linux-kernel - Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE

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Date:   Tue, 23 Oct 2018 12:19:35 +0100
From:   Alan Jenkins <alan.christopher.jenkins@...il.com>
To:     David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk
Cc:     torvalds@...ux-foundation.org, ebiederm@...ssion.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        mszeredi@...hat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE
 [ver #12]

On 21/09/2018 17:30, David Howells wrote:
> From: Al Viro <viro@...iv.linux.org.uk>
> 
> Allow a detached tree created by open_tree(..., OPEN_TREE_CLONE) to be
> attached by move_mount(2).
> 
> If by the time of final fput() of OPEN_TREE_CLONE-opened file its tree is
> not detached anymore, it won't be dissolved.  move_mount(2) is adjusted
> to handle detached source.
> 
> That gives us equivalents of mount --bind and mount --rbind.
> 
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
> 
>   fs/namespace.c |   26 ++++++++++++++++++++------
>   1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index dd38141b1723..caf5c55ef555 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1785,8 +1785,10 @@ void dissolve_on_fput(struct vfsmount *mnt)
>   {
>   	namespace_lock();
>   	lock_mount_hash();
> -	mntget(mnt);
> -	umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
> +	if (!real_mount(mnt)->mnt_ns) {
> +		mntget(mnt);
> +		umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
> +	}
>   	unlock_mount_hash();
>   	namespace_unlock();
>   }
> @@ -2393,6 +2395,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	struct mount *old;
>   	struct mountpoint *mp;
>   	int err;
> +	bool attached;
>   
>   	mp = lock_mount(new_path);
>   	err = PTR_ERR(mp);
> @@ -2403,10 +2406,19 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	p = real_mount(new_path->mnt);
>   
>   	err = -EINVAL;
> -	if (!check_mnt(p) || !check_mnt(old))
> +	/* The mountpoint must be in our namespace. */
> +	if (!check_mnt(p))
> +		goto out1;
> +	/* The thing moved should be either ours or completely unattached. */
> +	if (old->mnt_ns && !check_mnt(old))
>   		goto out1;
>   
> -	if (!mnt_has_parent(old))
> +	attached = mnt_has_parent(old);
> +	/*
> +	 * We need to allow open_tree(OPEN_TREE_CLONE) followed by
> +	 * move_mount(), but mustn't allow "/" to be moved.
> +	 */
> +	if (old->mnt_ns && !attached)
>   		goto out1;
>   
>   	if (old->mnt.mnt_flags & MNT_LOCKED)
> @@ -2421,7 +2433,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	/*
>   	 * Don't move a mount residing in a shared parent.
>   	 */
> -	if (IS_MNT_SHARED(old->mnt_parent))
> +	if (attached && IS_MNT_SHARED(old->mnt_parent))
>   		goto out1;
>   	/*
>   	 * Don't move a mount tree containing unbindable mounts to a destination
> @@ -2435,7 +2447,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   			goto out1;
>   
>   	err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
> -				   &parent_path);
> +				   attached ? &parent_path : NULL);
>   	if (err)
>   		goto out1;
>   

I think there's another small hole.  It is possible to move a sub-mount 
from a detached tree (instead of moving the root of the tree).  Then 
do_move_mount() calls attach_recursive_mnt() with a non-NULL parent_path.

This causes it to skip count_mounts().  So, it doesn't count the number 
of attached mounts, and it allows you to exceed sysctl_mount_max.

Regards
Alan

(I've tested to confirm the code lets you move a sub-mount.  I didn't 
test whether it allows exceeding sysctl_mount_max.

# unshare -m --propagation private
# mkdir -p /tmp/mnt
# mount --bind /tmp/mnt /tmp/mnt
# open_tree_clone 3</tmp 3 sh
# cd /proc/self/fd/3
# mount --move mnt /mnt
# exit
exit
# exit
logout
#