pager: set $LESSSECURE whenver we invoke a pager

Some extra safety when invoked via "sudo". With this we address a
genuine design flaw of sudo, and we shouldn't need to deal with this.
But it's still a good idea to disable this surface given how exotic it
is.

Prompted by #5666

(cherry picked from commit 612ebf6)

Related: RHEL-18791

man/less-variables.xml

@@ -36,5 +36,13 @@
       the invoking terminal is determined to be UTF-8 compatible).</para></listitem>
     </varlistentry>
 
+    <varlistentry id='lesssecure'>
+      <term><varname>$SYSTEMD_LESSSECURE</varname></term>
+
+      <listitem><para>Takes a boolean argument. Overrides the <varname>$LESSSECURE</varname> environment
+      variable when invoking the pager, which controls the "secure" mode of less (which disables commands
+      such as <literal>|</literal> which allow to easily shell out to external command lines). By default
+      less secure mode is enabled, with this setting it may be disabled.</para></listitem>
+    </varlistentry>
     </variablelist>
 </refsect1>

man/systemctl.xml

@@ -2010,6 +2010,7 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
     <xi:include href="less-variables.xml" xpointer="pager"/>
     <xi:include href="less-variables.xml" xpointer="less"/>
     <xi:include href="less-variables.xml" xpointer="lesscharset"/>
+    <xi:include href="less-variables.xml" xpointer="lesssecure"/>
   </refsect1>
 
   <refsect1>

man/systemd.xml

@@ -862,6 +862,8 @@
         </listitem>
       </varlistentry>
 
+      <xi:include href="less-variables.xml" xpointer="lesssecure"/>
+
       <varlistentry>
         <term><varname>$LISTEN_PID</varname></term>
         <term><varname>$LISTEN_FDS</varname></term>

src/basic/pager.c

@@ -11,6 +11,7 @@
 #include <unistd.h>
 
 #include "copy.h"
+#include "env-util.h"
 #include "fd-util.h"
 #include "locale-util.h"
 #include "log.h"
@@ -94,8 +95,7 @@ int pager_open(bool no_pager, bool jump_to_end) {
                 if (setenv("LESS", less_opts, 1) < 0)
                         _exit(EXIT_FAILURE);
 
-                /* Initialize a good charset for less. This is
-                 * particularly important if we output UTF-8
+                /* Initialize a good charset for less. This is particularly important if we output UTF-8
                  * characters. */
                 less_charset = getenv("SYSTEMD_LESSCHARSET");
                 if (!less_charset && is_locale_utf8())
@@ -104,6 +104,25 @@ int pager_open(bool no_pager, bool jump_to_end) {
                     setenv("LESSCHARSET", less_charset, 1) < 0)
                         _exit(EXIT_FAILURE);
 
+                /* People might invoke us from sudo, don't needlessly allow less to be a way to shell out
+                 * privileged stuff. */
+                r = getenv_bool("SYSTEMD_LESSSECURE");
+                if (r == 0) { /* Remove env var if off */
+                        if (unsetenv("LESSSECURE") < 0) {
+                                log_error_errno(errno, "Failed to uset environment variable LESSSECURE: %m");
+                                _exit(EXIT_FAILURE);
+                        }
+                } else {
+                        /* Set env var otherwise */
+                        if (r < 0)
+                                log_warning_errno(r, "Unable to parse $SYSTEMD_LESSSECURE, ignoring: %m");
+
+                        if (setenv("LESSSECURE", "1", 1) < 0) {
+                                log_error_errno(errno, "Failed to set environment variable LESSSECURE: %m");
+                                _exit(EXIT_FAILURE);
+                        }
+                }
+
                 if (pager) {
                         execlp(pager, pager, NULL);
                         execl("/bin/sh", "sh", "-c", pager, NULL);