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

[Lakshmi Ramasubramanian]

On 9/8/20 4:58 AM, Stephen Smalley wrote:
> On Tue, Sep 8, 2020 at 12:44 AM Lakshmi Ramasubramanian
> <nramas@xxxxxxxxxxxxxxxxxxx> wrote:
> > 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.
>
> No, we want to measure policy whenever there is a policy to measure.
> Just move the taking of the mutex to the callers of
> selinux_measure_state() so that it can be unconditional.

Will do.

 -lakshmi