On Wed, Nov 15, 2017 at 6:03 AM, Elena Reshetova <elena.reshetova@xxxxxxxxx> 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 This also looks correct to me. Andrew, you appear to be the person for kcov changes. :) Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> -Kees > > 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> > --- > kernel/kcov.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > diff --git a/kernel/kcov.c b/kernel/kcov.c > index 15f33fa..343288c 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); > } > @@ -311,7 +312,7 @@ 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 > -- Kees Cook Pixel Security
