linux-kernel - Re: [RFC PATCH 2/5] ima: Add ns_status for storing namespaced iint data

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Message-Id: <1501013725.27413.27.camel@linux.vnet.ibm.com>
Date:   Tue, 25 Jul 2017 16:15:25 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>,
        Mehmet Kayaalp <mkayaalp@...ux.vnet.ibm.com>
Cc:     ima-devel <linux-ima-devel@...ts.sourceforge.net>,
        containers <containers@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Tycho Andersen <tycho@...ker.com>,
        Yuqiong Sun <sunyuqiong1988@...il.com>,
        David Safford <david.safford@...com>,
        Mehmet Kayaalp <mkayaalp@...binghamton.edu>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 2/5] ima: Add ns_status for storing namespaced iint
 data

On Tue, 2017-07-25 at 14:43 -0500, Serge E. Hallyn wrote:
> ...
> > +static void free_ns_status_cache(struct ima_namespace *ns)
> > +{
> > +	struct ns_status *status, *next;
> > +
> > +	write_lock(&ns->ns_status_lock);
> > +	rbtree_postorder_for_each_entry_safe(status, next,
> > +					     &ns->ns_status_tree, rb_node)
> > +		kmem_cache_free(ns->ns_status_cache, status);
> > +	ns->ns_status_tree = RB_ROOT;
> > +	write_unlock(&ns->ns_status_lock);
> > +	kmem_cache_destroy(ns->ns_status_cache);
> > +}
> > +
> >  static void destroy_ima_ns(struct ima_namespace *ns)
> >  {
> >  	put_user_ns(ns->user_ns);
> >  	ns_free_inum(&ns->ns);
> > +	free_ns_status_cache(ns);
> >  	kfree(ns);
> >  }
> >  
> > @@ -181,3 +198,106 @@ struct ima_namespace init_ima_ns = {
> >  	.parent = NULL,
> >  };
> >  EXPORT_SYMBOL(init_ima_ns);
> > +
> > +/*
> > + * __ima_ns_status_find - return the ns_status associated with an inode
> > + */
> > +static struct ns_status *__ima_ns_status_find(struct ima_namespace *ns,
> > +					      struct inode *inode)
> > +{
> > +	struct ns_status *status;
> > +	struct rb_node *n = ns->ns_status_tree.rb_node;
> > +
> > +	while (n) {
> > +		status = rb_entry(n, struct ns_status, rb_node);
> > +
> > +		if (inode < status->inode)
> > +			n = n->rb_left;
> > +		else if (inode->i_ino > status->i_ino)
> > +			n = n->rb_right;
> > +		else
> > +			break;
> > +	}
> > +	if (!n)
> > +		return NULL;
> > +
> > +	return status;
> > +}
> > +
> > +/*
> > + * ima_ns_status_find - return the ns_status associated with an inode
> > + */
> > +static struct ns_status *ima_ns_status_find(struct ima_namespace *ns,
> > +					    struct inode *inode)
> > +{
> > +	struct ns_status *status;
> > +
> > +	read_lock(&ns->ns_status_lock);
> > +	status = __ima_ns_status_find(ns, inode);
> > +	read_unlock(&ns->ns_status_lock);
> > +
> > +	return status;
> > +}
> ...
> > +
> > +struct ns_status *ima_get_ns_status(struct ima_namespace *ns,
> > +				    struct inode *inode)
> > +{
> > +	struct ns_status *status;
> > +	int skip_insert = 0;
> > +
> > +	status = ima_ns_status_find(ns, inode);
> > +	if (status) {
> > +		/*
> > +		 * Unlike integrity_iint_cache we are not free'ing the
> > +		 * ns_status data when the inode is free'd. So, in addition to
> > +		 * checking the inode pointer, we need to make sure the
> > +		 * (i_generation, i_ino) pair matches as well. In the future
> > +		 * we might want to add support for lazily walking the rbtree
> > +		 * to clean it up.
> > +		 */
> > +		if (inode->i_ino == status->i_ino &&
> > +		    inode->i_generation == status->i_generation)
> > +			return status;
> > +
> > +		/* Same inode number is reused, overwrite the ns_status */
> > +		skip_insert = 1;
> > +	} else {
> > +		status = kmem_cache_alloc(ns->ns_status_cache, GFP_NOFS);
> > +		if (!status)
> > +			return ERR_PTR(-ENOMEM);
> > +	}
> 
> What prevents the status from being freed between the read_lock
> in ima_ns_status_find() and the write_lock in the following line?
> 
> IIUC it's that ns is always current's ima_ns, which will pin the ns
> and cause no statuses to be freed.  But then the ns should probably
> not be passed in here?  Or a comment should say that ns must be
> pinned?
> 
> Just trying to make sure I understand the locking.

iint's are only freed after the last reference to the inode is deleted
in __fput().  Refer to ima_file_free().  ns_status is a bit different
in that they are freed on namespace cleanup.

Mimi

> > +	write_lock(&ns->ns_status_lock);
> > +
> > +	if (!skip_insert)
> > +		insert_ns_status(ns, inode, status);
> > +
> > +	status->inode = inode;
> > +	status->i_ino = inode->i_ino;
> > +	status->i_generation = inode->i_generation;
> > +	status->flags = 0UL;
> > +	write_unlock(&ns->ns_status_lock);
> > +
> > +	return status;
> > +}
> > -- 
> > 2.9.
>