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@aisec.fraunhofer.de>
Tested-by: Harshal Gohel <hg@simonwunderlich.de>
Signed-off-by: Johannes Wiesböck <johannes.wiesboeck@aisec.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

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@aisec.fraunhofer.de>
> Tested-by: Harshal Gohel <hg@simonwunderlich.de>
> Signed-off-by: Johannes Wiesböck <johannes.wiesboeck@aisec.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@blackwall.org>

On Wed, Oct 15, 2025 at 10:15:43PM +0200, 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@aisec.fraunhofer.de>
> Tested-by: Harshal Gohel <hg@simonwunderlich.de>
> Signed-off-by: Johannes Wiesböck <johannes.wiesboeck@aisec.fraunhofer.de>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>