Quoting sukadev@us.ibm.com (sukadev@us.ibm.com):
> 
> From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Subject: [PATCH 6/7]: Check for user-space mount of /dev/pts
> 
> When the pts namespace is cloned, the /dev/pts is not useful unless it
> is remounted from the user space.
> 
> If user-space clones pts namespace but does not remount /dev/pts, it
> would end up using the /dev/pts mount from parent-pts-ns but allocate
> the pts indices from current pts ns.

So why not use the allocated_ptys from the parent ptsns?  It's what
userspace asked for and it's safe to do.

> This patch (hack ?) prevents creation of PTYs in user space unless
> user-space mounts /dev/pts.
> 
> (While this patch can be folded into others, keeping this separate
> for now for easier review (and to highlight the hack :-)
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> ---
>  fs/devpts/inode.c         |   25 +++++++++++++++++++++++--
>  include/linux/devpts_fs.h |   20 +++++++++++++++++++-
>  2 files changed, 42 insertions(+), 3 deletions(-)
> 
> Index: 2.6.25-rc5-mm1/include/linux/devpts_fs.h
> ===================================================================
> --- 2.6.25-rc5-mm1.orig/include/linux/devpts_fs.h	2008-03-24 20:08:33.000000000 -0700
> +++ 2.6.25-rc5-mm1/include/linux/devpts_fs.h	2008-03-24 20:08:57.000000000 -0700
> @@ -23,6 +23,7 @@ struct pts_namespace {
>  	struct kref kref;
>  	struct idr allocated_ptys;
>  	struct vfsmount *mnt;
> +	int user_mounted;
>  };
> 
>  extern struct pts_namespace init_pts_ns;
> @@ -30,6 +31,8 @@ extern struct pts_namespace init_pts_ns;
>  #define DEVPTS_SUPER_MAGIC 	0x1cd1
>  static inline struct pts_namespace *pts_ns_from_inode(struct inode *inode)
>  {
> +	struct pts_namespace *ns;
> +
>  	/*
>  	 * Need this bug-on for now to catch any cases in tty_open()
>  	 * or release_dev() I may have missed.
> @@ -43,7 +46,22 @@ static inline struct pts_namespace *pts_
>  	 * should not need a lock here.
>  	 */
> 
> -	return (struct pts_namespace *)inode->i_sb->s_fs_info;
> +	ns = (struct pts_namespace *)inode->i_sb->s_fs_info;
> +
> +	/*
> +	 * If user-space did not mount pts ns after cloning pts namespace,
> +	 * the child process would end up accessing devpts mount of the
> +	 * parent but use allocated_ptys from the cloned pts ns.
> +	 *
> +	 * This check prevents creating ptys unless user-space mounts
> +	 * devpts  in the new pts namespace.
> +	 *
> +	 * Is there a cleaner way to prevent this ?
> +	 */
> +	if (!ns->user_mounted)
> +		return NULL;
> +
> +	return ns;
>  }
> 
>  static inline struct pts_namespace *current_pts_ns(void)
> Index: 2.6.25-rc5-mm1/fs/devpts/inode.c
> ===================================================================
> --- 2.6.25-rc5-mm1.orig/fs/devpts/inode.c	2008-03-24 20:08:33.000000000 -0700
> +++ 2.6.25-rc5-mm1/fs/devpts/inode.c	2008-03-24 20:08:57.000000000 -0700
> @@ -201,8 +201,11 @@ static int devpts_get_sb(struct file_sys
>  	if (IS_ERR(sb))
>  		return PTR_ERR(sb);
> 
> -	if (sb->s_root)
> +	if (sb->s_root) {
> +		if (!(flags & MS_KERNMOUNT))
> +			ns->user_mounted = 1;
>  		return simple_set_mnt(mnt, sb);
> +	}
> 
>  	sb->s_flags = flags;
>  	err = devpts_fill_super(sb, data, flags & MS_SILENT ? 1 : 0);
> @@ -248,6 +251,10 @@ int devpts_new_index(struct pts_namespac
>  	int index;
>  	int idr_ret;
> 
> +	if (!pts_ns || !pts_ns->user_mounted) {
> +		printk(KERN_ERR "devpts_new_index() without user_mount\n");
> +		return -ENOSYS;
> +	}
>  retry:
>  	if (!idr_pre_get(&pts_ns->allocated_ptys, GFP_KERNEL)) {
>  		return -ENOMEM;
> @@ -273,7 +280,7 @@ retry:
> 
>  void devpts_kill_index(struct pts_namespace *pts_ns, int idx)
>  {
> -
> +	BUG_ON(!pts_ns->user_mounted);
>  	down(&allocated_ptys_lock);
>  	idr_remove(&pts_ns->allocated_ptys, idx);
>  	up(&allocated_ptys_lock);
> @@ -293,6 +300,11 @@ int devpts_pty_new( struct pts_namespace
>  	BUG_ON(driver->type != TTY_DRIVER_TYPE_PTY);
>  	BUG_ON(driver->subtype != PTY_TYPE_SLAVE);
> 
> +	if (!pts_ns || !pts_ns->user_mounted) {
> +		printk(KERN_ERR "devpts_pty_new() without user_mount\n");
> +		return -ENOSYS;
> +	}
> +
>  	mnt = pts_ns->mnt;
>  	root = mnt->mnt_root;
> 
> @@ -332,6 +344,11 @@ struct tty_struct *devpts_get_tty(struct
>  	struct dentry *dentry;
>  	struct tty_struct *tty;
> 
> +	if (!pts_ns || !pts_ns->user_mounted) {
> +		printk(KERN_ERR "devpts_get_tty() without user_mount\n");
> +		return ERR_PTR(-ENOSYS);
> +	}
> +
>  	mnt = pts_ns->mnt;
> 
>  	dentry = get_node(mnt->mnt_root, number);
> @@ -353,6 +370,10 @@ void devpts_pty_kill(struct pts_namespac
>  	struct dentry *dentry;
>  	struct dentry *root;
> 
> +	if (!pts_ns || !pts_ns->user_mounted) {
> +		printk(KERN_ERR "devpts_pty_kill() without user_mount\n");
> +		BUG_ON(1);
> +	}
>  	root = pts_ns->mnt->mnt_root;
> 
>  	dentry = get_node(root, number);
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