On Fri, 1 Sep 2017, Peter Zijlstra wrote: > 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. So the changelogs should reflect that. The current one suggests that this is a one to one replacement for atomic_t just with the extra sanity checks added. Thanks, tglx
