Re: [PATCH] rcu: kasan: record and print kvfree_call_rcu call stack

[Paul E. McKenney]

On Fri, Nov 20, 2020 at 03:44:04PM +0100, Dmitry Vyukov wrote:
> On Fri, Nov 20, 2020 at 3:34 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Fri, Nov 20, 2020 at 09:51:15AM +0100, Dmitry Vyukov wrote:
> > > On Thu, Nov 19, 2020 at 10:49 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Nov 18, 2020 at 11:53:09AM +0800, qiang.zhang@xxxxxxxxxxxxx wrote:
> > > > > From: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> > > > >
> > > > > Add kasan_record_aux_stack function for kvfree_call_rcu function to
> > > > > record call stacks.
> > > > >
> > > > > Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> > > >
> > > > Thank you, but this does not apply on the "dev" branch of the -rcu tree.
> > > > See file:///home/git/kernel.org/rcutodo.html for more info.
> > > >
> > > > Adding others on CC who might have feedback on the general approach.
> > > >
> > > >                                                         Thanx, Paul
> > > >
> > > > > ---
> > > > >  kernel/rcu/tree.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > index da3414522285..a252b2f0208d 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -3506,7 +3506,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > > >               success = true;
> > > > >               goto unlock_return;
> > > > >       }
> > > > > -
> > > > > +     kasan_record_aux_stack(ptr);
> > > > >       success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
> > > > >       if (!success) {
> > > > >               run_page_cache_worker(krcp);
> > >
> > > kvfree_call_rcu is intended to free objects, right? If so this is:
> >
> > True, but mightn't there still be RCU readers referencing this object for
> > some time, as in up to the point that the RCU grace period ends?  If so,
> > won't adding this cause KASAN to incorrectly complain about those readers?
> >
> > Or am I missing something here?
> 
> kvfree_call_rcu does not check anything, not poison the object for
> future accesses (it is also called in call_rcu which does not
> necessarily free the object).
> It just notes the current stack to provide in reports later.
> The problem is that the free stack is pointless for objects freed by
> rcu. In such cases we want call_rcu/kvfree_call_rcu stack in
> use-after-free reports.

OK, sounds good, thank you!

I will take this patch with your ack and Uladzislau's reviewed-by.
I had to forward-port this to -rcu brach "dev", and along the way I
updated the commit log to make Dmitry's point above, so please let me
know if I messed anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit 3ce23b2df528877623ffc9c9cc2b6885eb3ae9db
Author: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
Date:   Fri Nov 20 06:53:11 2020 -0800

    rcu: Record kvfree_call_rcu() call stack for KASAN
    
    This commit adds a call to kasan_record_aux_stack() in kvfree_call_rcu()
    in order to record the call stack of the code that caused the object
    to be freed.  Please note that this function does not update the
    allocated/freed state, which is important because RCU readers might
    still be referencing this object.
    
    Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
    Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
    Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1d956f9..4aa7745 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3514,6 +3514,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 		return;
 	}
 
+	kasan_record_aux_stack(ptr);
 	success = add_ptr_to_bulk_krc_lock(&krcp, &flags, ptr, !head);
 	if (!success) {
 		run_page_cache_worker(krcp);