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

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

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 <firo.yang@suse.com>
> ---
>  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

> +
>  	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
>