Re: [PATCH net-next] ipvs: allow some sysctls in non-init user namespaces

[Aleksandr Mikhalitsyn]

On Wed, Apr 17, 2024 at 3:02 PM Julian Anastasov <ja@xxxxxx> wrote:
>
>
>         Hello,

Dear Julian,

>
> On Tue, 16 Apr 2024, Alexander Mikhalitsyn wrote:
>
> > Let's make all IPVS sysctls visible and RO even when
> > network namespace is owned by non-initial user namespace.
> >
> > Let's make a few sysctls to be writable:
> > - conntrack
> > - conn_reuse_mode
> > - expire_nodest_conn
> > - expire_quiescent_template
> >
> > I'm trying to be conservative with this to prevent
> > introducing any security issues in there. Maybe,
> > we can allow more sysctls to be writable, but let's
> > do this on-demand and when we see real use-case.
> >
> > This list of sysctls was chosen because I can't
> > see any security risks allowing them and also
> > Kubernetes uses [2] these specific sysctls.
> >
> > This patch is motivated by user request in the LXC
> > project [1].
> >
> > [1] https://github.com/lxc/lxc/issues/4278
> > [2] https://github.com/kubernetes/kubernetes/blob/b722d017a34b300a2284b890448e5a605f21d01e/pkg/proxy/ipvs/proxier.go#L103
> >
> > Cc: Stéphane Graber <stgraber@xxxxxxxxxxxx>
> > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > Cc: Julian Anastasov <ja@xxxxxx>
> > Cc: Simon Horman <horms@xxxxxxxxxxxx>
> > Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> > Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx>
> > Cc: Florian Westphal <fw@xxxxxxxxx>
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
> > ---
> >  net/netfilter/ipvs/ip_vs_ctl.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/netfilter/ipvs/ip_vs_ctl.c b/net/netfilter/ipvs/ip_vs_ctl.c
> > index 143a341bbc0a..92a818c2f783 100644
> > --- a/net/netfilter/ipvs/ip_vs_ctl.c
> > +++ b/net/netfilter/ipvs/ip_vs_ctl.c
> > @@ -4285,10 +4285,22 @@ static int __net_init ip_vs_control_net_init_sysctl(struct netns_ipvs *ipvs)
>
>         As the list of privileged vars is short I prefer
> to use a bool and to make only some vars read-only:
>
>         bool unpriv = false;
>
> >               if (tbl == NULL)
> >                       return -ENOMEM;
> >
> > -             /* Don't export sysctls to unprivileged users */
> > +             /* Let's show all sysctls in non-init user namespace-owned
> > +              * net namespaces, but make them read-only.
> > +              *
> > +              * Allow only a few specific sysctls to be writable.
> > +              */
> >               if (net->user_ns != &init_user_ns) {
>
>         Here we should just set: unpriv = true;
>
> > -                     tbl[0].procname = NULL;
> > -                     ctl_table_size = 0;
> > +                     for (idx = 0; idx < ARRAY_SIZE(vs_vars); idx++) {
> > +                             if (!tbl[idx].procname)
> > +                                     continue;
> > +
> > +                             if (!((strcmp(tbl[idx].procname, "conntrack") == 0) ||
> > +                                   (strcmp(tbl[idx].procname, "conn_reuse_mode") == 0) ||
> > +                                   (strcmp(tbl[idx].procname, "expire_nodest_conn") == 0) ||
> > +                                   (strcmp(tbl[idx].procname, "expire_quiescent_template") == 0)))
> > +                                     tbl[idx].mode = 0444;
> > +                     }
> >               }
> >       } else
> >               tbl = vs_vars;
>
>         And below at every place to use:
>
>         if (unpriv)
>                 tbl[idx].mode = 0444;
>
>         for the following 4 privileged sysctl vars:
>
> - sync_qlen_max:
>         - allocates messages in kernel context
>         - this needs better tunning in another patch
>
> - sync_sock_size:
>         - allocates messages in kernel context
>
> - run_estimation:
>         - for now, better init ns to decide if to use est stats
>
> - est_nice:
>         - for now, better init ns to decide the value
>
> - debug_level:
>         - already set to 0444
>
>         I.e. these vars allocate resources (mem, CPU) without
> proper control, so for now we will just copy them from init ns
> without allowing writing. And they are vars that are not tuned
> often. Also we do not know which netns is supposed to be the
> privileged one, some solutions move all devices out of init_net,
> so we can not decide where to use lower limits.

I agree. I have also decided to forbid "est_cpulist" for unprivileged users.

>
>         OTOH, "amemthresh" is not privileged but needs single READ_ONCE
> for sysctl_amemthresh in update_defense_level() due to the possible
> div by zero if we allow writing to anyone, eg.:
>
>         int amemthresh = max(READ_ONCE(ipvs->sysctl_amemthresh), 0);
>         ...
>         nomem = availmem < amemthresh;
>         ... use only amemthresh
>
>         All other vars can be writable.

Have fixed this and sent it as a separate patch! ;-)

Thanks a lot for such a quick review!

Kind regards,
Alex

>
> Regards
>
> --
> Julian Anastasov <ja@xxxxxx>