Re: [PATCH 2/3] Security: Implement disablenetwork semantics. (v4)

From: David Wagner
Date: Sun Jan 10 2010 - 20:30:19 EST

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

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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