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

[Kees Cook]

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 ...)

-- 
Kees Cook