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

Paul E. McKenney
2020-11-20T14:54:47Z

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@kernel.org> 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@kernel.org> wrote:
> > > >
> > > > On Wed, Nov 18, 2020 at 11:53:09AM +0800, qiang.zhang@windriver.com wrote:
> > > > > From: Zqiang <qiang.zhang@windriver.com>
> > > > >
> > > > > Add kasan_record_aux_stack function for kvfree_call_rcu function to
> > > > > record call stacks.
> > > > >
> > > > > Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> > > >
> > > > 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@windriver.com>
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@google.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Zqiang <qiang.zhang@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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);