On Wed, Oct 10, 2018 at 12:08 AM Aleksa Sarai [off-list ref] wrote:

On 2018-10-09, Andy Lutomirski [off-list ref] wrote:

quoted

On Mon, Oct 8, 2018 at 11:53 PM Aleksa Sarai [off-list ref] wrote:

quoted

* AT_NO_PROCLINK: Disallows ->get_link "symlink" jumping. This is a very
  specific restriction, and it exists because /proc/$pid/fd/...
  "symlinks" allow for access outside nd->root and pose risk to
  container runtimes that don't want to be tricked into accessing a host
  path (but do want to allow no-funny-business symlink resolution).

Can you elaborate on the use case?

If I'm set up a container namespace and walk it for real (through the
outside /proc/PID/root or otherwise starting from an fd that points
into that namespace), and I walk through that namespace's /proc, I'm
going to see the same thing that the processes in the namespace would
see.  So what's the issue?

Similarly, if I somehow manage to walk into the outside /proc, then
I've pretty much lost regardless of the links.

Well, there's a couple of reasons:

* The original AT_NO_JUMPS patchset similarly disabled "proclinks" but
  it was sort of all contained within AT_NO_JUMPS. In order to have a
  precise 1:1 feature mapping we need this in *some* form (and in v1 the
  only way to get it was to add a separate flag). According to the
  original O_BENEATH changelog, both you and Al pushed for this to be
  part of O_BENEATH. :P

:)

Now that you mention it, I *think* my reasoning involved a rather
different use case: sandboxing.  If a task is Capsicum-ified or
seccomp()ed such that it can *only* use O_BENEATH or AT_BENEATH, this
restriction considerably strengthens the resulting security.