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

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Date:   Fri, 28 Apr 2017 18:08:07 +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

2017-04-28 17:50 GMT+02:00 Stephen Smalley <sds@...ho.nsa.gov>:
> You seem to be conflating kernel policy with userspace policy.
> security_load_policy() is provided with the kernel policy image, which
> is the result of linking the kernel-relevant portions of all policy
> modules together. A hash of that image will change if you insert a
> policy module that affects the kernel policy in any way.  But a change
> that only affects userspace policy isn't ever going to be reflected in
> the kernel.  It doesn't matter where or when you compute your checksum
> within the kernel; it isn't ever going to reflect those userspace
> policy changes.

Here is the content of the module is used for my tests:

#============= user_t ==============
allow user_t mnt_t:dir { write add_name };
allow user_t mnt_t:file { write create };

After loading the .pp corresponding to it, I can see that with the
method of computing the checksum on the (data, len) pair on entry to
security_load_policy(), the checksum does not change. However, when
using the (data, len) pair got from
security_read_policy(), the checksum changes. And when I remove the
module, the checksum is back to its previous value.
So this is what makes me think there is a difference. Am I missing something?