Merge patch series "ns: fixes for namespace iteration and active reference counting"

Christian Brauner <brauner@kernel.org> says:

* Make sure to initialize the active reference count for the initial
  network namespace and prevent __ns_common_init() from returning too
  early.

* Make sure that passive reference counts are dropped outside of rcu
  read locks as some namespaces such as the mount namespace do in fact
  sleep when putting the last reference.

* The setns() system call supports:

  (1) namespace file descriptors (nsfd)
  (2) process file descriptors (pidfd)

  When using nsfds the namespaces will remain active because they are
  pinned by the vfs. However, when pidfds are used things are more
  complicated.

  When the target task exits and passes through exit_nsproxy_namespaces()
  or is reaped and thus also passes through exit_cred_namespaces() after
  the setns()'ing task has called prepare_nsset() but before the active
  reference count of the set of namespaces it wants to setns() to might
  have been dropped already:

    P1                                                              P2

    pid_p1 = clone(CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS)
                                                                    pidfd = pidfd_open(pid_p1)
                                                                    setns(pidfd, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS)
                                                                    prepare_nsset()

    exit(0)
    // ns->__ns_active_ref        == 1
    // parent_ns->__ns_active_ref == 1
    -> exit_nsproxy_namespaces()
    -> exit_cred_namespaces()

    // ns_active_ref_put() will also put
    // the reference on the owner of the
    // namespace. If the only reason the
    // owning namespace was alive was
    // because it was a parent of @ns
    // it's active reference count now goes
    // to zero... --------------------------------
    //                                           |
    // ns->__ns_active_ref        == 0           |
    // parent_ns->__ns_active_ref == 0           |
                                                 |                  commit_nsset()
                                                 -----------------> // If setns()
                                                                    // now manages to install the namespaces
                                                                    // it will call ns_active_ref_get()
                                                                    // on them thus bumping the active reference
                                                                    // count from zero again but without also
                                                                    // taking the required reference on the owner.
                                                                    // Thus we get:
                                                                    //
                                                                    // ns->__ns_active_ref        == 1
                                                                    // parent_ns->__ns_active_ref == 0

    When later someone does ns_active_ref_put() on @ns it will underflow
    parent_ns->__ns_active_ref leading to a splat from our asserts
    thinking there are still active references when in fact the counter
    just underflowed.

  So resurrect the ownership chain if necessary as well. If the caller
  succeeded to grab passive references to the set of namespaces the
  setns() should simply succeed even if the target task exists or gets
  reaped in the meantime.

  The race is rare and can only be triggered when using pidfs to setns()
  to namespaces. Also note that active reference on initial namespaces are
  nops.

  Since we now always handle parent references directly we can drop
  ns_ref_active_get_owner() when adding a namespace to a namespace tree.
  This is now all handled uniformly in the places where the new namespaces
  actually become active.

* patches from https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-0-ae8a4ad5a3b3@kernel.org:
  selftests/namespaces: test for efault
  selftests/namespaces: add active reference count regression test
  ns: add asserts for active refcount underflow
  ns: handle setns(pidfd, ...) cleanly
  ns: return EFAULT on put_user() error
  ns: make sure reference are dropped outside of rcu lock
  ns: don't increment or decrement initial namespaces
  ns: don't skip active reference count initialization

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-0-ae8a4ad5a3b3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>