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List:       sssd-users
Subject:    =?utf-8?q?=5BSSSD-users=5D?= Re: Offline caching of group names and memberships?
From:       Lukas Slebodnik <lslebodn () redhat ! com>
Date:       2019-09-27 9:49:22
Message-ID: 20190927094922.GB856477 () 10 ! 4 ! 128 ! 1
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On (26/09/19 08:45), Simo Sorce wrote:
>On Wed, 2019-09-25 at 17:29 +0200, Lukas Slebodnik wrote:
>> On (25/09/19 09:05), Simo Sorce wrote:
>> > On Wed, 2019-09-25 at 11:07 +0200, Lukas Slebodnik wrote:
>> > > On (24/09/19 13:46), Simo Sorce wrote:
>> > > > On Tue, 2019-09-24 at 17:58 +0200, Lukas Slebodnik wrote:
>> > > > > On (24/09/19 09:26), Simo Sorce wrote:
>> > > > > > On Tue, 2019-09-24 at 10:56 +0200, Lukas Slebodnik wrote:
>> > > > > > > On (23/09/19 18:04), Simo Sorce wrote:
>> > > > > > > > On Mon, 2019-09-23 at 22:53 +0200, Lukas Slebodnik wrote:
>> > > > > > > > > On (23/09/19 15:55), Simo Sorce wrote:
>> > > > > > > > > > On Mon, 2019-09-23 at 14:39 -0500, Spike White wrote:
>> > > > > > > > > > > All,
>> > > > > > > > > > > 
>> > > > > > > > > > > Our cybersecurity team doesn't allow Linux sysadmins to directly log in as
>> > > > > > > > > > > root.  (violates accountability, auditability and traceability).  We log in
>> > > > > > > > > > > with an ADM account, which is then eligible to become root via ‘sudo su –‘.
>> > > > > > > > > > > 
>> > > > > > > > > > > That is, all members of a particular group are allowed to sudo to root.
>> > > > > > > > > > > 
>> > > > > > > > > > > This is preferred because with modern sudo versions all sudo sessions are
>> > > > > > > > > > > session-logged.
>> > > > > > > > > > > 
>> > > > > > > > > > > Anyway, if I log in with my ADM account and someone shuts down sssd, it no
>> > > > > > > > > > > longer knows what groups I'm in.  That is, the session is still there – but
>> > > > > > > > > > > it cannot look up the group names.
>> > > > > > > > > > > 
>> > > > > > > > > > > [admspike_white@zzzdmsdev06 ~]$ id
>> > > > > > > > > > > 
>> > > > > > > > > > > uid=2025431 gid=1002 groups=1002,2284295
>> > > > > > > > > > > 
>> > > > > > > > > > > 
>> > > > > > > > > > > 
>> > > > > > > > > > > Because the sudo privs are based on group name, it doesn't allow Linux
>> > > > > > > > > > > sysadmins to become root and thus start sssd.
>> > > > > > > > > > > 
>> > > > > > > > > > > 
>> > > > > > > > > > > 
>> > > > > > > > > > > Is there a way to cache those group names and memberships?  Say with nscd?
>> > > > > > > > > > > So that if sssd is (temporarily) shut down, we can become root and start up?
>> > > > > > > > > > 
>> > > > > > > > > > sssd already caches user and group tables for fast lookup, but those
>> > > > > > > > > > caches are not very big, so if you have very many groups you may need
>> > > > > > > > > > to increase the size.
>> > > > > > > > > > 
>> > > > > > > > > > Also these caches have somewhat strict timeouts, I forget if they stop
>> > > > > > > > > > returning anything at all if the timeout is expired.
>> > > > > > > > > > 
>> > > > > > > > > > 
>> > > > > > > > > 
>> > > > > > > > > The behaviour of fast mmap cache is to fall back to daemon in case of
>> > > > > > > > > expired entry. Which is by default just 5 minutes.
>> > > > > > > > > And if sssd is not running then it will not return anything.
>> > > > > > > > > 
>> > > > > > > > > > > Obviously, we can go look up the root password for the particular server –
>> > > > > > > > > > > but that's a painful portal.  It'd be better if we could cache group names
>> > > > > > > > > > > and memberships, if sssd is temporarily down or offline.
>> > > > > > > > > > 
>> > > > > > > > > > Perhaps an RFE to return whatever was in cachi, even if expired, if
>> > > > > > > > > > sssd daemons are unresponsive may be opened, should that be the
>> > > > > > > > > > behavior when caches timed out.
>> > > > > > > > > > 
>> > > > > > > > > > 
>> > > > > > > > > 
>> > > > > > > > > I do not see a reason why sssd should be temporarily down.
>> > > > > > > > > If there is a crash then it should be restarted by systemd.
>> > > > > > > > > If sssd is running but in offline mode then it should return even
>> > > > > > > > > expired entries from the cache.
>> > > > > > > > > 
>> > > > > > > > > I would say the biggest problem in the description is
>> > > > > > > > > "someone shuts down sssd". And just somebody with root privileges can do that.
>> > > > > > > > > But if sb has root(sudo) access then it can break anything there (even sshd)
>> > > > > > > > > And thus nobody can connect there. What would you do in such situation?
>> > > > > > > > 
>> > > > > > > > Not sure what would you do with a rouge admin, but there can definitely
>> > > > > > > > be cases where sssd will refuse to start, for example if an admin fat-
>> > > > > > > > fingers the config file, in that case allowing the fast cache to be
>> > > > > > > > used would save the day.
>> > > > > > > > 
>> > > > > > > 
>> > > > > > > `sssctl config-check should help
>> > > > > > > 
>> > > > > > > Admin should be careful when touching critical critical services sssd/sshd
>> > > > > > > and be prepared for recovery.
>> > > > > > > 
>> > > > > > > It is not a problem of daemons but admins.
>> > > > > > 
>> > > > > > We build tools for admins, not for platonic perfections though...
>> > > > > > 
>> > > > > 
>> > > > > I thought there was assumption that sssd will never handle root
>> > > > > because it is a prerequisite to run sssd itself. (chicken and egg problem)
>> > > > > And the issue with sudo and group membership is almost like that.
>> > > > 
>> > > > SSSD could handle root just fine, we chose not to because SSSD
>> > > > initially was for network identities.
>> > > > 
>> > > > Now that we have support for the files provider though, it is possible
>> > > > SSSD focus can shift toward playing with root accounts too. 
>> > > > 
>> > > > > > > 
>> > > > > > > > So I think that regardless of how sssd can end up in a state where it
>> > > > > > > > is not running it may be useful to allow to return whatever information
>> > > > > > > > we have so that the system is more recoverable, after all the
>> > > > > > > > information there may be stale, but it is not incorrect.
>> > > > > > > > 
>> > > > > > > > That said if sudo rules are served via SSSD there may be issues there
>> > > > > > > > too, but that is another story.
>> > > > > > > > 
>> > > > > > > 
>> > > > > > > sudo rules do not have fast memory cache and thus relying on
>> > > > > > > users and groups from fast memory cache is not enough in case of not-running
>> > > > > > > sssd.
>> > > > > > 
>> > > > > > Yes but for this case probably sudo rules are hardcoded in the sudoers
>> > > > > > file.
>> > > > > > 
>> > > > > 
>> > > > > OK that would be reasonable. But would be good to get info from reporter :-)
>> > > > > 
>> > > > > 
>> > > > > > > IMHO, there still should be a way how to do disaster recovery
>> > > > > > > in case of unresponsive sshd/sssd. I cannot see any issue in sssd itself here.
>> > > > > > 
>> > > > > > The issue is in not using the fast cache when there is no reason not
>> > > > > > to.
>> > > > > > 
>> > > > > 
>> > > > > You cannot rely on fast cache it might be half populated and admins
>> > > > > need to be lucky to get right group membersip in case of "unresponsive"
>> > > > > sssd. The only reliable way would be to query ldb cache.
>> > > > > But then either sssd_nss is running or sssd nsswitch plugin would need to know
>> > > > > hot to get data from ldb cache,
>> > > > > 
>> > > > > So it is not clear to me what do you suggest.
>> > > > > How would you solve such special case in sssd?
>> > > > 
>> > > > So we have a quite a few options.
>> > > > One option would be indeed to link nss_sss with the ldb code so it ould
>> > > > do direct queries if the user has at least read access, not a very
>> > > > interesting case, given users generally do not have access to the ldb
>> > > > caches.
>> > > > 
>> > > > Another option is to allow admins to mark some groups as important and
>> > > > make sure to never kick them out of the fast cache. This is actually
>> > > > potentially a good performance tuning option, for setups where there
>> > > > are large amounts of groups but only a few are really important to
>> > > > servers. (even better if we could somehow auto-learn what groups are
>> > > > critical, but an option would be the next best thing).
>> > > > 
>> > > > Setting important group may also trigger a timer within sssd so that it
>> > > > regularly refreshes the user/group fast caches, this would avoid
>> > > > periodic performance hits to critical applications when the fast cache
>> > > > expires.
>> > > > 
>> > > 
>> > > Could you file an upstream issue?
>> > 
>> > Ok.
>> > 
>> > > And there will be another prerequisite for this task.
>> > > Fast memory cache should work with case insensitive names.
>> > > Otherwise you cannot rely on it in "disaster" case.
>> > 
>> > This seem like a separate issue, where we should mark an entry as "case
>> > insensitive" or case sensitive, and I do not see it as a pre-requisite,
>> > but a nice to have.
>> > 
>> 
>> If you check other mails from Spike you will see lot of questions about AD.
>> Which is by default case insensitive
>> And thus it won't work for him without it :-)
>> So implementing feature would not be enough.
>
>See Jakub's reply, all you need is AD's responder behavior and then by
>default all names are lowecased in the cache, so they will match common
>sense use, which is sufficient in recovery situations as, I hope, Linux
>admins are sane and reference everything via lowercased names ?
>

You never know. And nobody said that just linux admins will connect to the
machine in case of recovery. If you get used to ignoring case-sensitivity
then you might be really surprised why things does not work.

You try to see that case from positive perspective.
But real world is not ideal.

LS
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