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

[Ondrej Mosnacek]

On Fri, Feb 26, 2021 at 5:08 AM Hillf Danton <hdanton@xxxxxxxx> wrote:
> On Thu, 25 Feb 2021 20:06:45 -0500 Paul Moore wrote:
> > On Wed, Feb 24, 2021 at 4:35 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> 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? ;)
>
> If the convert pointer can be derefered without lock, we can opt to
> convert context after building sidtab with the risk of AB BA deadlock
> cut. Below is the minimum change I can think of along your direction.

We could fix this a bit more easily by just having a shared spinlock
for both (well, *all*) sidtabs. Yes, we'd need to have it all the way
up in selinux_state and pass it through to sidtab_init(), but IMHO
that's less bad than trying to get it right with two locks.

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