Merge tag 'for-linus-2021-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git...

Pull detached mounts fix from Christian Brauner:
 "Creating a series of detached mounts, attaching them to the
  filesystem, and unmounting them can be used to trigger an integer
  overflow in ns->mounts causing the kernel to block any new mounts in
  count_mounts() and returning ENOSPC because it falsely assumes that
  the maximum number of mounts in the mount namespace has been reached,
  i.e. it thinks it can't fit the new mounts into the mount namespace
  anymore.

  Without this fix heavy use of the new mount API with move_mount() will
  cause the host to become unuseable and thus blocks some xfstest
  patches I want to resend.

  Depending on the number of mounts in your system, this can be
  reproduced on any kernel that supportes open_tree() and move_mount().

  A reproducer has been sent for inclusion with xfstests. It takes care
  to do this in another mount namespace, not in the host's mount
  namespace so there shouldn't be any risk in running it but if one did
  run it on the host it would require a reboot in order to be able to
  mount again. See

      https://lore.kernel.org/fstests/20210309121041.753359-1-christian.brauner@ubuntu.com

  The root cause of this is that detached mounts aren't handled
  correctly when source and target mount are identical and reside on a
  shared mount causing a broken mount tree where the detached source
  itself is propagated which propagation prevents for regular
  bind-mounts and new mounts.

  This ultimately leads to a miscalculation of the number of mounts in
  the mount namespace.

  Detached mounts created via 'open_tree(fd, path, OPEN_TREE_CLONE)' are
  essentially like an unattached bind-mount. They can then later on be
  attached to the filesystem via move_mount() which calls into
  attach_recursive_mount().

  Part of attaching it to the filesystem is making sure that mounts get
  correctly propagated in case the destination mountpoint is MS_SHARED,
  i.e. is a shared mountpoint. This is done by calling into
  propagate_mnt() which walks the list of peers calling propagate_one()
  on each mount in this list making sure it receives the propagation
  event. The propagate_one() function thereby skips both new mounts and
  bind mounts to not propagate them "into themselves". Both are
  identified by checking whether the mount is already attached to any
  mount namespace in mnt->mnt_ns. The is what the IS_MNT_NEW() helper is
  responsible for.

  However, detached mounts have an anonymous mount namespace attached to
  them stashed in mnt->mnt_ns which means that IS_MNT_NEW() doesn't
  realize they need to be skipped causing the mount to propagate "into
  itself" breaking the mount table and causing a disconnect between the
  number of mounts recorded as being beneath or reachable from the
  target mountpoint and the number of mounts actually recorded/counted
  in ns->mounts ultimately causing an overflow which in turn prevents
  any new mounts via the ENOSPC issue.

  So teach propagation to handle detached mounts by making it aware of
  them. I've been tracking this issue down for the last couple of days
  and then verifying that the fix is correct by unmounting everything in
  my current mount table leaving only /proc and /sys mounted and running
  the reproducer above overnight verifying the number of mounts counted
  in ns->mounts. With this fix the counts are correct and the ENOSPC
  issue can't be reproduced.

  This change will only have an effect on mounts created with the new
  mount API since detached mounts cannot be created with the old mount
  API so regressions are extremely unlikely.

  Here's an illustration:

    #### mount():
    ubuntu@f1-vm:~$ sudo mount --bind /mnt/ /mnt/
    ubuntu@f1-vm:~$ findmnt  | grep -i mnt
    ├─/mnt                                /dev/sda2[/mnt] ext4       rw,relatime

    #### open_tree(OPEN_TREE_CLONE) + move_mount() with bug:
    ubuntu@f1-vm:~$ sudo ./mount-new /mnt/ /mnt/
    ubuntu@f1-vm:~$ findmnt  | grep -i mnt
    ├─/mnt                                /dev/sda2[/mnt] ext4       rw,relatime
    │ └─/mnt                              /dev/sda2[/mnt] ext4       rw,relatime

    #### open_tree(OPEN_TREE_CLONE) + move_mount() with the fix:
    ubuntu@f1-vm:~$ sudo ./mount-new /mnt /mnt
    ubuntu@f1-vm:~$ findmnt | grep -i mnt
    └─/mnt                                /dev/sda2[/mnt] ext4       rw,relatime"

* tag 'for-linus-2021-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  mount: fix mounting of detached mounts onto targets that reside on shared mounts