Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount_t

[Peter Zijlstra]

On Fri, Sep 01, 2017 at 09:39:50AM +0200, Thomas Gleixner wrote:
> On Wed, 30 Aug 2017, Elena Reshetova wrote:
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> >  - counter is initialized to 1 using atomic_set()
> >  - a resource is freed upon counter reaching zero
> >  - once counter reaches zero, its further
> >    increments aren't allowed
> >  - counter schema uses basic atomic operations
> >    (set, inc, inc_not_zero, dec_and_test, etc.)
> > 
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows
> > and underflows. This is important since overflows and underflows
> > can lead to use-after-free situation and be exploitable.
> > 
> > The variable futex_pi_state.refcount is used as pure
> > reference counter. Convert it to refcount_t and fix up
> > the operations.
> > 
> > Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> > Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> 
> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

So the thing to be careful with for things like futex and some of the
other core kernel code is the memory ordering.

atomic_dec_and_test() provides a full smp_mb() before and after,
refcount_dec_and_test() only provides release semantics.

This is typically sufficient, and I would argue that if we rely on more
than that, there _should_ be a comment, however reality isn't always as
nice.

That said, I think this conversion is OK, pi_state->refcount isn't
relied upon to provide additional memory ordering above and beyond what
refcounting requires.