kernel-packages team mailing list archive

[Seth Forshee <seth.forshee+lp@xxxxxxxxxxxxx>]

Hmm. In this case it's really the kernel that is writing the xattr, so
in that case #2 kind of makes sense. It's also more than a bit scary,
assigning CAP_SYS_ADMIN in init_user_ns to a task from a user namespace.
Now we're already doing it for unprivileged users in init_user_ns which
isn't all that different, except for the fact that in the user namespace
that unprivileged user can also create the overlay mount, and that
leaves me feeling a bit uneasy. I'm not familiar enough with overlayfs
to decide whether or not this really presents an opportunity for someone
to do something malicious to the lower fs.

With #1, I don't think we have a way to distinguish between overlayfs
trying to write this xattr and userspace writing it directly, do we?
This also might present an opportunity for a user to do something mildly
malicious.

I can't comment on #3, I just don't know enough about overlayfs.

I don't really any other ideas. #2 seems the most logical to me if we
can be sure that it's safe.

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https://bugs.launchpad.net/bugs/1531747

Title:
  overlay: mkdir fails if directory exists in lowerdir in a user
  namespace

Status in linux package in Ubuntu:
  Triaged
Status in linux source package in Wily:
  Triaged
Status in linux source package in Xenial:
  Triaged

Bug description:
  If a directory exists in the lowerdir but not in the mounted
  overlay, then mkdir of the directory in the target dir results
  in a mysterious -EPERM.  I've seen this both in wily kernel
  (4.2.0-22-generic #27-Ubuntu) and in a hand-built xenial
  master-next (with unrelated patches added).

  =====================================================
  #!/bin/sh -ex
  dir=`mktemp -d`
  cleanup() {
   umount -l $dir/t
   rm -rf $dir
  }

  trap cleanup EXIT

  echo "dir is $dir"
  mkdir -p $dir/l $dir/u $dir/w $dir/t
  mkdir $dir/l/dev
  mount -t overlay -o lowerdir=$dir/l,upperdir=$dir/u,workdir=$dir/w o $dir/t
  stat $dir/t/dev
  rmdir $dir/t/dev
  mkdir $dir/t/dev
  echo $?
  echo "mkdir should have succeeded"
  =====================================================

  The above will work on the host, but fail in a user namespace, i.e
  in a regular lxd container.

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