netdev - Re: [PATCH net 1/1] net: sched: fix cleanup NULL pointer exception in act

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Date:   Thu, 21 Mar 2019 11:40:39 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     John Hurley <john.hurley@...ronome.com>,
        Jiri Pirko <jiri@...lanox.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Vlad Buslov <vladbu@...lanox.com>, oss-drivers@...ronome.com
Subject: Re: [PATCH net 1/1] net: sched: fix cleanup NULL pointer exception
 in act_mirr

On Thu, 21 Mar 2019 11:26:56 -0700, Cong Wang wrote:
> On Thu, Mar 21, 2019 at 11:22 AM Jakub Kicinski wrote:
> > On Thu, 21 Mar 2019 11:05:21 -0700, Cong Wang wrote:  
> > > On Wed, Mar 20, 2019 at 11:12 AM John Hurley wrote:  
> > > >
> > > > A new mirred action is created by the tcf_mirred_init function. This
> > > > contains a list head struct which is inserted into a global list on
> > > > successful creation of a new action. However, after a creation, it is
> > > > still possible to error out if the egress device does not exist. This
> > > > calls the act_mirr cleanup function via __tcf_idr_release and
> > > > __tcf_action_put. This cleanup function tries to delete the list entry
> > > > which is as yet uninitialised, leading to a NULL pointer exception.  
> > >
> > > Hmm, good catch but can this be just fixed by initializing it before
> > > taking the netdevice refcnt? Like this:
> > >
> > > @@ -163,6 +163,9 @@ static int tcf_mirred_init(struct net *net, struct
> > > nlattr *nla,
> > >         m->tcf_action = parm->action;
> > >         m->tcfm_eaction = parm->eaction;
> > >
> > > +       if (ret == ACT_P_CREATED)
> > > +               INIT_LIST_HEAD(&m->tcfm_list);
> > > +
> > >         if (parm->ifindex) {
> > >                 dev = dev_get_by_index(net, parm->ifindex);
> > >                 if (!dev) {
> > >
> > > which is also much simpler.  
> >
> > That's the initial way John fixed it, but I asked him to go back to the
> > previous way this code was written.
> >
> > I think having the parameters validated before any allocations happen
> > is less error prone, especially with the strange way actions get the
> > release call even when init failed.  It's just a more reliable code
> > patter for actions' init callback.  
> 
> The point is a simpler version is better for -net and -stable,
> and for review too.
> 
> You can always refactor it for net-next if you feel necessary.

I personally prefer to push a slightly bigger fix (30 lines of change
is hardly large after all) if it means we can have the same code in
net-next as in LTS kernels (4.19 in this case).  Especially for
relatively stable code.

But yeah, I was on the fence too, so given your preference we can go
back to init head.