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

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Message-ID: <bd16ced2-32b2-4f94-aedf-cfa8a955fa7e@blackwall.org>
Date: Thu, 16 Oct 2025 14:45:41 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Johannes Wiesböck
 <johannes.wiesboeck@...ec.fraunhofer.de>,
 "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,
 Michael Weiß <michael.weiss@...ec.fraunhofer.de>,
 Harshal Gohel <hg@...onwunderlich.de>
Subject: Re: [PATCH net v2] rtnetlink: Allow deleting FDB entries in user
 namespace

On 10/15/25 23:15, Johannes Wiesböck wrote:
> 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);

Reviewed-by: Nikolay Aleksandrov <razor@...ckwall.org>