for the response Colleen, John. After reading the above mails,
what I understood is to pass list of group names specific to a
keystone in to a JSON schema prepending a tag. e.g - groups_names =
"JSON:{groups_names:[a,b,c]}". IN SAML assertion Attribute_name can be
openstack_groups and Attribute_value will be
"JSON:{groups_names:[a,b,c]}".
Vishakha
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