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Subject: Re: [PATCH 2/2] SELinux: Measure state and hash of policy using IMA
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Lakshmi Ramasubramanian <nramas@linux.microsoft.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>
Cc:     Tyler Hicks <tyhicks@linux.microsoft.com>,
        tusharsu@linux.microsoft.com, sashal@kernel.org,
        James Morris <jmorris@namei.org>,
        linux-integrity@vger.kernel.org,
        SElinux list <selinux@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        paul Moore <paul@paul-moore.com>
Date:   Mon, 17 Aug 2020 19:11:53 -0400
In-Reply-To: <089ca24d-863b-ca84-4859-d2d6e4f09b4c@linux.microsoft.com>
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On Mon, 2020-08-17 at 15:33 -0700, Lakshmi Ramasubramanian wrote:
> On 8/17/20 3:00 PM, Casey Schaufler wrote:
> > On 8/17/2020 2:31 PM, Mimi Zohar wrote:
> > > On Thu, 2020-08-13 at 14:13 -0400, Stephen Smalley wrote:
> > > > On Thu, Aug 13, 2020 at 2:03 PM Lakshmi Ramasubramanian
> > > > <nramas@linux.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@linux.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.
> > > With the new ima_measure_critical_data() hook, I agree with you and
> > > Casey it doesn't make sense for each caller to have to write their own
> > > function.  Casey suggested exporting IMA's hash function or defining a
> > > new common hash function.   There's nothing specific to IMA.
> > 
> > Except that no one is going to use the function unless they're
> > doing an IMA operation.
> 
> Can we do the following instead:
> 
> In ima_measure_critical_data() IMA hook, we can add another param for 
> the caller to indicate whether
> 
>   => The contents of "buf" needs to be measured
>      OR
>   => Hash of the contents of "buf" needs to be measured.
> 
> This way IMA doesn't need to export any new function to meet the hashing 
> requirement.

I'm not sure overloading the parameters is a good idea, but extending
ima_measure_critical_data() to calculate a simple buffer hash should be
fine.

Mimi