netdev - Re: [Patch net v5 4/9] net_sched: Prevent using netem duplication in non-initial user namespace

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Message-ID: <20251201164042.5ef12ced@phoenix.local>
Date: Mon, 1 Dec 2025 16:40:53 -0800
From: Stephen Hemminger <stephen@...workplumber.org>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, kuba@...nel.org, Cong Wang
 <cwang@...tikernel.io>
Subject: Re: [Patch net v5 4/9] net_sched: Prevent using netem duplication
 in non-initial user namespace

On Wed, 26 Nov 2025 11:52:39 -0800
Cong Wang <xiyou.wangcong@...il.com> wrote:

> From: Cong Wang <cwang@...tikernel.io>
> 
> The netem qdisc has a known security issue with packet duplication
> that makes it unsafe to use in unprivileged contexts. While netem
> typically requires CAP_NET_ADMIN to load, users with "root" privileges
> inside a user namespace also have CAP_NET_ADMIN within that namespace,
> allowing them to potentially exploit this feature.
> 
> To address this, we need to restrict the netem duplication to only the
> initial user namespace.
> 
> Fixes: 0afb51e72855 ("[PKT_SCHED]: netem: reinsert for duplication")
> Signed-off-by: Cong Wang <cwang@...tikernel.io>
> ---

Duplication logic should be fixed rather than trying to build
a wall around it.