netdev - Re: [RFC][PATCH] netlink: Only check file credentials for implicit destinations

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Message-ID: <CALCETrVezgCY61wSO_5kTJz-tX5HdYwbEPpS-HAy8bg7KEqibA@mail.gmail.com>
Date:	Sun, 25 May 2014 09:50:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Jorge Boncompte [DTI2]" <jorge@...2.net>,
	Jiri Benc <jbenc@...hat.com>,
	David Miller <davem@...emloft.net>,
	Vivek Goyal <vgoyal@...hat.com>,
	Simo Sorce <ssorce@...hat.com>,
	"security@...nel.org" <security@...nel.org>,
	Network Development <netdev@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Subject: Re: [RFC][PATCH] netlink: Only check file credentials for implicit destinations

On Sat, May 24, 2014 at 10:38 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> It was possible to get a setuid root or setcap executable to write to
> it's stdout or stderr (which has been set made a netlink socket) and
> inadvertently reconfigure the networking stack.
>
> To prevent this we check that both the creator of the socket and
> the currentl applications has permission to reconfigure the network
> stack.
>
> Unfortunately this breaks Zebra which always uses sendto/sendmsg
> and creates it's socket without any privileges.
>
> To keep Zebra working don't bother checking if the creator of the
> socket has privilege when a destination address is specified.  Instead
> rely exclusively on the privileges of the sender of the socket.
>

Cute.

> +       NETLINK_SKB_DST         = 0x8,          /* Packet not socket destination */

How about "sendto/sendmsg with explicit destination"

Whatever we settle on, I think this'll need to end up in the man
pages.  Cc: Michael Kerrisk.  I hereby volunteer to write something
up.

Michael, for background: Pre-linux-3.15, sending netlink messages to
the kernel checked the credentials of the sender.  This is a security
bug: the sender might be a setuid-root program with stdout or stderr
redirected to a netlink socket (or an SCM_RIGHTS user, etc).

The proposal in this patch is that doing privileged things using a
netlink socket will require the sender to have capabilities and
(either sendto/sendmsg with an explicit destination or a connected
socket that was created by a privileged user).

This is still not great from a security POV: if you can get a hold of
a privileged socket (i.e. a socket created with CAP_NET_ADMIN
available), then you can connect it and try to attack the kernel.
This issue would go away if we hooked netlink_connect.  I can try
writing up that version of the patch tomorrow.

--Andy
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