linux-kernel - Re: [PATCH v4 16/16] ima: Setup securityfs for IMA namespace

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Message-ID: <20211208144634.rqwn3ccizrbzdq52@wittgenstein>
Date:   Wed, 8 Dec 2021 15:46:34 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Stefan Berger <stefanb@...ux.ibm.com>,
        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 v4 16/16] ima: Setup securityfs for IMA namespace

On Wed, Dec 08, 2021 at 09:11:09AM -0500, James Bottomley wrote:
> On Wed, 2021-12-08 at 13:58 +0100, Christian Brauner wrote:
> > On Tue, Dec 07, 2021 at 03:21:27PM -0500, Stefan Berger wrote:
> [...]
> > > @@ -69,6 +74,11 @@ static int securityfs_init_fs_context(struct
> > > fs_context *fc)
> > >  
> > >  static void securityfs_kill_super(struct super_block *sb)
> > >  {
> > > +	struct user_namespace *ns = sb->s_fs_info;
> > > +
> > > +	if (ns != &init_user_ns)
> > > +		ima_fs_ns_free_dentries(ns);
> > 
> > Say securityfs is unmounted. Then all the inodes and dentries become
> > invalid. It's not allowed to hold on to any dentries or inodes after
> > the super_block is shut down. So I just want to be sure that nothing
> > in ima can access these dentries after securityfs is unmounted.
> > 
> > To put it another way: why are they stored in struct ima_namespace in
> > the first place? If you don't pin a filesystem when creating files or
> > directories like you do for securityfs in init_ima_ns then you don't
> > need to hold on to them as they will be automatically be wiped during
> > umount.
> 
> For IMA this is true because IMA can't be a module.  However, a modular

This thread is about ima and its stashing of dentries in struct
ima_namespace. That things might be different for other consumers is
uninteresting for this specific case, I think.

> consumer, like the TPM, must be able to remove its entries from a
> mounted securityfs because the code that serves the operations is going
> away.  In order to do this removal, it needs the dentries somewhere. 

That still doesn't require you to take an additional reference on the
dentry per se.
Aside from this brings in a whole different and way bigger issue as that
requires way more fundamental work since this is about a (pseudo or
proper) device. It's not even clear that this should have entries
outside of init_user_ns-securityfs.

> The current convention seems to be everything has a directory in the
> top level, so we could call d_genocide() on this directory and not have
> to worry about storing the dentries underneath, but I think we can't
> avoid storing the dentry for the top level directory.

I have not heard an argument why ima needs to stash these dentries as it
doesn't remove them once created until umount.