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From: daw@cs.berkeley.edu (David Wagner)
Newsgroups: isaac.lists.linux-kernel
Subject: Re: [PATCH 2/3] Security: Implement disablenetwork semantics. (v4)
Date: Mon, 11 Jan 2010 01:29:43 +0000 (UTC)
Organization: University of California, Berkeley
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References: <alpine.LRH.2.00.1001110811350.25601@tundra.namei.org> <20100110215848.GA26609@elf.ucw.cz>
Reply-To: daw-news@taverner.cs.berkeley.edu (David Wagner)
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Pavel Machek  wrote:
>Scenario 2:
>
>Mallory calls disablenetwork, calls sendmail as the first user after
>boot; sendmail can't deliver anything (its network is disabled), but
>starts forking and taking requests for other users, DoSing the mail
>delivery.

On my system, sendmail is started by trusted boot scripts before
a "Mallory" would have a chance to do that, so the premise does not
apply.  I cannot guarantee this is the case on every system, but I'm
not familiar with any important exceptions.

>Scenario 3:
>
>Mallory calls disablenetwork, then keeps hammering on su, knowing that
>su can no longer send data to audit subsystem and so he will not get caught.

And then what?  I don't see how this gets Mallory very far.
She can keep hammering on su and keep getting denied access to
root, and it's not going to help her much.

(Note: If root's password is guessable, then there's a fair chance you're
screwed even without disablenetwork.  If root has a guessable password,
then Mallory can keep trying until she guesses right, then when she
gets it right, go and retroactively edit the logs to eliminate the
log entries if necessary -- if those log entries are ever looked at,
which is often dubious.  It's very difficult to build a secure system
if the root password is guessable.  So in my opinion, the root password
must be unguessable if you want to have a secure system, and we should
analyze disablenetwork under the assumption that sysadmins have done so.
And if the system administrators do choose an unguessable password,
then your Scenario 3 doesn't seem to help Mallory.)

The impact here seems pretty minor.

>You can trivialy make disablenetwork disable setuid exec, too. That
>will introduce better isolation facilities, but not introduce any new
>security problems.

Yup, this is probably the compromise that must be made, for political
reasons, to get this into the kernel.

But I just want to document that it's not clear to me that this decision
is well justified on technical grounds.
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