[SSSD] [PATCH] GPO: Fix crash with GPO and missing security descriptor

Jakub Hrozek jhrozek at redhat.com
Fri Apr 24 12:07:57 UTC 2015


On Fri, Apr 24, 2015 at 02:01:11PM +0200, Lukas Slebodnik wrote:
> On (24/04/15 12:43), Jakub Hrozek wrote:
> >On Thu, Apr 23, 2015 at 07:29:07AM -0400, Stephen Gallagher wrote:
> >> On Thu, 2015-04-23 at 08:14 +0200, Lukas Slebodnik wrote:
> >> > On (20/04/15 14:38), Stephen Gallagher wrote:
> >> > > On Mon, 2015-04-20 at 08:53 +0200, Lukas Slebodnik wrote:
> >> > > > ehlo,
> >> > > > 
> >> > > > attached patch fixes crash in
> >> > > > https://fedorahosted.org/sssd/ticket/2629
> >> > > > 
> >> > > 
> >> > > 
> >> > > Nack. I'd rather we fixed the root of this problem. I did some 
> >> > > digging
> >> > > this afternoon and tracked the issue back to ad_gpo.c line 3499 (in
> >> > > current master). If we get back a NULL result or num_results == 0,
> >> > > then we just skip over this item in the list and start processing 
> >> > > the
> >> > > next one. Unfortunately, that leaves an item in the candidate_gpos
> >> > > list that was never properly constructed.
> >> > > 
> >> > You are right.
> >> > 
> >> > We got a referral to GPO and therefore we do not find any attributes.
> >> > 
> >> > [sdap_sd_search_send] (0x0400): Searching entry [cn={2BA15B73-9524-
> >> > 419F-B4B7-185E1F0D3DCF},cn=policies,cn=system,DC=example,DC=com] 
> >> > using SD
> >> > [sdap_print_server] (0x2000): Searching 10.1.1.14
> >> > [sdap_get_generic_ext_step] (0x0400): calling ldap_search_ext with 
> >> > [(objectclass=*)][cn={2BA15B73-9524-419F-B4B7-
> >> > 185E1F0D3DCF},cn=policies,cn=system,DC=example,DC=com].
> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
> >> > [nTSecurityDescriptor]
> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: [cn]
> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
> >> > [gPCFileSysPath]
> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
> >> > [gPCMachineExtensionNames]
> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: 
> >> > [gPCFunctionalityVersion]
> >> > [sdap_get_generic_ext_step] (0x1000): Requesting attrs: [flags]
> >> > [sdap_get_generic_ext_step] (0x2000): ldap_search_ext called, msgid =
> >> > 14
> >> > [sdap_process_result] (0x2000): Trace: sh[0x7f5d409a8c60], 
> >> > connected[1], ops[0x7f5d409d2c10], ldap[0x7f5d409a9ed0]
> >> > [sdap_process_result] (0x2000): Trace: ldap_result found nothing!
> >> > [sdap_process_result] (0x2000): Trace: sh[0x7f5d409a8c60], 
> >> > connected[1], ops[0x7f5d409d2c10], ldap[0x7f5d409a9ed0]
> >> > [sdap_process_message] (0x4000): Message type: 
> >> > [LDAP_RES_SEARCH_RESULT] 
> >> > [sdap_get_generic_op_finished] (0x0400): Search result: 
> >> > Referral(10), 0000202B: RefErr: DSID-0310063C, data 0, 1 access 
> >> > points
> >> >         ref 1: 'lzb.hq'
> >> > 
> >> > [sdap_get_generic_op_finished] (0x1000): Ref: 
> >> > ldap://lzb.hq/cn=%7B2BA15B73-9524-419F-B4B7-
> >> > 185E1F0D3DCF%7D,cn=policies,cn=system,DC=example,DC=com
> >> > [ad_gpo_get_gpo_attrs_done] (0x0040): no attrs found for GPO; try 
> >> > next GPO.
> >> > 
> >> > > Under what circumstances can the secinfo_dacl search return success
> >> > > but with zero results? Is there a bug or a race here (such as the 
> >> > > AD
> >> > > server has updated the GPO since we got the list of candidate 
> >> > > GPOs?).
> >> > > How best to handle this?
> >> > > 
> >> > > With your patch here, it looks like we're assuming that it's okay 
> >> > > to
> >> > > just skip over this GPO. If that's the case, then what we really 
> >> > > need
> >> > > to be doing in ad_gpo_get_gpo_attrs_step() is to mark the gp_gpo as
> >> > > being invalid and then after we've gone through them all, shrink 
> >> > > the
> >> > > array, removing all of the invalid entries. This will be more 
> >> > > future-
> >> > > proof, as it's not just the gpo_sd that is uninitialized here. 
> >> > > Every
> >> > > member of this gp_gpo is NULL except for the DN.
> >> > > 
> >> > > If it's *not* okay that we've gotten no results for this lookup 
> >> > > (such
> >> > > as in the race case; we don't want to be skipping over a GPO that
> >> > > might properly be denying users), we may need to restart 
> >> > > processing at
> >> > > least a couple times to try to avoid the race and go offline if we
> >> > > can't complete the processing (so we at least stick to our cached
> >> > > rules).
> >> > 
> >> > I'm sorry I didn't noticed it in log files the first time.
> >> > Now we know the root of problem.
> >> > Which of your proposal do you prefer now?
> >> > 
> >> > LS
> >> 
> >> OK, I hadn't considered the referral scenario. (I wasn't actually 
> >> aware that GPOs could *be* referred to elsewhere). Is this referral 
> >> pointing to a different machine than the host is enrolled to? It's 
> >> trying to reach a DN at lzb.hq that is identical to the one we just 
> >> requested.
> >> 
> >> I'd really like to know how to reproduce the setup for this situation; 
> >> is it inheriting a GPO from a parent domain somehow?
> >> 
> >> Neither of my above proposals are a proper solution for this case, 
> >> unfortunately. The first one would make the crash go away, but if this 
> >> GPO really exists and is restrictive, not processing it is *really 
> >> bad*. Right now we're actually slightly lucky in that the crash has a 
> >> net result of causing an access denial rather than erroneously 
> >> resulting in a user login, but if we were to "solve" this problem the 
> >> way Lukas's original patch intended (or my first alternative above), 
> >> it would be introducing a security issue. So let's not do either of 
> >> those.
> >> 
> >> The second option I suggested there is invalid as well, since the 
> >> entry *exists*, it's just been converted to a referral. There are 
> >> therefore only two safe options at this time:
> >> 
> >> 1) Quick-and-ugly: If we encounter a referral, skip straight to denial 
> >> and loudly warn in the logs that this is a situation we are unable to 
> >> handle yet. This will avoid the crash, but will leave the GPO behavior 
> >> equivalently bad to the current state. Might be acceptable if this is 
> >> only happening on a very small subset of systems. 
> >
> >I'm afraid we need to go this way now.
> >
> >> Users experiencing 
> >> this might be able to set "ldap_referrals = True" on the affected 
> >> systems to quietly solve the problem at the expense of all the other 
> >> side-effects to that (particularly performance)
> >
> >I wouldn't even suggest enabling referrals. I've seen the LDAP provider
> >misbehave completely with referrals enabled, openldap-libs is not really
> >great at chasing them and LDAP provider would go offline etc..
> >So enabling referrals doesn't just degrade performance, but also
> >functionality..
> >
> >I also tested that using Global Catalog wouldn't help here, some
> >attributes we require are not replicated to GC, such as gPCFileSysPath.
> >
> >Are you sure we're using the right connection to the right domain? We
> >can select the right connection based on the DN that we know.
> >I'm sorry, but I couldn't reproduce the bug myself.
> >
> Stephen wrote a reproducer into ticket
> https://fedorahosted.org/sssd/ticket/2629

I know, that's what I tried, but I couldn't get the setup to break. For
some reason SSSD didn't examine the GPO from the parent domain if it was
linked to a site the client was at..

> 
> We agreed to do the "Quick-and-ugly" way for 1.12 + documentation
> but he mentioned that it is walid use-case and it would be good to
> implement it in next release.

Yes, the LDAP provider is in a big need of refactoring, but I think even
for 1.13 it's too late.



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