netdev - Re: [PATCH linux 2/2] net sched actions: fix refcount decrement on error

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Date:   Sat, 15 Apr 2017 20:48:55 +0200 (CEST)
From:   Wolfgang Bumiller <w.bumiller@...xmox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Roman Mashak <mrv@...atatu.com>
Subject: Re: [PATCH linux 2/2] net sched actions: fix refcount decrement on
 error


> On April 15, 2017 at 8:20 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> 
> 
> On Fri, Apr 14, 2017 at 2:08 AM, Wolfgang Bumiller
> <w.bumiller@...xmox.com> wrote:
> > Before I do that - trying to wrap my head around the interdependencies
> > here better to be thorough - I noticed that tcf_hash_release() can
> > return ACT_P_DELETED. The ACT_P_CREATED case means tcf_hash_create()
> > was used, in the other case the tc_action's ref & bind count is bumped
> > by tcf_hash_check() and then also decremented by tcf_hash_release() if
> > it existed, iow. kept at 1, but not always: It does always happen in
> > act_police.c but in other files such as act_bpf.c or act_connmark.c if
> > eg. bind is set they return without decrementing, so both ref&bind count
> > are bumped when they return - the refcount logic isn't easy to follow
> > for a newcomer. Now there are two uses of __tcf_hash_release() in
> > act_api.c which check for a return value of ACT_P_DELETED, in which case
> > they call module_put().
> 
> 
> That's the nasty part... IIRC, Jamal has fixed two bugs on action refcnt'ing.
> We really need to clean up the code.
> 
> > So I'm not sure exactly how the module and tc_action counts are related
> > (and I usually like to understand my own patches ;-) ).
> 
> 
> Each action holds a refcnt to its module, each filter holds a refcnt to
> its bound or referenced (unbound) action.
> 
> 
> > Maybe I'm missing something obvious but I'm currently a bit confused as
> > to whether the tcf_hash_release() call there is okay, or should have its
> > return value checked or should depend on ->init()'s ACT_P_CREATED value
> > as well?
> >
> 
> I think it's the same? If we have ACT_P_CREATED here, tcf_hash_release()
> will return ACT_P_DELETED for sure because the newly created action has
> refcnt==1?

Makes sense on the one hand, but for ACT_P_DELETED both ref and bind
count need to reach 0, so I'm still concerned that the different behaviors
I mentioned above might be problematic if we use ACT_P_CREATED only.
(It also means my patches still leak a count - which is probably still
better than the previous underflow, but ultimately doesn't satisfy me.)
Should I still resend it this way for the record with the Acked-bys?
(Since given the fact that with unprivileged containers it's possible to
trigger this access and potentially crash the kernel I strongly feel that
some version of this should end up in the 4.11 release.)