dev-manual: Updates to the "Making Images More Secure" section.

Fixes [YOCTO #5482]

Applied extensive review edist from Paul Eggleton throughout
this section.

(From yocto-docs rev: aa0bcd9199c83b43bad2390ff2292a8c2abe7455)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -3841,14 +3841,46 @@
         <title>Making Images More Secure</title>
 
         <para>
-            If securing your image is of concern, there are steps, tools,
+            Security for a device is always a concern.
+            Consider the issues and problems discussed in just this
+            sampling of work found across the Internet:
+            <itemizedlist>
+                <listitem><para><emphasis>
+                    "<ulink url='https://www.schneier.com/blog/archives/2014/01/security_risks_9.html'>Security Risks of Embedded Systems</ulink>"</emphasis>
+                    by Bruce Schneier
+                    </para></listitem>
+                <listitem><para><emphasis>
+                    "<ulink url='http://internetcensus2012.bitbucket.org/paper.html'>Internet Census 2012</ulink>"</emphasis>
+                    by Carna Botnet</para></listitem>
+                <listitem><para><emphasis>
+                    "<ulink url='http://elinux.org/images/6/6f/Security-issues.pdf'>Security Issues for Embedded Devices</ulink>"</emphasis>
+                    by Jake Edge
+                    </para></listitem>
+                <listitem><para><emphasis>
+                    "<ulink url='https://www.nccgroup.com/media/18475/exploiting_security_gateways_via_their_web_interfaces.pdf'>They ought to know better: Exploiting Security
+Gateways via their Web Interfaces</ulink>"</emphasis>
+                    by Ben Williams
+                    </para></listitem>
+            </itemizedlist>
+        </para>
+
+        <para>
+            When securing your image is of concern, there are steps, tools,
             and variables that you can consider to help you reach the
             security goals you need for your particular device.
             Not all situations are identical when it comes to making an
             image secure.
             Consequently, this section provides some guidance and suggestions
             for consideration when you want to make your image more secure.
-            The section does not offer a complete solution.
+        </para>
+
+        <para>
+            Because the security requirements and risks are
+            different for every type of device, this section cannot
+            provide a complete reference on securing your custom OS.
+            It is strongly recommended that you also consult other sources
+            of information on embedded Linux system hardening and on
+            security.
         </para>
 
         <section id='general-considerations'>
@@ -3895,7 +3927,10 @@
                     <listitem><para>
                         Ensure you remove or disable debugging functionality
                         before producing the final image.
-                    </para></listitem>
+                        For information on how to do this, see the
+                        "<link linkend='considerations-specific-to-the-openembedded-build-system'>Considerations Specific to the OpenEmbedded Build System</link>
+                        section.
+                        </para></listitem>
                     <listitem><para>
                         Ensure you have no network services listening that
                         are not needed.
@@ -3929,6 +3964,7 @@
             </para>
 
             <para>
+<!--
                 The GCC/LD flags in <filename>security_flags.inc</filename>
                 enable more secure code generation.
                 By including the <filename>security_flags.inc</filename>
@@ -3938,10 +3974,11 @@
                     The GCC/LD flags are enabled by default in the
                     <filename>poky-lsb</filename> distribution.
                 </note>
+-->
                 Use the following line in your
-                <filename>local.conf</filename> file
+                <filename>local.conf</filename> file or in your custom
-                to enable the security compiler and
+                distribution configuration file to enable the security
-                linker flags to your build:
+                compiler and linker flags to your build:
                 <literallayout class='monospaced'>
      require conf/distro/include/security_flags.inc
                 </literallayout>
@@ -3966,38 +4003,65 @@
                         sure that it does not have "debug-tweaks" before
                         producing your final image.
                         Among other things, leaving this in place sets the
-                        root password as blank.
+                        root password as blank, which makes logging in for
+                        debugging or inspection easy during
+                        development but also means anyone can easily log in
+                        during production.
                         </para></listitem>
                     <listitem><para>
                         It is possible to set a root password for the image.
-                        For information on how to do that, see the
+                        When you set up root passwords for multiple images,
-                        <ulink url='https://wiki.yoctoproject.org/wiki/FAQ:How_do_I_set_or_change_the_root_password'>How do I set or change the root password</ulink>
+                        you should not duplicate them.
-                        Wiki page.
+                        See the note on passwords at the end of this list.
+                        </para>
+                        <para>
+                        To set up a root password,
+                        use the <filename>extrausers</filename>
+                        class, which is the preferred method.
+                        For an example on how to set up the root password,
+                        see the
+                        "<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-extrausers'><filename>extrausers.bbclass</filename></ulink>"
+                        section.
+                        <note>
+                            You can also see the the example in the
+                            <ulink url='https://wiki.yoctoproject.org/wiki/FAQ:How_do_I_set_or_change_the_root_password'>How do I set or change the root password Wiki</ulink>
+                            page as an alternative method.
+                        </note>
                         </para></listitem>
                     <listitem><para>
                         It is possible to add an additional user account
-                        for later administrative or service access using the
+                        for later administrative or service access.
-                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-extrausers'><filename>extrausers</filename></ulink>
+                        As with root passwords, be sure to not duplicate
-                        class or the
+                        passwords for generic users (e.g. tester, qa, and
-                        <ulink url='&YOCTO_DOCS_REF_URL;#var-ROOTFS_POSTPROCESS_COMMAND'><filename>ROOTFS_POSTPROCESS_COMMAND</filename></ulink>
+                        so forth) across multiple devices.
-                        variable.
+                        See the note on passwords following this list.
-                        For an example on how to add users, see the
-                        "<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-extrausers'><filename>extrausers.bbclass</filename></ulink>"
-                        section.
                         </para>
-                        <para>If you do add extra user accounts,
+                        <para>
-                        be cautious about setting
+                        As with the root password, you also use the
-                        the same password for every device.
+                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-extrausers'><filename>extrausers</filename></ulink>
-                        If you want the device to remain secure
+                        class.
-                        from unauthorized access, and the password set on
+                        </para></listitem>
-                        all devices becomes compromised, then every device
+                    <listitem><para>
-                        becomes compromised.
+                        Consider enabling a Mandatory Access Control (MAC)
-                        If you need this access but want to ensure security,
+                        framework (such as SMACK or SELinux) and tuning it
-                        consider setting a different, random password for each
+                        appropriately for your device's usage.
-                        device.
                         </para></listitem>
                 </itemizedlist>
             </para>
+
+            <para>
+                <note><title>A Note about Passwords</title>
+                    When adding extra user accounts or deciding on root
+                    passwords for multiple devices, be cautious about setting
+                    the same passwords for multiple users or devices.
+                    If you want the device to remain secure from unauthorized
+                    access, and the password set on all devices becomes
+                    compromised, then every device becomes compromised.
+                    If you need this access but want to ensure security,
+                    consider setting a different, random passwords for each
+                    user or device.
+                </note>
+            </para>
         </section>
 
         <section id='tools-for-hardening-your-image'>