doc: warn about security issues around log files

Reported-by: Simon Riggs

Discussion: https://postgr.es/m/CANP8+jJESuuXYq9Djvf-+tx2vY2OFLmfEuu+UvwHNJ1RT7iJCQ@mail.gmail.com

Author: Simon Riggs

Backpatch-through: 10

doc/src/sgml/config.sgml

@@ -6649,6 +6649,13 @@ local0.*    /var/log/postgresql
      <sect2 id="runtime-config-logging-what">
      <title>What to Log</title>
 
+     <note>
+      <para>
+       What you choose to log can have security implications;  see
+       <xref linkend="logfile-maintenance"/>.
+      </para>
+     </note>
+
      <variablelist>
 
      <varlistentry id="guc-application-name" xreflabel="application_name">
@@ -7241,6 +7248,10 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
          planning).  Set <varname>log_min_error_statement</varname> to
          <literal>ERROR</literal> (or lower) to log such statements.
         </para>
+        <para>
+         Logged statements might reveal sensitive data and even contain
+         plaintext passwords.
+        </para>
        </note>
       </listitem>
      </varlistentry>

doc/src/sgml/maintenance.sgml

@@ -958,7 +958,25 @@ analyze threshold = analyze base threshold + analyze scale factor * number of tu
    It is a good idea to save the database server's log output
    somewhere, rather than just discarding it via <filename>/dev/null</filename>.
    The log output is invaluable when diagnosing
-   problems.  However, the log output tends to be voluminous
+   problems.
+  </para>
+
+  <note>
+   <para>
+    The server log can contain sensitive information and needs to be protected,
+    no matter how or where it is stored, or the destination to which it is routed.
+    For example, some DDL statements might contain plaintext passwords or other
+    authentication details. Logged statements at the <literal>ERROR</literal>
+    level might show the SQL source code for applications
+    and might also contain some parts of data rows. Recording data, events and
+    related information is the intended function of this facility, so this is
+    not a leakage or a bug. Please ensure the server logs are visible only to
+    appropriately authorized people.
+   </para>
+  </note>
+
+  <para>
+   Log output tends to be voluminous
    (especially at higher debug levels) so you won't want to save it
    indefinitely.  You need to <emphasis>rotate</emphasis> the log files so that
    new log files are started and old ones removed after a reasonable