[PATCH net v2] rtnetlink: add missing netlink_ns_capable() check for peer netns

[Nikolaos Gkarlis <nickgarlis@gmail.com>]

rtnl_newlink() lacks a CAP_NET_ADMIN capability check on the peer
network namespace when creating paired devices (veth, vxcan,
netkit). This allows an unprivileged user with a user namespace
to create interfaces in arbitrary network namespaces, including
init_net.

Add a netlink_ns_capable() check for CAP_NET_ADMIN in the peer
namespace before allowing device creation to proceed.

Fixes: 81adee47dfb6 ("net: Support specifying the network namespace upon device creation.")
Signed-off-by: Nikolaos Gkarlis <nickgarlis@gmail.com>
---
v2:
 - Removed "Reported-by" tag
 - Fixed "Fixes" tag with the help of Kuniyuki Iwashima (thanks !)

 net/core/rtnetlink.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index fae8034efbf..ee7d9d65b2c 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -4059,8 +4059,15 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 				ret = PTR_ERR(peer_net);
 				goto put_ops;
 			}
-			if (peer_net)
+			if (peer_net) {
+				if (!netlink_ns_capable(skb, peer_net->user_ns,
+							CAP_NET_ADMIN)) {
+					put_net(peer_net);
+					ret = -EPERM;
+					goto put_ops;
+				}
 				rtnl_nets_add(&rtnl_nets, peer_net);
+			}
 		}
 	}
 
-- 
2.34.1