Subject: Re: [PATCH 1/1] sctp: sysctl: referring the correct net namespace

From: Firo Yang <[email-masked]>
Date: Thu, 24 Nov 2022 14:29:38 +0800

The 11/23/2022 10:00, Marcelo Ricardo Leitner wrote:
> On Wed, Nov 23, 2022 at 05:44:06PM +0800, Firo Yang wrote:
> > Recently, a customer reported that from their container whose
> > net namespace is different to the host's init_net, they can't set
> > the container's net.sctp.rto_max to any value smaller than
> > init_net.sctp.rto_min.
> > 
> > For instance,
> > Host:
> > sudo sysctl net.sctp.rto_min
> > net.sctp.rto_min = 1000
> > 
> > Container:
> > echo 100 > /mnt/proc-net/sctp/rto_min
> > echo 400 > /mnt/proc-net/sctp/rto_max
> > echo: write error: Invalid argument
> > 
> > This is caused by the check made from this'commit 4f3fdf3bc59c
> > ("sctp: add check rto_min and rto_max in sysctl")'
> > When validating the input value, it's always referring the boundary
> > value set for the init_net namespace.
> > 
> > Having container's rto_max smaller than host's init_net.sctp.rto_min
> > does make sense. Considering that the rto between two containers on the
> > same host is very likely smaller than it for two hosts.
> 
> Makes sense. And also, here, it is not using the init_net as
> boundaries for the values themselves. I mean, rto_min in init_net
> won't be the minimum allowed for rto_min in other netns. Ditto for
> rto_max.
> 
> More below.
> 
> > 
> > So to fix this problem, just referring the boundary value from the net
> > namespace where the new input value came from shold be enough.
> > 
> > Signed-off-by: Firo Yang <[email-masked]>
> > ---
> >  net/sctp/sysctl.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
> > index b46a416787ec..e167df4dc60b 100644
> > --- a/net/sctp/sysctl.c
> > +++ b/net/sctp/sysctl.c
> > @@ -429,6 +429,9 @@ static int proc_sctp_do_rto_min(struct ctl_table *ctl, int write,
> >  	else
> >  		tbl.data = &net->sctp.rto_min;
> >  
> > +	if (net != &init_net)
> > +		max = net->sctp.rto_max;
> 
> This also affects other sysctls:
> 
> $ grep -e procname -e extra sysctl.c | grep -B1 extra.*init_net
>                 .extra1         = SYSCTL_ONE,
>                 .extra2         = &init_net.sctp.rto_max
>                 .procname       = "rto_max",
>                 .extra1         = &init_net.sctp.rto_min,
> --
>                 .extra1         = SYSCTL_ZERO,
>                 .extra2         = &init_net.sctp.ps_retrans,
>                 .procname       = "ps_retrans",
>                 .extra1         = &init_net.sctp.pf_retrans,
> 
> And apparently, SCTP is the only one doing such dynamic limits. At
> least in networking.
> 
> While the issue you reported is fixable this way, for ps/pf_retrans,
> it is not, as it is using proc_dointvec_minmax() and it will simply
> consume those values (with no netns translation).
> 
> So what about patching sctp_sysctl_net_register() instead, to update
> these pointers during netns creation? Right after where it update the
> 'data' one in there:
> 
>         for (i = 0; table[i].data; i++)
>                 table[i].data += (char *)(&net->sctp) - (char *)&init_net.sctp;

Thanks Marcelo. It's better. So you mean something like the following?

--- a/net/sctp/sysctl.c
+++ b/net/sctp/sysctl.c
@@ -586,6 +586,11 @@ int sctp_sysctl_net_register(struct net *net)
        for (i = 0; table[i].data; i++)
                table[i].data += (char *)(&net->sctp) - (char *)&init_net.sctp;
 
+#define SCTP_RTO_MIN_IDX 1
+#define SCTP_RTO_MAX_IDX 2
+       table[SCTP_RTO_MIN_IDX].extra2 = &net->sctp.rto_max;
+       table[SCTP_RTO_MAX_IDX].extra1 = &net->sctp.rto_min;
+
        net->sctp.sysctl_header = register_net_sysctl(net, "net/sctp", table);
        if (net->sctp.sysctl_header == NULL) {
                kfree(table);

> 
> Thanks,
> Marcelo
> 
> > +
> >  	ret = proc_dointvec(&tbl, write, buffer, lenp, ppos);
> >  	if (write && ret == 0) {
> >  		if (new_value > max || new_value < min)
> > @@ -457,6 +460,9 @@ static int proc_sctp_do_rto_max(struct ctl_table *ctl, int write,
> >  	else
> >  		tbl.data = &net->sctp.rto_max;
> >  
> > +	if (net != &init_net)
> > +		min = net->sctp.rto_min;
> > +
> >  	ret = proc_dointvec(&tbl, write, buffer, lenp, ppos);
> >  	if (write && ret == 0) {
> >  		if (new_value > max || new_value < min)
> > -- 
> > 2.26.2
> >