[PATCH v9 04/23] securityfs: Extend securityfs with namespacing support

[Mimi Zohar]

On Tue, 2022-01-25 at 17:46 -0500, Stefan Berger wrote:
> From: Stefan Berger <stefanb at linux.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 at linux.ibm.com>
> Signed-off-by: James Bottomley <James.Bottomley at HansenPartnership.com>
> Suggested-by: Christian Brauner <brauner at kernel.org>

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

Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>