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From:   Thomas Pedersen <thomas@eero.com>
Date:   Fri, 24 May 2019 20:46:22 -0700
Message-ID: <CADjYELz_4VC+14Ko0Lp2zxpMGDSF9SboqN_-LKDjFXmdsxqvXw@mail.gmail.com>
Subject: Re: [PATCH] mac80211: mesh: fix RCU warning
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     linux-wireless@vger.kernel.org,
        Peter Oh <peter.oh@bowerswilkins.com>
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On Fri, May 24, 2019 at 1:29 AM Johannes Berg <johannes@sipsolutions.net> wrote:
>
> On Wed, 2019-05-15 at 15:21 -0700, Thomas Pedersen wrote:
> > ifmsh->csa was being dereferenced without the RCU read
> > lock held.
>
> > +++ b/net/mac80211/mesh.c
> > @@ -1220,10 +1220,12 @@ int ieee80211_mesh_finish_csa(struct ieee80211_sub_if_data *sdata)
> >       ifmsh->chsw_ttl = 0;
> >
> >       /* Remove the CSA and MCSP elements from the beacon */
> > +     rcu_read_lock();
> >       tmp_csa_settings = rcu_dereference(ifmsh->csa);
> >       RCU_INIT_POINTER(ifmsh->csa, NULL);
> >       if (tmp_csa_settings)
> >               kfree_rcu(tmp_csa_settings, rcu_head);
> > +     rcu_read_unlock();
>
> This seems wrong to me.
>
> Really this code is the *writer* side, so you should do something like
> this:

Thanks this looks correct. I should've thought about this a tiny bit more ;)

> diff --git a/net/mac80211/mesh.c b/net/mac80211/mesh.c
> index 766e5e5bab8a..d578147ad7e8 100644
> --- a/net/mac80211/mesh.c
> +++ b/net/mac80211/mesh.c
> @@ -1220,7 +1220,8 @@ int ieee80211_mesh_finish_csa(struct
> ieee80211_sub_if_data *sdata)
>         ifmsh->chsw_ttl = 0;
>
>         /* Remove the CSA and MCSP elements from the beacon */
> -       tmp_csa_settings = rcu_dereference(ifmsh->csa);
> +       tmp_csa_settings = rcu_dereference_protected(ifmsh->csa,
> +                               lockdep_is_held(&sdata->wdev.mtx));
>         RCU_INIT_POINTER(ifmsh->csa, NULL);
>         if (tmp_csa_settings)
>                 kfree_rcu(tmp_csa_settings, rcu_head);
> @@ -1242,6 +1243,8 @@ int ieee80211_mesh_csa_beacon(struct
> ieee80211_sub_if_data *sdata,
>         struct mesh_csa_settings *tmp_csa_settings;
>         int ret = 0;
>
> +       lockdep_assert_held(&sdata->wdev.mtx);
> +
>         tmp_csa_settings = kmalloc(sizeof(*tmp_csa_settings),
>                                    GFP_ATOMIC);
>         if (!tmp_csa_settings)
>
>
> Can you test that and send a proper patch?
>
> johannes
>


-- 
thomas