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

[Martin Kosek]

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+.

Martin