Re: [PATCH] selinux: convert struct sidtab count to refcount_t

[Jann Horn]

On Wed, Jul 24, 2019 at 12:17 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, Jul 23, 2019 at 04:53:47PM +0200, Jann Horn wrote:
> > On Mon, Jul 22, 2019 at 3:44 PM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
> > > On Mon, Jul 22, 2019 at 1:35 PM NitinGote <nitin.r.gote@xxxxxxxxx> wrote:
> > > > refcount_t type and corresponding API should be
> > > > used instead of atomic_t when the variable is used as
> > > > a reference counter. This allows to avoid accidental
> > > > refcounter overflows that might lead to use-after-free
> > > > situations.
> > > >
> > > > Signed-off-by: NitinGote <nitin.r.gote@xxxxxxxxx>
> > >
> > > Nack.
> > >
> > > The 'count' variable is not used as a reference counter here. It
> > > tracks the number of entries in sidtab, which is a very specific
> > > lookup table that can only grow (the count never decreases). I only
> > > made it atomic because the variable is read outside of the sidtab's
> > > spin lock and thus the reads and writes to it need to be guaranteed to
> > > be atomic. The counter is only updated under the spin lock, so
> > > insertions do not race with each other.
> >
> > Probably shouldn't even be atomic_t... quoting Documentation/atomic_t.txt:
> >
> > | SEMANTICS
> > | ---------
> > |
> > | Non-RMW ops:
> > |
> > | The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
> > | implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
> > | smp_store_release() respectively. Therefore, if you find yourself only using
> > | the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
> > | and are doing it wrong.
> >
> > So I think what you actually want here is a plain "int count", and then:
> >  - for unlocked reads, either READ_ONCE()+smp_rmb() or smp_load_acquire()
> >  - for writes, either smp_wmb()+WRITE_ONCE() or smp_store_release()
> >
> > smp_load_acquire() and smp_store_release() are probably the nicest
> > here, since they are semantically clearer than smp_rmb()/smp_wmb().
>
> Perhaps we need a "statistics" counter type for these kinds of counters?
> "counter_t"? I bet there are a lot of atomic_t uses that are just trying
> to be counters. (likely most of atomic_t that isn't now refcount_t ...)

This isn't a statistics counter though; this thing needs ordered
memory accesses, which you wouldn't need for statistics.