Re: [PATCH] uprobes: convert uprobe.ref to refcount_t

[Masami Hiramatsu]

On Wed, 16 Jan 2019 08:44:52 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> 
> [ Cc'ing Masami as he maintains uprobes (we need to add uprobes to
>   the MAINTAINERS file ]

Thanks Steve, I think it is maintained mainly by Srikar and Oleg.
Srikar, Oleg, could you update MAINTAINERS file to add UPROBES entry?
And ack this change?

Thank you,

> 
> -- Steve
> 
> On Wed, 16 Jan 2019 13:20:27 +0200
> 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 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 <keescook@xxxxxxxxxxxx>
> > Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> > Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> > ---
> >  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;
> >  }
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>