linux-kernel - Re: [PATCH v5 15/16] ima: Move dentries into ima

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Message-ID: <fb99af21f029b8072435e35731b919f4ec98f89d.camel@linux.ibm.com>
Date:   Thu, 09 Dec 2021 10:30:25 -0500
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        Stefan Berger <stefanb@...ux.ibm.com>
Cc:     linux-integrity@...r.kernel.org, zohar@...ux.ibm.com,
        serge@...lyn.com, containers@...ts.linux.dev,
        dmitry.kasatkin@...il.com, ebiederm@...ssion.com,
        krzysztof.struczynski@...wei.com, roberto.sassu@...wei.com,
        mpeters@...hat.com, lhinds@...hat.com, lsturman@...hat.com,
        puiterwi@...hat.com, jamjoom@...ibm.com,
        linux-kernel@...r.kernel.org, paul@...l-moore.com, rgb@...hat.com,
        linux-security-module@...r.kernel.org, jmorris@...ei.org
Subject: Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

On Thu, 2021-12-09 at 15:37 +0100, Christian Brauner wrote:
> On Thu, Dec 09, 2021 at 03:34:28PM +0100, Christian Brauner wrote:
> > On Wed, Dec 08, 2021 at 05:18:17PM -0500, Stefan Berger wrote:
> > > Move the dentries into the ima_namespace for reuse by virtualized
> > > SecurityFS. Implement function freeing the dentries in order of
> > > files and symlinks before directories.
> > > 
> > > Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
> > > ---
> > 
> > This doesn't work as implemented, I think.
> > 
> > What I would have preferred and what I tried to explain in the
> > earlier review was:
> > Keep the dentry stashing global since it is only needed for
> > init_ima_ns.
> > Then struct ima_namespace becomes way smaller and simpler.
> > If you do that then it makes sense to remove the additional dget()
> > in securityfs_create_dentry() for non-init_ima_ns.
> > Then you can rely on auto-cleanup in .kill_sb() or on
> > ima_securityfs_init() failure and you only need to call
> > ima_fs_ns_free_dentries() if ns != init_ima_ns.
> > 
> > IIuc, it seems you're currently doing one dput() too many since
> > you're calling securityfs_remove() in the error path for non-
> > init_ima_ns which relies on the previous increased dget() which we
> > removed.
> 
> If you really want to move the dentry stashing into struct
> ima_namespace even though it's really unnecessary then you may as
> well not care about the auto-cleanup and keep that additional
> ima_fs_ns_free_dentries(ns) call in .kill_sb(). But I really think
> not dragging dentry stashing into struct ima_namespace is the correct
> way to go about this.

We, unfortunately, do have one case we can't avoid stashing for the
policy file.  It's this code in ima_release_policy:

> #if !defined(CONFIG_IMA_WRITE_POLICY) &&
> !defined(CONFIG_IMA_READ_POLICY)
> 	securityfs_remove(ns->dentry[IMAFS_DENTRY_IMA_POLICY]);
> 	ns->dentry[IMAFS_DENTRY_IMA_POLICY] = NULL;
> 

What it does is that in certain config options, the policy file entry
gets removed from the securityfs ima directory after you write to it.

James