[RFC][PATCH] ima: Use sequence number to wait for policy updates

[Mimi Zohar]

Hi Roberto,

On Tue, 2024-05-07 at 11:32 +0200, Roberto Sassu wrote:
> On Tue, 2024-05-07 at 11:28 +0200, Roberto Sassu wrote:
> > From: Roberto Sassu <roberto.sassu at huawei.com>
> > 
> > Maintain a global sequence number, and set it to individual policy rules,
> > when they are created.
> 
> Just did an attempt, to see if this path is viable.
> 
> This patch would be an alternative to:
> 
> [PATCH v3] ima: Avoid blocking in RCU read-side critical section

Stephen had said,
   "Sidebar: the refactoring of the SELinux policy loading logic may have
   made it possible to revisit the approaches here to permit holding a
   reference to the policy from which the rule was derived so that we
   don't have to return -ESTALE in this scenario."

Removing -ESTALE would be the best solution.  We could then remove the -ESTALE
detection.

I assume the change would be in selinux_policy_commit(). Instead of freeing the
old policy, define and increment a per policy reference count for each
registered notifier callback.

        /* Free the old policy */
        synchronize_rcu();
        selinux_policy_free(oldpolicy);
        kfree(load_state->convert_data);

        /* Notify others of the policy change */
        selinux_notify_policy_change(seqno);

Mimi