[Devel] Re: [PATCH 05/10] Containers(V10): Add container_clone() interface

[Andrew Morton]

On Tue, 29 May 2007 06:01:09 -0700 menage at google.com wrote:

> This patch adds support for container_clone(), a speculative interface
> to creating new containers intended to be used for systems such as
> namespace unsharing.
> 
> ...
>
> +
> +static atomic_t namecnt;
> +static void get_unused_name(char *buf)
> +{
> +	sprintf(buf, "node%d", atomic_inc_return(&namecnt));
> +}

A stupid thing, but a sufficiently determined attacker could cause this to
wrap.

> +/**
> + * container_clone - duplicate the current container in the hierarchy
> + * that the given subsystem is attached to, and move this task into
> + * the new child
> + */
> +int container_clone(struct task_struct *tsk, struct container_subsys *subsys)
> +{
> +	struct dentry *dentry;
> +	int ret = 0;
> +	char nodename[32];
> +	struct container *parent, *child;
> +	struct inode *inode;
> +	struct css_group *cg;
> +	struct containerfs_root *root;
> +
> +	/* We shouldn't be called by an unregistered subsystem */
> +	BUG_ON(!subsys->active);
> +
> +	/* First figure out what hierarchy and container we're dealing
> +	 * with, and pin them so we can drop container_mutex */
> +	mutex_lock(&container_mutex);
> + again:
> +	root = subsys->root;
> +	if (root == &rootnode) {
> +		printk(KERN_INFO
> +		       "Not cloning container for unused subsystem %s\n",
> +		       subsys->name);
> +		mutex_unlock(&container_mutex);
> +		return 0;
> +	}
> +	cg = &tsk->containers;
> +	parent = task_container(tsk, subsys->subsys_id);
> +	/* Pin the hierarchy */
> +	atomic_inc(&parent->root->sb->s_active);
> +
> +	mutex_unlock(&container_mutex);
> +
> +	/* Now do the VFS work to create a container */
> +	get_unused_name(nodename);
> +	inode = parent->dentry->d_inode;
> +
> +	/* Hold the parent directory mutex across this operation to
> +	 * stop anyone else deleting the new container */
> +	mutex_lock(&inode->i_mutex);
> +	dentry = container_get_dentry(parent->dentry, nodename);
> +	if (IS_ERR(dentry)) {
> +		printk(KERN_INFO
> +		       "Couldn't allocate dentry for %s: %ld\n", nodename,
> +		       PTR_ERR(dentry));
> +		ret = PTR_ERR(dentry);
> +		goto out_release;

Which I guess could cause a failure here.

Perhaps we could go back and rerun the get_unused_name() if EEXIST, if
we're bothered.

I hope this is the biggest problem ;)

> +	}
> +
> +	/* Create the container directory, which also creates the container */
> +	ret = vfs_mkdir(inode, dentry, S_IFDIR | 0755);
> +	child = __d_cont(dentry);
> +	dput(dentry);
> +	if (ret) {
> +		printk(KERN_INFO
> +		       "Failed to create container %s: %d\n", nodename,
> +		       ret);
> +		goto out_release;
> +	}
> +
> +	if (!child) {
> +		printk(KERN_INFO
> +		       "Couldn't find new container %s\n", nodename);
> +		ret = -ENOMEM;
> +		goto out_release;
> +	}
> +
> +	/* The container now exists. Retake container_mutex and check
> +	 * that we're still in the same state that we thought we
> +	 * were. */
> +	mutex_lock(&container_mutex);
> +	if ((root != subsys->root) ||
> +	    (parent != task_container(tsk, subsys->subsys_id))) {
> +		/* Aargh, we raced ... */
> +		mutex_unlock(&inode->i_mutex);
> +
> +		deactivate_super(parent->root->sb);
> +		/* The container is still accessible in the VFS, but
> +		 * we're not going to try to rmdir() it at this
> +		 * point. */
> +		printk(KERN_INFO
> +		       "Race in container_clone() - leaking container %s\n",
> +		       nodename);
> +		goto again;
> +	}
> +
> +	/* All seems fine. Finish by moving the task into the new container */
> +	ret = attach_task(child, tsk);
> +	mutex_unlock(&container_mutex);
> +
> + out_release:
> +	mutex_unlock(&inode->i_mutex);
> +	deactivate_super(parent->root->sb);
> +	return ret;
> +}