update lxc-attach manpage

- explain rationale behind allocation of pty
- briefly explain how a pty is allocated
- add a short note that describes the changed behavior for lxc-attach when the
  user is not placed in a writeable cgroup at login

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>

doc/lxc-attach.sgml.in

@@ -78,6 +78,23 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
       inside the container or the container does not have a working
       nsswitch mechanism.
     </para>
+    <para>
+    Previous versions of <command>lxc-attach</command> simply attached to the
+    specified namespaces of a container and ran a shell or the specified
+    command without allocating a pseudo terminal. This made them vulnerable to
+    input faking via a TIOCSTI <command>ioctl</command> call after switching
+    between userspace execution contexts with different privilegel levels. Newer
+    versions of <command>lxc-attach</command> will try to allocate a pseudo
+    terminal master/slave pair and attach any standard file descriptors which
+    refer to a terminal to the slave side of the pseudo terminal before
+    executing a shell or command. <command>lxc-attach</command> will first try
+    to allocate a pseudo terminal in the container. Should this fail it will try
+    to allocate a pseudo terminal on the host before finally giving up. Note,
+    that if none of the standard file descriptors refer to a terminal
+    <command>lxc-attach</command> will not try to allocate a pseudo terminal.
+    Instead it will simply attach to the containers namespaces and run a shell
+    or the specified command.
+    </para>
 
   </refsect1>
 
@@ -311,6 +328,16 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
       except for the <replaceable>/proc</replaceable> and
       <replaceable>/sys</replaceable> filesystems.
     </para>
+    <para>
+      Previous versions of <command>lxc-attach</command> suffered a bug whereby
+      a user could attach to a containers namespace without being placed in a
+      writeable cgroup for some critical subsystems. Newer versions of
+      <command>lxc-attach</command> will check whether a user is in a writeable
+      cgroup for those critical subsystems. <command>lxc-attach</command> might
+      thus fail unexpectedly for some users (E.g. on systems where an
+      unprivileged user is not placed in a writeable cgroup in critical
+      subsystems on login.). However, this behavior is correct and more secure.
+    </para>
   </refsect1>
 
   <refsect1>