linux-kernel - Re: [PATCH 23/34] union-mount: Prevent topmost file system from being mounted elsewhere

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Date:	Thu, 30 Sep 2010 11:37:48 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	viro@...iv.linux.org.uk, miklos@...redi.hu, hch@...radead.org,
	agruen@...e.de, npiggin@...nel.dk, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, vaurora@...hat.com
Subject: Re: [PATCH 23/34] union-mount: Prevent topmost file system from being mounted elsewhere

On Thu, 16 Sep 2010, Valerie Aurora wrote:
> The device underlying the topmost read-write layer of a file system
> cannot be mounted anywhere else on the system.  We keep a pointer to
> the union stack in the dentry of the topmost directory, so that dentry
> can't be part of a different mount, since dentries are shared between
> different mounts of the same device.
> 
> Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> ---
>  fs/namespace.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 61256e6..26efaf3 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1998,6 +1998,11 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
>  	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
>  		goto unlock;
>  
> +	/* Top layers of union mounts can't be mounted elsewhere */
> +	err = -EBUSY;
> +	if (newmnt->mnt_sb->s_union_lower_mnts)
> +		goto unlock;
> +

This is insufficient: the super block may be mounted elsewhere later.
And no, preventing bind mounts is not enough.

BTW, what about CLONE_NEWNS?  I think it's a rather big limitation if
that doesn't work...

Thanks,
Miklos
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