Re: [PATCH] Smack: Simplified Mandatory Access Control Kernel

[Casey Schaufler <casey@schaufler-ca.com>]

--- Andi Kleen <andi@firstfloor.org> wrote:

> Casey Schaufler <casey@schaufler-ca.com> writes:
> 
> > Smack is the Simplified Mandatory Access Control Kernel.
> 
> I like the simplified part.
> 
> > +static int smk_get_access(smack_t sub, smack_t obj)
> > +{
> > +	struct smk_list_entry *sp = smack_list;
> > +
> > +	for (; sp != NULL; sp = sp->smk_next)
> > +		if (sp->smk_rule.smk_subject == sub &&
> > +		    sp->smk_rule.smk_object == obj)
> > +			return sp->smk_rule.smk_access;
> 
> Do I miss something, or is there really no locking for the reader side
> of the list? That looks dangerous. Of course a global lock for readers 
> would be likely a scaling disaster. You could use RCU.

Entries are never deleted, although they can be modified.

> Or if you assume rules are changed only very infrequently it might
> be more cache friendly to compile all the rules into a linear buffer
> and then just replace the whole buffer atomically with a RCU
> grace period on cahnges.

Individual entries can be modified without changing the whole
thing, but they shouldn't change often.

> It doesn't look like it would scale to larger numbers of rules though.
> Is that intended? Would caching of decisions fit into the design?

I put some thought into clever schemes for supporting large rule sets
well but decided to go ahead with the simplest possible mechanism
because I expect that in real deployments the number of rules will
be small. In fact, experiance says that virtually all access choices
will be covered either by the subject==object case or the subject can
read floor case. Cacheing, hashing, and 2D structures are all
possibilties that I would be happy to entertain as enhancements.

> Also in general code style would need some improvements;
> e.g. no externs in .c; no ../.. include hacks etc.
> You also seem weak on the Documentation front.

Yes, it is pretty sparse.

> Other than that it looks reasonably clean (haven't read all of it)

Thank you for your comments. I think the next version will be improved.


Casey Schaufler
casey@schaufler-ca.com