linux-kernel - Re: Rcceiving a generic netlink multicast - should be restricted to the root user?

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Message-ID: <CAPQBh9R5_br9AiWTYNAh27b=hnNg83T+w3SNmWt55H=NMqzxVg@mail.gmail.com>
Date:	Sat, 7 Mar 2015 21:12:24 -0700
From:	Craig Davison <craig65535@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: Rcceiving a generic netlink multicast - should be restricted to
 the root user?

Some code to reproduce this is at https://github.com/craig65535/mcast-exmpl.

The kernel module hooks TCP connects via a jprobe, and multicasts a
generic netlink message on every connect. So after insmod'ing the
module, just connect anywhere to send a multicast.

The client receives multicasts, even when it's not running as root.

Again, the documentation (man 7 netlink) does not agree with this. Is
this just a documentation bug, or a real bug?

On Fri, Mar 6, 2015 at 7:08 PM, Craig Davison <craig65535@...il.com> wrote:
> I wrote a kernel module that sends generic Netlink multicasts, and
> wrote a userland client using libmnl that receives them.
>
> That all works fine, but my client works even when it's not the root user.
>
> man 7 netlink says:
>
> Only processes with an effective UID of 0 or the CAP_NET_ADMIN capability
> may send or listen to a netlink multicast group.
>
> The listen part of this is seemingly not true. I've tried this on
> kernels 3.13 (Ubuntu 14.04), 2.6.32 (CentOS 6) and 2.6.18 (CentOS 5).
>
> Is this a bug?
>
> If not: I know that restricting receiving generic netlink commands
> incoming to the kernel to being only from root is possible with
> GENL_ADMIN_PERM flag, but is it possible to send multicasts from the
> kernel that can only be received by root?
>
> Thank you
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