Re: [PATCH net] psp: fix NULL genl_sock deref race with concurrent netns teardown

[Wei Wang]

On Mon, Jul 6, 2026 at 6:46 AM Daniel Zahka <daniel.zahka@xxxxxxxxx> wrote:
>
> >
>
> On 7/3/26 7:24 AM, Kiran Kella wrote:
> > The race occurs between network namespace removal and PSP device
> > unregistration.  When a netns is deleted while a PSP device associated
> > with that netns is concurrently being removed, psp_dev_unregister()
> > triggers psp_nl_notify_dev() to send a device change notification.
> > Concurrently, cleanup_net() running in the netns workqueue calls
> > genl_pernet_exit(), which sets net->genl_sock to NULL. If
> > genl_pernet_exit() wins the race, two sites in psp_nl_multicast_per_ns()
> > then dereference the NULL socket and crash:
> >
> > CPU 0 (netns teardown)       CPU 1 (PSP device unregister)
> > ======================       =============================
> > cleanup_net [workqueue]
> >    genl_pernet_exit()         psp_dev_unregister()
> >      net->genl_sock = NULL      psp_nl_notify_dev()
> >                                   psp_nl_multicast_per_ns()
> >                                     build_ntf()
> >                                       -> netlink_has_listeners(NULL)
> >                                       /* crash */
> >                                     genlmsg_multicast_netns()
> >                                       -> nlmsg_multicast_filtered(NULL)
> >                                       /* crash */
> >
> > Both the main_net path (derived from psd->main_netdev) and each
> > assoc_net entry in psd->assoc_dev_list are affected.
> >
> > Fix by replacing the bare dev_net() calls with maybe_get_net().
> > maybe_get_net() returns NULL if the namespace is already dying.
> > Holding the reference ensures genl_sock remains valid across both the
> > build_ntf() and genlmsg_multicast_netns() calls.
> >
> > Fixes: 00c94ca2b99e ("psp: base PSP device support")
> > Fixes: 06c2dce2d0f6 ("psp: add new netlink cmd for dev-assoc and dev-disassoc")
> > Signed-off-by: Kiran Kella <kiran.kella@xxxxxxxxxxxx>
> > Reviewed-by: Ajit Khaparde <ajit.khaparde@xxxxxxxxxxxx>
> > Reviewed-by: Vikas Gupta <vikas.gupta@xxxxxxxxxxxx>
> > Reviewed-by: Bhargava Marreddy <bhargava.marreddy@xxxxxxxxxxxx>
>
>
> Thanks. I was able to repro myself by inserting a delay after the old:
>
> main_net = dev_net(psd->main_netdev);
>
> and then executing nsim_psp_rereg_write() in parallel with destroying
> the netns that I placed a netdevsim dev into.
>
> Tested-by: Daniel Zahka <daniel.zahka@xxxxxxxxx>
>
> Reviewed-by: Daniel Zahka <daniel.zahka@xxxxxxxxx>
>
Thanks for the fix!

Reviewed-by: Wei Wang <weibunny@xxxxxx>
> > ---
> >   net/psp/psp_nl.c | 33 +++++++++++++++++++++------------
> >   1 file changed, 21 insertions(+), 12 deletions(-)
> >
> > diff --git a/net/psp/psp_nl.c b/net/psp/psp_nl.c
> > index 9610d8c456ff..24ab626a9e8a 100644
> > --- a/net/psp/psp_nl.c
> > +++ b/net/psp/psp_nl.c
> > @@ -62,36 +62,45 @@ psp_nl_multicast_per_ns(struct psp_dev *psd, unsigned int group,
> >       struct net *main_net;
> >       struct sk_buff *ntf;
> >
> > -     main_net = dev_net(psd->main_netdev);
> > +     main_net = maybe_get_net(dev_net(psd->main_netdev));
> > +     if (!main_net)
> > +             return;
> > +
> >       xa_init(&sent_nets);
> >
> >       list_for_each_entry(entry, &psd->assoc_dev_list, dev_list) {
> >               struct net *assoc_net = dev_net(entry->assoc_dev);
> > +             struct net *net;
> >               int ret;
> >
> >               if (net_eq(assoc_net, main_net))
> >                       continue;
> >
> > -             ret = xa_insert(&sent_nets, (unsigned long)assoc_net, assoc_net,
> > -                             GFP_KERNEL);
> > -             if (ret == -EBUSY)
> > +             net = maybe_get_net(assoc_net);
> > +             if (!net)
> >                       continue;
> >
> > -             ntf = build_ntf(psd, assoc_net, ctx);
> > -             if (!ntf)
> > +             ret = xa_insert(&sent_nets, (unsigned long)assoc_net, assoc_net,
> > +                             GFP_KERNEL);
> > +             if (ret == -EBUSY) {
> > +                     put_net(net);
> >                       continue;
> > +             }
> >
> > -             genlmsg_multicast_netns(&psp_nl_family, assoc_net, ntf, 0,
> > -                                     group, GFP_KERNEL);
> > +             ntf = build_ntf(psd, net, ctx);
> > +             if (ntf)
> > +                     genlmsg_multicast_netns(&psp_nl_family, net, ntf, 0,
> > +                                             group, GFP_KERNEL);
> > +             put_net(net);
> >       }
>
> some optional nits if you wanted to respin:
>
> You could eliminate the extra struct net *net, by just doing something
> like if (!maybe_get_net(assoc_net)) directly.
>
> You could get away with a single put_net(net); call site, if you reorder
> the ops that could fail so that maybe_get_net(assoc_net) happens last
> before the build_ntf(psd, net, ctx)
>
> >       xa_destroy(&sent_nets);
> >
> >       /* Send to main device netns */
> >       ntf = build_ntf(psd, main_net, ctx);
> > -     if (!ntf)
> > -             return;
> > -     genlmsg_multicast_netns(&psp_nl_family, main_net, ntf, 0, group,
> > -                             GFP_KERNEL);
> > +     if (ntf)
> > +             genlmsg_multicast_netns(&psp_nl_family, main_net, ntf, 0, group,
> > +                                     GFP_KERNEL);
> > +     put_net(main_net);
> >   }
> >
> >   static struct sk_buff *psp_nl_clone_ntf(struct psp_dev *psd, struct net *net,