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

[James Bottomley <jejb@linux.ibm.com>]

On Tue, 2021-12-07 at 07:48 -0800, Casey Schaufler wrote:
> 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_not
> > > > > ifier,
> > > > > +					     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.

Well securityfs has EXPORT_MODULE_GPL() across all its dentry creation
functions ... that does mean it expects to be called by a module.

However, it does appear to be it's only TPM that may use it as a module
... this is still going to cause a problem eventually because now we'll
have to require some of the TPM code be built in once we want to attach
vTPMs to containers.

James