fs/namespace: fix unprivileged mount propagation

When propagating mounts across mount namespaces owned by different user
namespaces it is not possible anymore to move or umount the mount in the
less privileged mount namespace.

Here is a reproducer:

  sudo mount -t tmpfs tmpfs /mnt
  sudo --make-rshared /mnt

  # create unprivileged user + mount namespace and preserve propagation
  unshare -U -m --map-root --propagation=unchanged

  # now change back to the original mount namespace in another terminal:
  sudo mkdir /mnt/aaa
  sudo mount -t tmpfs tmpfs /mnt/aaa

  # now in the unprivileged user + mount namespace
  mount --move /mnt/aaa /opt

Unfortunately, this is a pretty big deal for userspace since this is
e.g. used to inject mounts into running unprivileged containers.
So this regression really needs to go away rather quickly.

The problem is that a recent change falsely locked the root of the newly
added mounts by setting MNT_LOCKED. Fix this by only locking the mounts
on copy_mnt_ns() and not when adding a new mount.

Fixes: 3bd045cc9c4b ("separate copying and locking mount tree on cross-userns copies")
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Tested-by: Christian Brauner <christian@brauner.io>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 5dc137a22d406c0f20d276adb7895dbb602f3546..7660c2749c96f075c721b701e0ee4839fe62cb82 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2105,6 +2105,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
                /* Notice when we are propagating across user namespaces */
                if (child->mnt_parent->mnt_ns->user_ns != user_ns)
                        lock_mnt_tree(child);
+               child->mnt.mnt_flags &= ~MNT_LOCKED;
                commit_tree(child);
        }
        put_mountpoint(smp);

diff --git a/fs/pnode.c b/fs/pnode.c
index 595857a1883e1a68a207fb32beef9c20a8bcd0e2..49f6d7ff21394f5e15e440ade00ba3363e27cf68 100644 (file)
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -261,7 +261,6 @@ static int propagate_one(struct mount *m)
        child = copy_tree(last_source, last_source->mnt.mnt_root, type);
        if (IS_ERR(child))
                return PTR_ERR(child);
-       child->mnt.mnt_flags &= ~MNT_LOCKED;
        mnt_set_mountpoint(m, mp, child);
        last_dest = m;
        last_source = child;