On Wed, Jan 16, 2019 at 11:27 AM Elena Reshetova <[email protected]> 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 <[email protected]> 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); > 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]> > --- > 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 >
