[RFC v2 19/19] ima: Setup securityfs for IMA namespace

[James Bottomley]

On Thu, 2021-12-02 at 21:31 -0500, Stefan Berger wrote:
[...]
>  static int securityfs_init_fs_context(struct fs_context *fc)
>  {
> +	int rc;
> +
> +	if (fc->user_ns->ima_ns->late_fs_init) {
> +		rc = fc->user_ns->ima_ns->late_fs_init(fc->user_ns);
> +		if (rc)
> +			return rc;
> +	}
>  	fc->ops = &securityfs_context_ops;
>  	return 0;
>  }

I know I suggested this, but to get this to work in general, it's going
to have to not be specific to IMA, so it's going to have to become
something generic like a notifier chain.  The other problem is it's
only working still by accident:

> +int ima_fs_ns_init(struct ima_namespace *ns)
> +{
> +	ns->mount = securityfs_ns_create_mount(ns->user_ns);

This actually triggers on the call to securityfs_init_fs_context, but
nothing happens because the callback is null.  Every subsequent use of
fscontext will trigger this.  The point of a keyed supeblock is that
fill_super is only called once per key, that's the place we should be
doing this.   It should also probably be a blocking notifier so any
consumer of securityfs can be namespaced by registering for this
notifier.

> +	if (IS_ERR(ns->mount)) {
> +		ns->mount = NULL;
> +		return -1;
> +	}
> +	ns->mount_count = 1;

This is a bit nasty, too: we're spilling the guts of mount count
tracking into IMA instead of encapsulating it inside securityfs.

> +
> +	/* Adjust the trigger for user namespace's early teardown of
> dependent
> +	 * namespaces. Due to the filesystem there's an additional
> reference
> +	 * to the user namespace.
> +	 */
> +	ns->user_ns->refcount_teardown += 1;
> +
> +	ns->late_fs_init = ima_fs_ns_late_init;
> +
> +	return 0;
> +}

I think what should be happening is that we shouldn't so the
simple_pin_fs, which creates the inodes, ahead of time; we should do it
inside fill_super using a notifier, meaning it gets called once per
key, creates the root dentry then triggers the notifier which
instantiates all the namespaced entries.  We can still use
simple_pin_fs for this because there's no locking across fill_super. 
This would mean fill_super would be called the first time the
securityfs is mounted inside the namespace.

If we do it this way, we can now make securityfs have its own mount and
mount_count inside the user namespace, which it uses internally to the
securityfs code, thus avoiding exposing them to ima or any other
namespaced consumer.

I also think we now don't need the securityfs_ns_ duplicated functions
because the callback via the notifier chain now ensures we can use the
namespace they were created in to distinguish between non namespaced
and namespaced entries.

So non-namespaced consumers of securityfs would do what they do now
(calling the securityfs_create on initialization) and namespaced
consumers would register a callback on the notifier which would get
called once for every namespace the securityfs gets mounted in.

I also theorize if we do it with notifiers, we could have a notifier on
kill_sb to tear down all the entires.  If we do this, I think we don't
have to pin any more.

James