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Date:   Tue, 11 Aug 2020 13:58:35 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Johan Knöös <jknoos@...gle.com>,
        Gregory Rose <gvrose8192@...il.com>,
        bugs <bugs@...nvswitch.org>, Netdev <netdev@...r.kernel.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        rcu <rcu@...r.kernel.org>
Subject: Re: [ovs-discuss] Double free in recent kernels after memleak fix

On Tue, Aug 11, 2020 at 12:08 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Mon, Aug 10, 2020 at 8:27 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> >
> > On Tue, Aug 11, 2020 at 10:24 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > >
> > > On Mon, Aug 10, 2020 at 6:16 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> > > > Hi all, I send a patch to fix this. The rcu warnings disappear. I
> > > > don't reproduce the double free issue.
> > > > But I guess this patch may address this issue.
> > > >
> > > > http://patchwork.ozlabs.org/project/netdev/patch/20200811011001.75690-1-xiangxia.m.yue@gmail.com/
> > >
> > > I don't see how your patch address the double-free, as we still
> > > free mask array twice after your patch: once in tbl_mask_array_realloc()
> > > and once in ovs_flow_tbl_destroy().
> > Hi Cong.
> > Before my patch, we use the ovsl_dereference
> > (rcu_dereference_protected) in the rcu callback.
> > ovs_flow_tbl_destroy
> > ->table_instance_destroy
> > ->table_instance_flow_free
> > ->flow_mask_remove
> > ASSERT_OVSL(will print warning)
> > ->tbl_mask_array_del_mask
> > ovsl_dereference(rcu usage warning)
> >
>
> I understand how your patch addresses the RCU annotation issue,
> which is different from double-free.
>
>
> > so we should invoke the table_instance_destroy or others under
> > ovs_lock to avoid (ASSERT_OVSL and rcu usage warning).
>
> Of course... I never doubt it.
>
>
> > with this patch, we reallocate the mask_array under ovs_lock, and free
> > it in the rcu callback. Without it, we  reallocate and free it in the
> > rcu callback.
> > I think we may fix it with this patch.
>
> Does it matter which context tbl_mask_array_realloc() is called?
> Even with ovs_lock, we can still double free:
>
> ovs_lock()
> tbl_mask_array_realloc()
>  => call_rcu(&old->rcu, mask_array_rcu_cb);
> ovs_unlock()
> ...
> ovs_flow_tbl_destroy()
>  => call_rcu(&old->rcu, mask_array_rcu_cb);
>
> So still twice, right? To fix the double-free, we have to eliminate one
> of them, don't we? ;)
No
Without my patch: in rcu callback:
ovs_flow_tbl_destroy
->call_rcu(&ma->rcu, mask_array_rcu_cb);
->table_instance_destroy
->tbl_mask_array_realloc(Shrink the mask array if necessary)
->call_rcu(&old->rcu, mask_array_rcu_cb);

With the patch:
ovs_lock
table_instance_flow_flush (free the flow)
tbl_mask_array_realloc(shrink the mask array if necessary, will free
mask_array in rcu(mask_array_rcu_cb) and rcu_assign_pointer new
mask_array)
ovs_unlock

in rcu callback:
ovs_flow_tbl_destroy
call_rcu(&ma->rcu, mask_array_rcu_cb);(that is new mask_array)

>
> >
> > > Have you tried my patch which is supposed to address this double-free?
> > I don't reproduce it. but your patch does not avoid ruc usage warning
> > and ASSERT_OVSL.
>
> Sure, I never intend to fix anything else but double-free. The $subject is
> about double free, I double checked. ;)
>
> Thanks.



-- 
Best regards, Tonghao

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