linux-kernel - Re: [RFC][PATCH] ima: Use sequence number to wait for policy updates

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Date: Tue, 07 May 2024 16:27:33 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>, dmitry.kasatkin@...il.com,
        eric.snowberg@...cle.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, john.johansen@...onical.com,
        stephen.smalley.work@...il.com, casey@...aufler-ca.com,
        eparis@...hat.com
Cc: linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, guozihua@...wei.com, omosnace@...hat.com,
        audit@...r.kernel.org, apparmor@...ts.ubuntu.com,
        selinux@...r.kernel.org, Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [RFC][PATCH] ima: Use sequence number to wait for policy updates

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@...wei.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