linux-kernel - [PATCH net v2] rtnetlink: Allow deleting FDB entries in user namespace

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Message-ID: <20251015201548.319871-1-johannes.wiesboeck@aisec.fraunhofer.de>
Date: Wed, 15 Oct 2025 22:15:43 +0200
From: Johannes Wiesböck <johannes.wiesboeck@...ec.fraunhofer.de>
To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Kuniyuki Iwashima
	<kuniyu@...gle.com>, Stanislav Fomichev <sdf@...ichev.me>, Xiao Liang
	<shaw.leon@...il.com>, Vlad Yasevich <vyasevic@...hat.com>, Jitendra Kalsaria
	<jitendra.kalsaria@...gic.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
CC: <gyroidos@...ec.fraunhofer.de>, <sw@...onwunderlich.de>,
	Johannes Wiesböck
	<johannes.wiesboeck@...ec.fraunhofer.de>, Michael Weiß
	<michael.weiss@...ec.fraunhofer.de>, Harshal Gohel <hg@...onwunderlich.de>
Subject: [PATCH net v2] rtnetlink: Allow deleting FDB entries in user namespace

Creating FDB entries is possible from a non-initial user namespace when
having CAP_NET_ADMIN, yet, when deleting FDB entries, processes receive
an EPERM because the capability is always checked against the initial
user namespace. This restricts the FDB management from unprivileged
containers.

Drop the netlink_capable check in rtnl_fdb_del as it was originally
dropped in c5c351088ae7 and reintroduced in 1690be63a27b without
intention.

This patch was tested using a container on GyroidOS, where it was
possible to delete FDB entries from an unprivileged user namespace and
private network namespace.

Fixes: 1690be63a27b ("bridge: Add vlan support to static neighbors")
Reviewed-by: Michael Weiß <michael.weiss@...ec.fraunhofer.de>
Tested-by: Harshal Gohel <hg@...onwunderlich.de>
Signed-off-by: Johannes Wiesböck <johannes.wiesboeck@...ec.fraunhofer.de>
---
v2:
  - completely drop the capability check in favor of changing to
    netlink_net_capable
  - describe intended behavior already possible for adding FDB entries
v1: https://lore.kernel.org/all/20250923082153.60030-1-johannes.wiesboeck@aisec.fraunhofer.de/
---
 net/core/rtnetlink.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 8040ff7c356e4..576d5ec3bb364 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -4715,9 +4715,6 @@ static int rtnl_fdb_del(struct sk_buff *skb, struct nlmsghdr *nlh,
 	int err;
 	u16 vid;
 
-	if (!netlink_capable(skb, CAP_NET_ADMIN))
-		return -EPERM;
-
 	if (!del_bulk) {
 		err = nlmsg_parse_deprecated(nlh, sizeof(*ndm), tb, NDA_MAX,
 					     NULL, extack);
-- 
2.51.0