linux-kernel - Re: Allow ioctl TUNSETIFF without CAP_NET

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Message-ID: <66e9419c6c8f9_2561f32947d@willemb.c.googlers.com.notmuch>
Date: Tue, 17 Sep 2024 04:45:16 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Zach Walton <me@...h.us>, 
 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: Re: Allow ioctl TUNSETIFF without CAP_NET_ADMIN via seccomp?

Zach Walton wrote:
> 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)

In this case seccomp would not restrict capabilities, but actually
expand them, by bypassing the standard CAP_NET_ADMIN requirement.

That sounds like it might complicate reasoning about seccomp.

Is there prior art, where kernel restrictions are actually relaxed
when relying on a privileged process allow a privileged operation
through a seccomp policy?