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

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Date:	Sun, 25 May 2014 16:44:43 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
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\@kernel.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

Andy Lutomirski <luto@...capital.net> writes:

> 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"

That is a better comment. 

> 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.

The really important part technically is skipping the socket based check
in sendto and sendmsg as I have done, as it fixes the one known
regression, and yields comprehensible semantics.

But I agree that since connect on sockets is really the equivalent of
open on files, and unprivileged users can change where a socket is
connected to, using a struct cred captured at connect() time is better
than the struct cred captured at socket() time.

Eric

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