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SubjectRe: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
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On 21/09/2018 17:30, David Howells wrote:
> From: Al Viro <viro@zeniv.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@zeniv.linux.org.uk>
> Signed-off-by: David Howells <dhowells@redhat.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
#

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