netdev - Re: [PATCH net 2/5] vxlan: Don't double-free default FDB entry in __vxlan_dev

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Date:   Mon, 17 Dec 2018 07:52:50 -0800
From:   Roopa Prabhu <roopa@...ulusnetworks.com>
To:     Petr Machata <petrm@...lanox.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Ido Schimmel <idosch@...lanox.com>
Subject: Re: [PATCH net 2/5] vxlan: Don't double-free default FDB entry in __vxlan_dev_create()

On Mon, Dec 17, 2018 at 7:45 AM Roopa Prabhu <roopa@...ulusnetworks.com> wrote:
>
> On Mon, Dec 17, 2018 at 7:39 AM Petr Machata <petrm@...lanox.com> wrote:
> >
> > Roopa Prabhu <roopa@...ulusnetworks.com> writes:
> >
> > > On Mon, Dec 17, 2018 at 6:58 AM Petr Machata <petrm@...lanox.com> wrote:
> > >>
> > >> When a failure occurs in rtnl_configure_link(), the current code
> > >> calls unregister_netdevice() to roll back the earlier call to
> > >> register_netdevice(), and jumps to errout, which calls
> > >> vxlan_fdb_destroy().
> > >>
> > >> However unregister_netdevice() calls transitively ndo_uninit(),
> > >> which is vxlan_uninit(), and that already takes care of deleting
> > >> the default FDB entry by calling vxlan_fdb_delete_default().
> > >> Since the entry added earlier in __vxlan_dev_create() is exactly
> > >> the default entry, the cleanup code in the errout block always
> > >> leads to double free and thus a panic.
> > >>
> > >> Fix by skipping the rollback code and just returning.
> > >
> > > I trust your test case that pointed to a potential issue..., but can
> > > we add a comment there on
> > > why there is an fdb add but no destroy in the roll back code ?. Also,
> >
> > This is at a place in the code where the clean-up would normally be:
> >
> > >> +       /* the all_zeros_mac entry was deleted at vxlan_uninit */
> >
> > Is that what you meant?
> >
> > > the destroy code from uninit path does
> > > look for an entry being present before attempting a destroy, its
> > > unclear to me yet why that ends up doing a double free ?
> >
> > That code is called first, finds the FDB entry, destroys it. After it
> > returns, another destroy call is invoked from __vxlan_dev_create().
> >
>
> ok, I am wondering if we can just avoid calling the fdb destroy in
> error path if unregister has already happened on the device with a
> flag local to that function or add extra check for the fdb being
> present before calling destroy in the error path. This keeps this fdb
> destroy checks local to this function.

i think you probably just want a call to fdb destroy before
unregister_netdevice in the error path. that should avoid the delete
in vxlan_uninit and thus avoid patch3