linux-kernel - Re: [PATCH 2/3] selinux: add checksum to policydb

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Date:   Thu, 27 Apr 2017 19:12:17 +0200
From:   Sebastien Buisson <sbuisson.ddn@...il.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...ho.nsa.gov,
        serge@...lyn.com, james.l.morris@...cle.com,
        Eric Paris <eparis@...isplace.org>,
        Paul Moore <paul@...l-moore.com>,
        Daniel Jurgens <danielj@...lanox.com>,
        Sebastien Buisson <sbuisson@....com>
Subject: Re: [PATCH 2/3] selinux: add checksum to policydb

2017-04-27 17:18 GMT+02:00 Stephen Smalley <sds@...ho.nsa.gov>:
> Ok, that should work as long as you just want to validate that all the
> clients loaded the same policy file, and aren't concerned about non-
> persistent policy boolean changes.

As far as I understand, non-persistent policy boolean changes can
affect the way the policy is enforced. So that is a problem if the
checksum does not reflect it. We want to protect against someone
tampering the policy locally on a Lustre client, even if it does not
survive a reboot.
I just checked, with the method of computing the checksum on a (data,
len) pair on entry to security_load_policy() the checksum does not
change after using setsebool. So it seems I would need to call
security_read_policy() to retrieve the binary representation of the
policy as currently enforced by the kernel. Unless you can see another
way?

> You needed to get (global) enforcing mode too, didn't you?  That's
> separate from the policy.

Exactly, I also need to rework the patch I proposed about this, in
light of the comments I received.

> Make sure you make the hash algorithm explicit in both what is returned
> by the hook to lustre and by what is exported via selinuxfs.  Can
> likely just encode the hash algorithm name in the string when you
> generate it.

Sure, I will add "sha256:" at the beginning of the string.