From:   Dominick Grift <dac.override@gmail.com>
To:     Russell Coker <russell@coker.com.au>
Cc:     "selinux-refpolicy\@vger.kernel.org" 
        <selinux-refpolicy@vger.kernel.org>
Subject: Re: What is this GetDynamicUsers about?
References: <2151367.RAjrkxNSSX@liv>
Date:   Sat, 19 Jan 2019 13:30:25 +0100
In-Reply-To: <2151367.RAjrkxNSSX@liv> (Russell Coker's message of "Sat, 19 Jan
        2019 22:48:11 +1100")
Message-ID: <871s5825jy.fsf@gmail.com>
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Russell Coker <russell@coker.com.au> writes:

It is kind of like a mcstrans thingy except this is baked into glibc nss
via the nss-systemd module. it translates dymamic user id's to something
that is human readable.

dynamic users are temporary users identities that can be created by systemd
on the fly for your service. Theres only a limeted range of system user
identities (<1000) available and this allows one to just create an identity on the
fly for a service via the systemd service unit.

This is a pretty intrusive feature. Consider the following:

you have a service with a dynamicuser (say "myservice") this service
creates files for example a log file in /var/log. When the service exits
the uid no longer exists and so you have a file in /var/log with a
userid that does not exist eny longer.

This is why you see the "private" dirs in /var/lib, /var/cache and
/var/log. the services see the private dirs are the root for these
respective dirs. (its using a symlink: example: /var/lib ->
/var/lib/private) So the files that might end up with orphaned
identities are atleast kept separate on the filesystem.

So myservice maintains the log file in /var/log/private instead of
/var/log "transparently" (this all needs to be configured though in the
service unit)

There can also be a file in /etc/systemd called something like
"dont-synthesize-nobody" users of nss-systemd will look for that file
(just a get attributes) So you might see these processes atleast
traverse /etc/systemd, looking to see if the flag-file exists)

So yes fully implementing support for dynamic users is far-reaching (i
did this in dssp2-standard)

You can play with this feature with `systemd-run --system -p ... [...] -t`
To see how it behaves

But anyway back to your GetDynamicUsers question: users of
auth_use_nsswitch() (nss-systemd) need to potentially be able to resolve these dynamic
user id's , for example if they read state on a system with processes
that are associated with dynamic uids or if they need to stat files
associated with dynamic uids.

I hope this helps

> # msgtype=method_call interface=org.freedesktop.systemd1.Manager 
> member=GetDynamicUsers dest=org.freedesktop.systemd1
> init_dbus_chat(postfix_showq_t)
> dbus_system_bus_client(postfix_showq_t)
>
> # msgtype=method_call interface=org.freedesktop.systemd1.Manager 
> member=GetDynamicUsers dest=org.freedesktop.systemd1
> init_dbus_chat(dictd_t)
>
> The above is from my policy that hasn't yet seemed good enough for my Debian 
> tree.  What is this GetDynamicUsers about and why do programs like dictd 
> (dictionary server) and postfix showq need it?

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Dominick Grift