On 21/09/2018 17:30, David Howells wrote:
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---
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
#
