[PATCH 5.10.y] net: add missing ns_capable check for peer netns

[Maximilian Heyne]

The upstream commit 7b735ef81286 ("rtnetlink: add missing
netlink_ns_capable() check for peer netns") doesn't apply on older
stable kernels due to refactoring. Therefore, this patch is an attempt
to implement the same capability check just directly in the respective
interface types.

Approximate the netlink_ns_capable check with an ns_capable check. As
the newlink operation is synchronous this should result in the same
behavior.

Without this commit, for example, the following command creating a veth
device in network namespace of pid 1 succeeds:

$ unshare -U -r -n -- bash -c '
  ip link add veth0 type veth peer name foobar netns 1
  sleep 60' &
$ ip link show foobar
13: foobar@if2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 96:09:69:92:92:cc brd ff:ff:ff:ff:ff:ff link-netnsid 1

With this patch, it's returning -EPERM.

This fixes CVE-2026-31692

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 81adee47dfb6 ("net: Support specifying the network namespace upon device creation.")
Assisted-by: Kiro:claude
Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
---
 drivers/net/can/vxcan.c | 5 +++++
 drivers/net/veth.c      | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/drivers/net/can/vxcan.c b/drivers/net/can/vxcan.c
index 1bfede407270d..05fcbfacc3433 100644
--- a/drivers/net/can/vxcan.c
+++ b/drivers/net/can/vxcan.c
@@ -198,6 +198,11 @@ static int vxcan_newlink(struct net *net, struct net_device *dev,
 	if (IS_ERR(peer_net))
 		return PTR_ERR(peer_net);
 
+	if (!ns_capable(peer_net->user_ns, CAP_NET_ADMIN)) {
+		put_net(peer_net);
+		return -EPERM;
+	}
+
 	peer = rtnl_create_link(peer_net, ifname, name_assign_type,
 				&vxcan_link_ops, tbp, extack);
 	if (IS_ERR(peer)) {
diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 743716ebebdb9..bda3add65c76e 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -1341,6 +1341,11 @@ static int veth_newlink(struct net *src_net, struct net_device *dev,
 	if (IS_ERR(net))
 		return PTR_ERR(net);
 
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN)) {
+		put_net(net);
+		return -EPERM;
+	}
+
 	peer = rtnl_create_link(net, ifname, name_assign_type,
 				&veth_link_ops, tbp, extack);
 	if (IS_ERR(peer)) {
-- 
2.50.1




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