linux-kernel - Re: [PATCH 2/2] SELinux: Measure state and hash of policy using IMA

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Date:   Thu, 13 Aug 2020 14:13:44 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        tusharsu@...ux.microsoft.com, sashal@...nel.org,
        James Morris <jmorris@...ei.org>,
        linux-integrity@...r.kernel.org,
        SElinux list <selinux@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        paul Moore <paul@...l-moore.com>
Subject: Re: [PATCH 2/2] SELinux: Measure state and hash of policy using IMA

On Thu, Aug 13, 2020 at 2:03 PM Lakshmi Ramasubramanian
<nramas@...ux.microsoft.com> wrote:
>
> On 8/13/20 10:58 AM, Stephen Smalley wrote:
> > On Thu, Aug 13, 2020 at 1:52 PM Lakshmi Ramasubramanian
> > <nramas@...ux.microsoft.com> wrote:
> >>
> >> On 8/13/20 10:42 AM, Stephen Smalley wrote:
> >>
> >>>> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
> >>>> new file mode 100644
> >>>> index 000000000000..f21b7de4e2ae
> >>>> --- /dev/null
> >>>> +++ b/security/selinux/measure.c
> >>>> @@ -0,0 +1,204 @@
> >>>> +static int selinux_hash_buffer(void *buf, size_t buf_len,
> >>>> +                   void **buf_hash, int *buf_hash_len)
> >>>> +{
> >>>> +    struct crypto_shash *tfm;
> >>>> +    struct shash_desc *desc = NULL;
> >>>> +    void *digest = NULL;
> >>>> +    int desc_size;
> >>>> +    int digest_size;
> >>>> +    int ret = 0;
> >>>> +
> >>>> +    tfm = crypto_alloc_shash("sha256", 0, 0);
> >>>> +    if (IS_ERR(tfm))
> >>>> +        return PTR_ERR(tfm);
> >>> Can we make the algorithm selectable via kernel parameter and/or writing
> >>> to a new selinuxfs node?
> >>
> >> I can add a kernel parameter to select this hash algorithm.
> >
> > Also can we provide a Kconfig option for the default value like IMA does?
> >
>
> Would we need both - Kconfig and kernel param?
>
> The other option is to provide an IMA function to return the current
> hash algorithm used for measurement. That way a consistent hash
> algorithm can be employed by both IMA and the callers. Would that be better?

This is why I preferred just passing the serialized policy buffer to
IMA and letting it handle the hashing.  But apparently that approach
wouldn't fly.  IMA appears to support both a Kconfig option for
selecting a default algorithm and a kernel parameter for overriding
it.  I assume the idea is that the distros can pick a reasonable
default and then the end users can override that if they have specific
requirements.  I'd want the same for SELinux.  If IMA is willing to
export its hash algorithm to external components, then I'm willing to
reuse that but not sure if that's a layering violation.