linux-kernel - Re: [PATCH v9 04/23] securityfs: Extend securityfs with namespacing support

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Date:   Thu, 27 Jan 2022 11:53:51 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        linux-integrity@...r.kernel.org
Cc:     serge@...lyn.com, christian.brauner@...ntu.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, jejb@...ux.ibm.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,
        Stefan Berger <stefanb@...ux.ibm.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v9 04/23] securityfs: Extend securityfs with namespacing
 support

On Tue, 2022-01-25 at 17:46 -0500, Stefan Berger wrote:
> From: Stefan Berger <stefanb@...ux.ibm.com>
> 
> Enable multiple instances of securityfs by keying each instance with a
> pointer to the user namespace it belongs to.
> 
> Since we do not need the pinning of the filesystem for the virtualization
> case, limit the usage of simple_pin_fs() and simpe_release_fs() to the
> case when the init_user_ns is active. This simplifies the cleanup for the
> virtualization case where usage of securityfs_remove() to free dentries
> is not needed anymore.

Could you add a sentence here explaining why securityfs_remove() isn't
needed in the virtualization case?

> 
> For the initial securityfs, i.e. the one mounted in the host userns mount,
> nothing changes. The rules for securityfs_remove() are as before and it is
> still paired with securityfs_create(). Specifically, a file created via
> securityfs_create_dentry() in the initial securityfs mount still needs to
> be removed by a call to securityfs_remove(). Creating a new dentry in the
> initial securityfs mount still pins the filesystem like it always did.
> Consequently, the initial securityfs mount is not destroyed on
> umount/shutdown as long as at least one user of it still has dentries that
> it hasn't removed with a call to securityfs_remove().
> 
> Prevent mounting of an instance of securityfs in another user namespace
> than it belongs to. Also, prevent accesses to files and directories by
> a user namespace that is neither the user namespace it belongs to
> nor an ancestor of the user namespace that the instance of securityfs
> belongs to. Do not prevent access if securityfs was bind-mounted and
> therefore the init_user_ns is the owning user namespace.
> 
> Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
> Signed-off-by: James Bottomley <James.Bottomley@...senPartnership.com>
> Suggested-by: Christian Brauner <brauner@...nel.org>

Really nice!   A minor nit is that "Suggested-by" should go before your
"Signed-off-by".

Reviewed-by: Mimi Zohar <zohar@...ux.ibm.com>