netdev - Allow ioctl TUNSETIFF without CAP_NET

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Message-ID: <CABQG4PHGcZggTbDytM4Qq_zk2r3GPGAXEKPiFf9htjFpp+ouKg@mail.gmail.com>
Date: Mon, 16 Sep 2024 21:31:17 -0700
From: Zach Walton <me@...h.us>
To: linux-kernel@...r.kernel.org
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, Jason Wang <jasowang@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Allow ioctl TUNSETIFF without CAP_NET_ADMIN via seccomp?

I was debugging a seccomp profile that attempts to allow TUNSETIFF in
a container, relevant bits:

...
      {
            "names":[
                  "ioctl"
            ],
            "action":"SCMP_ACT_ALLOW",
            "args":[
                  {
                        "index":1,
                        "value":1074025674,
                        "op":"SCMP_CMP_EQ"
                  },
                  {
                        "index":1,
                        "value":2147767498,
                        "op":"SCMP_CMP_EQ"
                  }
            ]
      },
...

...but I get:

Tuntap IOCTL TUNSETIFF failed [0], errno operation not permitted

Looking at the code, it seems that there's an explicit check for
CAP_NET_ADMIN, which I'd prefer not to grant the container because the
permissions are excessive (yes, I can lock it down with seccomp but
still...): https://github.com/torvalds/linux/blob/3352633ce6b221d64bf40644d412d9670e7d56e3/drivers/net/tun.c#L2758-L2759

Is it possible to update this check to allow TUNSETIFF operations if a
seccomp profile allowing it is in place? (I am not a kernel developer
and it's unlikely I could safely contribute this)