Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

[James Bottomley]

On Thu, 2021-12-09 at 10:30 -0500, James Bottomley wrote:
> On Thu, 2021-12-09 at 15:37 +0100, Christian Brauner wrote:
> > On Thu, Dec 09, 2021 at 03:34:28PM +0100, Christian Brauner wrote:
> > > On Wed, Dec 08, 2021 at 05:18:17PM -0500, Stefan Berger wrote:
> > > > Move the dentries into the ima_namespace for reuse by
> > > > virtualized
> > > > SecurityFS. Implement function freeing the dentries in order of
> > > > files and symlinks before directories.
> > > > 
> > > > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > > > ---
> > > 
> > > This doesn't work as implemented, I think.
> > > 
> > > What I would have preferred and what I tried to explain in the
> > > earlier review was:
> > > Keep the dentry stashing global since it is only needed for
> > > init_ima_ns.
> > > Then struct ima_namespace becomes way smaller and simpler.
> > > If you do that then it makes sense to remove the additional
> > > dget() in securityfs_create_dentry() for non-init_ima_ns.
> > > Then you can rely on auto-cleanup in .kill_sb() or on
> > > ima_securityfs_init() failure and you only need to call
> > > ima_fs_ns_free_dentries() if ns != init_ima_ns.
> > > 
> > > IIuc, it seems you're currently doing one dput() too many since
> > > you're calling securityfs_remove() in the error path for non-
> > > init_ima_ns which relies on the previous increased dget() which
> > > we removed.
> > 
> > If you really want to move the dentry stashing into struct
> > ima_namespace even though it's really unnecessary then you may as
> > well not care about the auto-cleanup and keep that additional
> > ima_fs_ns_free_dentries(ns) call in .kill_sb(). But I really think
> > not dragging dentry stashing into struct ima_namespace is the
> > correct way to go about this.
> 
> We, unfortunately, do have one case we can't avoid stashing for the
> policy file.  It's this code in ima_release_policy:
> 
> > #if !defined(CONFIG_IMA_WRITE_POLICY) &&
> > !defined(CONFIG_IMA_READ_POLICY)
> > 	securityfs_remove(ns->dentry[IMAFS_DENTRY_IMA_POLICY]);
> > 	ns->dentry[IMAFS_DENTRY_IMA_POLICY] = NULL;
> > 
> 
> What it does is that in certain config options, the policy file entry
> gets removed from the securityfs ima directory after you write to it.

This is what I have incremental to v5 that corrects all of this.  It
actually keeps every dentry reference (including init_user_ns ones) at
1 so they can be reaped on unmount.  For the remove case it does
d_delete and then puts the only reference.  This means
securityfs_remove() works for the namespaced policy file as well.

I also got rid of the spurious initialized check in ima_securityfs_init
because it prevents you doing a mount;umount;mount on securityfs within
a namespace.

There's still the problem that if you write the policy, making the file
disappear then unmount and remount securityfs it will come back.  My
guess for fixing this is that we only stash the policy file reference,
create it if NULL but then set the pointer to PTR_ERR(-EINVAL) or
something and refuse to create it for that value.

James

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