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

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Message-ID: <689591ae-2680-4ff4-b742-a8653da546a7@redhat.com>
Date: Tue, 2 Dec 2025 10:16:55 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc: stephen@...workplumber.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 11/26/25 8:52 PM, Cong Wang 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>

If I read correctly, this will prevent any kind of duplication inside
some/most container orchestration platforms.

I think that nowadays completely disabling a feature for containers is
not much different than removing that feature entirely.

/P