Re: [PATCH] SELinux: Measure state and hash of policy using IMA

[Lakshmi Ramasubramanian]

On 9/7/20 3:32 PM, Stephen Smalley wrote:

> > Signed-off-by: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx>
> > Suggested-by: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx> # error: implicit declaration of function 'vfree'
> > Reported-by: kernel test robot <lkp@xxxxxxxxx> # error: implicit declaration of function 'crypto_alloc_shash'
> > Reported-by: kernel test robot <lkp@xxxxxxxxx> # sparse: symbol 'security_read_selinux_policy' was not declared. Should it be static?
>
> Not sure these Reported-by lines are useful since they were just on
> submitted versions of the patch not on an actual merged commit.

I'll remove them when I update the patch.

> > diff --git a/security/selinux/measure.c b/security/selinux/measure.c
> > new file mode 100644
> > index 000000000000..caf9107937d9
> > --- /dev/null
> > +++ b/security/selinux/measure.c
> <snip>
> > +void selinux_measure_state(struct selinux_state *state, bool policy_mutex_held)
> > +{
> <snip>
> > +
> > +       if (!policy_mutex_held)
> > +               mutex_lock(&state->policy_mutex);
> > +
> > +       rc = security_read_policy_kernel(state, &policy, &policy_len);
> > +
> > +       if (!policy_mutex_held)
> > +               mutex_unlock(&state->policy_mutex);
>
> This kind of conditional taking of a mutex is generally frowned upon
> in my experience.
> You should likely just always take the mutex in the callers of
> selinux_measure_state() instead.
> In some cases, it may be the caller of the caller.  Arguably selinuxfs
> could be taking it around all state modifying operations (e.g.
> enforce, checkreqprot) not just policy modifying ones although it
> isn't strictly for that purpose.

Since currently policy_mutex is not used to synchronize access to state 
variables (enforce, checkreqprot, etc.) I am wondering if 
selinux_measure_state() should measure only state if policy_mutex is not 
held by the caller - similar to how we skip measuring policy if 
initialization is not yet completed.

	/*
	 * Measure SELinux policy only after initialization is
         * completed.
	 */
	if (!initialized)
		goto out;

 -lakshmi