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

[Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>]

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@stgraber.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Julian Anastasov <ja@ssi.bg>
Cc: Simon Horman <horms@verge.net.au>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 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)
 		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) {
-			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;
-- 
2.34.1