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

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Message-Id: <93604a2b-cee3-7506-bf8d-ebcddac64e82@linux.vnet.ibm.com>
Date:   Tue, 25 Jul 2017 16:25:08 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        "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>
Subject: Re: [RFC PATCH 2/5] ima: Add ns_status for storing namespaced iint
 data

On 07/25/2017 04:15 PM, Mimi Zohar wrote:
> 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.

It should be possible to move the write_lock() above the

status = ima_ns_status_find(ns, inode);


and instead call __ima_ns_status_find() with the write_lock() held.

     Stefan


>
> 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.