namespace: refuse an automount below a mount that is in no namespace

In the Linux kernel, the following vulnerability has been resolved:

namespace: refuse an automount below a mount that is in no namespace

It's possible to add autmounts even when the parent mount isn't in the
mount namespace of the caller. The only requirement we have is that the
parent mount namespace must not be NULL, i.e., unmounted.

Problem is that clone_private_mount() has MNT_NS_INTERNAL which makes
that trivially true. So that passes the test and attach_recursive_mnt()
accepts that as a mount point and funny enough, count_mounts()
dereferences MNT_NS_INTERNAL. The problem is it is an error pointer...

So we can reach this in userspace via fanotify. A filesystem mark on the
lower filesystem of an overlay reports paths on the layer clone and
reading the event hands out a descriptor on it. For example with debugfs
as the lower layer it goes kaboom:

openat(evfd, "tracing", O_DIRECTORY)

Oops: general protection fault
KASAN: null-ptr-deref in range [0x1d0-0x1d7]
RIP: 0010:count_mounts+0x35/0x200
attach_recursive_mnt
finish_automount

And since that sleeping beauty happens under namespace_sem held for
writing every mount operation on the system blocks from then on.
Congrats.

Use is_mounted() instead which rejects unmounted and internal mounts
alike. The open fails with EINVAL just as it did before
clone_private_mount() used MNT_NS_INTERNAL.