Re: [PATCH] kcov: convert kcov.refcount to refcount_t

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Jan 16, 2019 at 1:51 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Wed, Jan 16, 2019 at 11:27 AM Elena Reshetova
> <elena.reshetova@intel.com> 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 kcov.refcount is used as pure reference counter.
> > Convert it to refcount_t and fix up the operations.
> >
> > **Important note for maintainers:
> >
> > Some functions from refcount_t API defined in lib/refcount.c
> > have different memory ordering guarantees than their atomic
> > counterparts.
> > The full comparison can be seen in
> > https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
> > in state to be merged to the documentation tree.
> > Normally the differences should not matter since refcount_t provides
> > enough guarantees to satisfy the refcounting use cases, but in
> > some rare cases it might matter.
> > Please double check that you don't have some undocumented
> > memory guarantees for this variable usage.
> >
> > For the kcov.refcount it might make a difference
> > in following places:
> >  - kcov_put(): decrement in refcount_dec_and_test() only
> >    provides RELEASE ordering and control dependency on success
> >    vs. fully ordered atomic counterpart
>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
>
> Thanks for improving this.
>
> KCOV uses refcounts in a very simple canonical way, so no hidden
> ordering implied.
>
> Am I missing something or refcount_dec_and_test does not in fact
> provide ACQUIRE ordering?
>
> +case 5) - decrement-based RMW ops that return a value
> +-----------------------------------------------------
> +
> +Function changes:
> +                atomic_dec_and_test() --> refcount_dec_and_test()
> +                atomic_sub_and_test() --> refcount_sub_and_test()
> +                no atomic counterpart --> refcount_dec_if_one()
> +                atomic_add_unless(&var, -1, 1) --> refcount_dec_not_one(&var)
> +
> +Memory ordering guarantees changes:
> +                fully ordered --> RELEASE ordering + control dependency
>
> I think that's against the expected refcount guarantees. When I
> privatize an  atomic_dec_and_test I would expect that not only stores,
> but also loads act on a quiescent object. But loads can hoist outside
> of the control dependency.
>
> Consider the following example, is it the case that the BUG_ON can still fire?
>
> struct X {
>   refcount_t rc; // == 2
>   int done1, done2; // == 0
> };
>
> // thread 1:
> x->done1 = 1;
> if (refcount_dec_and_test(&x->rc))
>   BUG_ON(!x->done2);
>
> // thread 2:
> x->done2 = 1;
> if (refcount_dec_and_test(&x->rc))
>   BUG_ON(!x->done1);

+more people knowledgeable in memory ordering

Unfortunately I can't find a way to reply to the
Documentation/core-api/refcount-vs-atomic.rst patch review thread.

The refcount_dec_and_test guarantees look too weak to me, see the
example above. Shouldn't refcount_dec_and_test returning true give the
object in fully quiescent state? Why only control dependency? Loads
can hoist across control dependency, no?



> > Suggested-by: Kees Cook <keescook@chromium.org>
> > Reviewed-by: David Windsor <dwindsor@gmail.com>
> > Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> > ---
> >  kernel/kcov.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/kcov.c b/kernel/kcov.c
> > index c2277db..051e86e 100644
> > --- a/kernel/kcov.c
> > +++ b/kernel/kcov.c
> > @@ -20,6 +20,7 @@
> >  #include <linux/debugfs.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/kcov.h>
> > +#include <linux/refcount.h>
> >  #include <asm/setup.h>
> >
> >  /* Number of 64-bit words written per one comparison: */
> > @@ -44,7 +45,7 @@ struct kcov {
> >          *  - opened file descriptor
> >          *  - task with enabled coverage (we can't unwire it from another task)
> >          */
> > -       atomic_t                refcount;
> > +       refcount_t              refcount;
> >         /* The lock protects mode, size, area and t. */
> >         spinlock_t              lock;
> >         enum kcov_mode          mode;
> > @@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
> >
> >  static void kcov_get(struct kcov *kcov)
> >  {
> > -       atomic_inc(&kcov->refcount);
> > +       refcount_inc(&kcov->refcount);
> >  }
> >
> >  static void kcov_put(struct kcov *kcov)
> >  {
> > -       if (atomic_dec_and_test(&kcov->refcount)) {
> > +       if (refcount_dec_and_test(&kcov->refcount)) {
> >                 vfree(kcov->area);
> >                 kfree(kcov);
> >         }
> > @@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode, struct file *filep)
> >         if (!kcov)
> >                 return -ENOMEM;
> >         kcov->mode = KCOV_MODE_DISABLED;
> > -       atomic_set(&kcov->refcount, 1);
> > +       refcount_set(&kcov->refcount, 1);
> >         spin_lock_init(&kcov->lock);
> >         filep->private_data = kcov;
> >         return nonseekable_open(inode, filep);
> > --
> > 2.7.4
> >