Re: [BUG] Race between policy reload sidtab conversion and live conversion

[Ondrej Mosnacek <omosnace@redhat.com>]

On Fri, Feb 26, 2021 at 2:07 AM Paul Moore <paul@paul-moore.com> wrote:
> On Wed, Feb 24, 2021 at 4:35 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > After the switch to RCU, we now have:
> > 1. Start live conversion of new entries.
> > 2. Convert existing entries.
> > 3. RCU-assign the new policy pointer to selinux_state.
> > [!!! Now actually both old and new sidtab may be referenced by
> > readers, since there is no synchronization barrier previously provided
> > by the write lock.]
> > 4. Wait for synchronize_rcu() to return.
> > 5. Now only the new sidtab is visible to readers, so the old one can
> > be destroyed.
> >
> > So the race can happen between 3. and 5., if one thread already sees
> > the new sidtab and adds a new entry there, and a second thread still
> > has the reference to the old sidtab and also tires to add a new entry;
> > live-converting to the new sidtab, which it doesn't expect to change
> > by itself. Unfortunately I failed to realize this when reviewing the
> > patch :/
>
> It is possible I'm not fully understanding the problem and/or missing
> an important detail - it is rather tricky code, and RCU can be very
> hard to reason at times - but I think we may be able to solve this
> with some lock fixes inside sidtab_context_to_sid().  Let me try to
> explain to see if we are on the same page here ...
>
> The problem is when we have two (or more) threads trying to
> add/convert the same context into a sid; the task with new_sidtab is
> looking to add a new sidtab entry, while the task with old_sidtab is
> looking to convert an entry in old_sidtab into a new entry in
> new_sidtab.  Boom.
>
> Looking at the code in sidtab_context_to_sid(), when we have two
> sidtabs that are currently active (old_sidtab->convert pointer is
> valid) and a task with old_sidtab attempts to add a new entry to both
> sidtabs it first adds it to the old sidtab then it also adds it to the
> new sidtab.  I believe the problem is that in this case while the task
> grabs the old_sidtab->lock, it never grabs the new_sidtab->lock which
> allows it to race with tasks that already see only new_sidtab.  I
> think adding code to sidtab_context_to_sid() which grabs the
> new_sidtab->lock when adding entries to the new_sidtab *should* solve
> the problem.
>
> Did I miss something important? ;)

Sadly, yes :) Consider this scenario (assuming we fix the locking at
sidtab level):

If it happens that a new SID (x) is added via the new sidtab and then
another one (y) via the old sidtab, to avoid clash of SIDs, we would
need to leave a "hole" in the old sidtab for SID x. And this will
cause trouble if the thread that has just added SID y, then tries to
translate the context string corresponding to SID x (without re-taking
the RCU read lock and refreshing the policy pointer). Even if we
handle skipping the "holes" in the old sidtab safely, the translation
would then end up adding a duplicate SID entry for the context already
represented by SID x - which is not a state we want to end up in.

This is why I said that to fully fix this, we'd need to have a
both-ways live conversion in place. (And that already starts to feel
like too much hacking for something that should probably go to
stable@...)

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.