[Freeipa-devel] #4534: SSSD deref processing fail when entryusn can be read and objectclass doesn't

[Martin Kosek]

On 09/11/2014 08:49 PM, Simo Sorce wrote:
> On Thu, 2014-09-11 at 20:28 +0200, Martin Kosek wrote:
>> On 09/11/2014 05:37 PM, Simo Sorce wrote:
>>> On Thu, 2014-09-11 at 17:03 +0200, Martin Kosek wrote:
>>>> Hello,
>>>>
>>>> We have another important issue to resolve. Current FreeIPA 4.0.2 ACI settings
>>>> cause older SSSD clients to fail as they get returned an LDAP deref call
>>>> results without objectclass attribute and just with entryusn attribute. Details
>>>> in https://fedorahosted.org/freeipa/ticket/4534. While I agree SSSD should be
>>>> more tolerant in this case, it still breaks old clients which we should prevent.
>>>>
>>>> There are 2 ways how to fix I currently see:
>>>>
>>>> 1) Return objectclass for any entry, just like we do with operational attributes
>>>>
>>>> This would include adding "objectclass" to "System: Read Timestamp and USN
>>>> Operational Attributes". However, I am rather cautious about this approach as
>>>> even though objectclass does not usually carry a secret information, we could
>>>> still leak some information about our DIT to unprivileged user.
>>>
>>> Our DIT is public and known, I see no problem.
>>
>> I rather meant the LDAP tree and it's contents (custom groups, sudo rules,
>> roles, ...).
>>
>> I did one more test and found out we cannot do this as it would undermine the
>> ACIs we have right now. As soon as objectclass is allowed globally, ldapsearch
>> returns every object even if no other attribute is returned:
>>
>> # ldapsearch -h `hostname` -Y GSSAPI -b cn=pbac,dc=mkosek-fedora20,dc=test
>> SASL/GSSAPI authentication started
>> SASL username: host/ipa.mkosek-fedora20.test at MKOSEK-FEDORA20.TEST
>> SASL SSF: 56
>> SASL data security layer installed.
>> ...
>> # User Administrators, privileges, pbac, mkosek-fedora20.test
>> dn: cn=User Administrators,cn=privileges,cn=pbac,dc=mkosek-fedora20,dc=test
>> objectClass: top
>> objectClass: groupofnames
>> objectClass: nestedgroup
>> ...
>>
>> It would not show any more info before that, but even the list of permissions,
>> privileges and roles along with it's names is a leaked information.
>>
>>
>>>> 2) Show objectclass+operational attributes in our Read ACIs
>>>> Other way I see is to update *all* our Read permissions allowing reading
>>>> objectclass attribute and also allow the chosen LDAP operational attribute.
>>>> This would let the LDAP caller to always get either all these discussed
>>>> attributes but none at all.
>>>>
>>>>
>>>> Do we want to do this? Any other (better) idea how to approach it?
>>>
>>> I honestly am not sure I understand the distinction between 1 and 2, can
>>> you provide the actual ACIs or a behavior example ?
>>>
>>> Simo.
>>
>> Currently, our ACI allows reading entryusn in any LDAP entry. So user (SSSD)
>> running LDAP deref call gets entryusn from object it does not have a read
>> access to:
>>
>> # ldapsearch -h `hostname` -Y GSSAPI -b
>> uid=admin,cn=users,cn=accounts,dc=mkosek-fedora20,dc=test -E
>> 'deref=memberof:objectclass,entryusn'
>> SASL/GSSAPI authentication started
>> SASL username: host/ipa.mkosek-fedora20.test at MKOSEK-FEDORA20.TEST
>> SASL SSF: 56
>> SASL data security layer installed.
>> ...
>> # memberof: <entryusn=845>;cn=replication administrators,cn=privileges,cn=pba
>>    c,dc=mkosek-fedora20,dc=test
>>
>> # memberof: <entryusn=75>;cn=add replication agreements,cn=permissions,cn=pba
>>    c,dc=mkosek-fedora20,dc=test
>> ...
>>
>> This confuses SSSD (sees entryusn but does not see objectclass attribute) + may
>> give out some information we do not want to give out. Fortunately, bare
>> ldapsearch does not show anything:
>>
>> # ldapsearch -h `hostname` -Y GSSAPI -b "cn=replication
>> administrators,cn=privileges,cn=pbac,dc=mkosek-fedora20,dc=test" entryusn
>> SASL/GSSAPI authentication started
>> SASL username: host/ipa.mkosek-fedora20.test at MKOSEK-FEDORA20.TEST
>> SASL SSF: 56
>> SASL data security layer installed.
>> # extended LDIF
>> #
>> # LDAPv3
>> # base <cn=replication
>> administrators,cn=privileges,cn=pbac,dc=mkosek-fedora20,dc=test> with scope subtree
>> # filter: (objectclass=*)
>> # requesting: entryusn
>> #
>>
>> # search result
>> search: 4
>> result: 0 Success
>>
>> # numResponses: 1
>>
>>
>> The idea behind Option 1 was to add ACI to allow reading objectclass attribute
>> globally, for our entire tree. (as noted above, not an option).
>>
>> The idea behind Option 2 was to:
>> - remove global ACI allowing reading entryusn (System: Read Timestamp and USN
>> Operational Attributes)
>> - update all our Read permissions to allow entryusn
>>
>> Then for example, if user (SSSD) is allowed to read RBAC role objects, he would
>> not be able to read either objectclass or entryusn attributes. This means users
>> would be only allowed to read entryusn for objects that they can really read
>> (i.e. for objects where they can read at least objectclass).
>>
>> Did that clarify the options?
>>
>> Of course, there is still option 3) to close as wontfix and let older SSSDs be
>> incompatible with FreeIPA 4.0+.
>
> No, 3 is definitely not on the table, I would rather do 1, but I guess 2
> is the only good way for now ?
>
> Simo.

Yeah - 1) would be easy to implement, but it would be a step back in our ACI 
model (global allowing ACI again...).

Something based on 2) is the only approach that I have in mind right now and 
that would work. It also looks as the right thing to do as then with changing 
visibility of objects by our permission system, visibility of entryusn would 
change too.

Programatically it should not be difficult to do, we could add these attributes 
by default to all read permissions which have allow objectclass attribute so we 
would not have to update all our read permissions by hand...

Martin