Re: [PATCH v3 00/16] ima: Namespace IMA with audit support in IMA-ns

[Casey Schaufler]

On 12/7/2021 7:40 AM, James Bottomley wrote:
> On Tue, 2021-12-07 at 10:16 -0500, James Bottomley wrote:
> > On Tue, 2021-12-07 at 15:59 +0100, Christian Brauner wrote:
> > > On Mon, Dec 06, 2021 at 04:14:15PM -0500, James Bottomley wrote:
> [...]
> > > >   static int securityfs_fill_super(struct super_block *sb, struct
> > > > fs_context *fc)
> > > >   {
> > > >   	static const struct tree_descr files[] = {{""}};
> > > >   	int error;
> > > > +	struct user_namespace *ns = fc->user_ns;
> > > >   
> > > >   	error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
> > > >   	if (error)
> > > >   		return error;
> > > >   
> > > > +	ns->securityfs_root = dget(sb->s_root);
> > > > +
> > > >   	sb->s_op = &securityfs_super_operations;
> > > >   
> > > > +	if (ns != &init_user_ns)
> > > > +		blocking_notifier_call_chain(&securityfs_ns_notifier,
> > > > +					     SECURITYFS_NS_ADD, ns);
> > > I would propose not to use the notifier logic. While it might be
> > > nifty it's over-engineered in my opinion.
> > The reason for a notifier is that this current patch set only
> > namespaces ima, but we also have integrity and evm to do.  Plus, as
> > Casey said, we might get apparmour and selinux.  Since each of those
> > will also want to add entries in fill_super, the notifier mechanism
> > seemed fairly tailor made for this.  The alternative is to have a
> > load of
> >
> > #if CONFIG_securityfeature
> > callback()
> > #endif
> >
> > Inside securityfs_fill_super which is a bit inelegant.
> >
> > >   The dentry stashing in struct user_namespace currently serves the
> > > purpose to make it retrievable in ima_fs_ns_init(). That doesn't
> > > justify its existence imho.
> > I can thread the root as part of the callback.  I think I can still
> > use the standard securityfs calls because the only reason for the
> > dentry in the namespace is so the callee can pass NULL and have the
> > dentry created at the top level.  We can insist in the namespaced use
> > case that the callee always pass in the dentry, even for the top
> > level.
> >
> > > There is one central place were all users of namespaced securityfs
> > > can create the files that they need to and that is in
> > > securityfs_fill_super(). (If you want to make that more obvious
> > > then give it a subdirectory securityfs and move inode.c in there.)
> > Right, that's what the patch does.
> >
> > > We simply will expect users to add:
> > >
> > > ima_init_securityfs()
> > > mylsm_init_securityfs()
> > Yes, plus all the #ifdefs because securityfs can exist independently
> > of each of the features.  We can hide the ifdefs in the header files
> > and make the functions static do nothing if not defined, but the
> > ifdeffery has to live somewhere.
> Actually, I've got a much better reason: securityfs is a bool; all the
> other LSMs and IMA are tristates.  We can't call module init functions
> from core code, it has to be done by something like a notifier.

Err, no. LSMs are not available as loadable modules.