cred: copy_process() should clear child->replacement_session_keyring

keyctl_session_to_parent(task) sets ->replacement_session_keyring,
it should be processed and cleared by key_replace_session_keyring().

However, this task can fork before it notices TIF_NOTIFY_RESUME and
the new child gets the bogus ->replacement_session_keyring copied by
dup_task_struct(). This is obviously wrong and, if nothing else, this
leads to put_cred(already_freed_cred).

change copy_creds() to clear this member. If copy_process() fails
before this point the wrong ->replacement_session_keyring doesn't
matter, exit_creds() won't be called.

Cc: <stable@vger.kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static void release_tgcred_rcu(struct rcu_head *rcu)
{
	struct thread_group_cred *tgcred =
		container_of(rcu, struct thread_group_cred, rcu);

	BUG_ON(atomic_read(&tgcred->usage) != 0);

	key_put(tgcred->session_keyring);
	key_put(tgcred->process_keyring);
	kfree(tgcred);
}

static void release_tgcred_rcu(struct rcu_head *rcu)
{
	struct thread_group_cred *tgcred =
		container_of(rcu, struct thread_group_cred, rcu);

	BUG_ON(atomic_read(&tgcred->usage) != 0);

	key_put(tgcred->session_keyring);
	key_put(tgcred->process_keyring);
	kfree(tgcred);
}
