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

From: Christian Brauner <christian.brauner@ubuntu.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>,
	linux-integrity@vger.kernel.org, zohar@linux.ibm.com,
	serge@hallyn.com, containers@lists.linux.dev,
	dmitry.kasatkin@gmail.com, ebiederm@xmission.com,
	krzysztof.struczynski@huawei.com, roberto.sassu@huawei.com,
	mpeters@redhat.com, lhinds@redhat.com, lsturman@redhat.com,
	puiterwi@redhat.com, jamjoom@us.ibm.com,
	linux-kernel@vger.kernel.org, paul@paul-moore.com,
	rgb@redhat.com, linux-security-module@vger.kernel.org,
	jmorris@namei.org
Subject: Re: [PATCH v4 16/16] ima: Setup securityfs for IMA namespace
Date: Wed, 8 Dec 2021 15:46:34 +0100	[thread overview]
Message-ID: <20211208144634.rqwn3ccizrbzdq52@wittgenstein> (raw)
In-Reply-To: <dd43783ae76ad3238d99f75d8aaf95e20ad28b79.camel@HansenPartnership.com>

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.