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 <[email protected]> > > Reviewed-by: David Windsor <[email protected]> > > Reviewed-by: Hans Liljestrand <[email protected]> > > Signed-off-by: Elena Reshetova <[email protected]> > > Reviewed-by: Thomas Gleixner <[email protected]>
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.
