Re: [PATCH v4] libselinux: use kernel status page by default

[Mike Palmiotto <mike.palmiotto@crunchydata.com>]

On Fri, Jul 24, 2020 at 4:25 PM Stephen Smalley
<stephen.smalley.work@gmail.com> wrote:
>
> On Fri, Jul 24, 2020 at 3:43 PM Mike Palmiotto
> <mike.palmiotto@crunchydata.com> wrote:
> >
> > On Fri, Jul 24, 2020 at 3:34 PM Mike Palmiotto
> > <mike.palmiotto@crunchydata.com> wrote:
> > >
> > > On Fri, Jul 24, 2020 at 2:26 PM Stephen Smalley
> > > <stephen.smalley.work@gmail.com> wrote:
> > > >
> > > > On Fri, Jul 24, 2020 at 12:23 PM Mike Palmiotto
> > > > <mike.palmiotto@crunchydata.com> wrote:
> > > > >
> > > > > On Fri, Jul 24, 2020 at 12:09 PM Stephen Smalley
> > > > > <stephen.smalley.work@gmail.com> wrote:
> > > > > >
> > > > > > On Fri, Jul 24, 2020 at 11:48 AM Stephen Smalley
> > > > > > <stephen.smalley.work@gmail.com> wrote:
> > > > > > >
> > > > > > > On Fri, Jul 24, 2020 at 11:34 AM Stephen Smalley
> > > > > > > <stephen.smalley.work@gmail.com> wrote:
> > > > > > > >
<snip>
> > > > > > So, options would appear to be:
> > > > > > 1) Drop the usage of avc_using_threads altogether, i.e. even if the
> > > > > > caller provided a thread callback, don't create another thread and
> > > > > > just call selinux_status_updated() on every avc_has_perm_noaudit()
> > > > > > unless avc_app_main_loop is set.  Rationale: dbus-daemon was only
> > > > > > using threads to avoid the overhead of avc_netlink_check_nb() on every
> > > > > > avc_has_perm_noaudit() call, and we've eliminated that via use of
> > > > > > sestatus, hence we don't need to create a separate thread at all.
> > > > > > -or-
> > > > > > 2) If using threads, then create the netlink socket during avc_init
> > > > > > and keep using the netlink loop for the thread.  This preserves the
> > > > > > blocking behavior.
> > > > > >
> > > > > > #1 seems more optimal to me and gets rid of threading for dbus-daemon,
> > > > > > which was something they didn't like anyway.
> > > > >
> > > > > Perhaps this is misguided, but it seems like avc_init is deprecated
> > > > > and along with it the ability to even set a custom thread callback.
> > > > > IOTW there does not appear to be a mechanism to set a thread callback
> > > > > while using avc_open (only avc_init). Perhaps we can just get rid of
> > > > > the default callback for avc_open and allow the (deprecated) avc_init
> > > > > to continue using it as it does?
> > > > >
> > > > > Is this basically what you were proposing for #2? I think I'd be more
> > > > > inclined to go with that approach, in case userspace object managers
> > > > > are doing other things in their thread callback.
> > > >
> > > > I think that's the same as #2 if I understood you currently.  That's
> > > > fine if you prefer it.
> > > > So then programs using avc_init() with non-NULL thread callbacks
> > > > (hence avc_using_threads == 1) will still create the netlink socket
> > > > and start a thread running avc_netlink_loop().  And programs using
> > > > avc_netlink_acquire_fd() will create the netlink socket if not already
> > > > created and can use it however they want.  Everything else will move
> > > > to using the status page.
> > >
> > > What do you think about moving avc_create_thread call (if
> > > avc_using_threads is set) into avc_netlink_acquire_fd().
> > >
> > > That way, if the caller is using avc_init with a create_thread
> > > callback, they can get their netlink socket and create the netlink
> > > thread and everything will function as before. In theory, this would
> > > also work for the sestatus netlink fallback.
> >
> > Alternatively, we could just move the thread creation into the
> > sestatus fallback, since, as you pointed out, the only reason for
> > creating a thread would be to avoid the avc_netlink_check_nb()
> > overhead.
>
> avc_netlink_acquire_fd() isn't called by dbus-daemon in its current
> release used in Fedora/RHEL.
> So adding it there won't help.  We could add it to
> selinux_status_open().  Just need to make sure we don't call
> avc_netlink_open() twice there (it is already called in the fallback
> case) or make it safe to do so.

I've given this a bit more thought, and I'm actually leaning toward
your option #1, Stephen.

Doing away with netlink threads (for non-fallback) should be fine. The
only real change in functionality would be handling of status events
on the next access check, rather than immediately in the thread.

I have a patch repaired to use this approach (and properly handle avc
threads for the sestatus fallback case). I just need to
rebase/review/re-test before submitting.

Thanks for all of the feedback and sorry for the delay.



-- 
Mike Palmiotto
https://crunchydata.com