[ Cc'ing Masami as he maintains uprobes (we need to add uprobes to the MAINTAINERS file ]
-- Steve On Wed, 16 Jan 2019 13:20:27 +0200 Elena Reshetova <elena.reshet...@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 uprobe.ref 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 uprobe.ref it might make a difference > in following places: > - put_uprobe(): decrement in refcount_dec_and_test() only > provides RELEASE ordering and control dependency on success > vs. fully ordered atomic counterpart > > Suggested-by: Kees Cook <keesc...@chromium.org> > Reviewed-by: David Windsor <dwind...@gmail.com> > Reviewed-by: Hans Liljestrand <ishkam...@gmail.com> > Signed-off-by: Elena Reshetova <elena.reshet...@intel.com> > --- > kernel/events/uprobes.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c > index ad415f7..750aece 100644 > --- a/kernel/events/uprobes.c > +++ b/kernel/events/uprobes.c > @@ -66,7 +66,7 @@ static struct percpu_rw_semaphore dup_mmap_sem; > > struct uprobe { > struct rb_node rb_node; /* node in the rb tree */ > - atomic_t ref; > + refcount_t ref; > struct rw_semaphore register_rwsem; > struct rw_semaphore consumer_rwsem; > struct list_head pending_list; > @@ -561,13 +561,13 @@ set_orig_insn(struct arch_uprobe *auprobe, struct > mm_struct *mm, unsigned long v > > static struct uprobe *get_uprobe(struct uprobe *uprobe) > { > - atomic_inc(&uprobe->ref); > + refcount_inc(&uprobe->ref); > return uprobe; > } > > static void put_uprobe(struct uprobe *uprobe) > { > - if (atomic_dec_and_test(&uprobe->ref)) { > + if (refcount_dec_and_test(&uprobe->ref)) { > /* > * If application munmap(exec_vma) before uprobe_unregister() > * gets called, we don't get a chance to remove uprobe from > @@ -658,7 +658,7 @@ static struct uprobe *__insert_uprobe(struct uprobe > *uprobe) > rb_link_node(&uprobe->rb_node, parent, p); > rb_insert_color(&uprobe->rb_node, &uprobes_tree); > /* get access + creation ref */ > - atomic_set(&uprobe->ref, 2); > + refcount_set(&uprobe->ref, 2); > > return u; > }
