pager: make pager secure when under euid is changed or explicitly requested

The variable is renamed to SYSTEMD_PAGERSECURE (because it's not just about
less now), and we automatically enable secure mode in certain cases, but not
otherwise.

This approach is more nuanced, but should provide a better experience for
users:

- Previusly we would set LESSSECURE=1 and trust the pager to make use of
  it. But this has an effect only on less. We need to not start pagers which
  are insecure when in secure mode. In particular more is like that and is a
  very popular pager.

- We don't enable secure mode always, which means that those other pagers can
  reasonably used.

- We do the right thing by default, but the user has ultimate control by
  setting SYSTEMD_PAGERSECURE.

Fixes #5666.

v2:
- also check $PKEXEC_UID

v3:
- use 'sd_pid_get_owner_uid() != geteuid()' as the condition

man/less-variables.xml

@@ -65,12 +65,30 @@
     </varlistentry>
 
     <varlistentry id='lesssecure'>
-      <term><varname>$SYSTEMD_LESSSECURE</varname></term>
+      <term><varname>$SYSTEMD_PAGERSECURE</varname></term>
 
-      <listitem><para>Takes a boolean argument. Overrides the <varname>$LESSSECURE</varname> environment
+      <listitem><para>Takes a boolean argument. When true, the "secure" mode of the pager is enabled; if
-      variable when invoking the pager, which controls the "secure" mode of less (which disables commands
+      false, disabled. If <varname>$SYSTEMD_PAGERSECURE</varname> is not set at all, secure mode is enabled
-      such as <literal>|</literal> which allow to easily shell out to external command lines). By default
+      if the effective UID is not the same as the owner of the login session, see <citerefentry
-      less secure mode is enabled, with this setting it may be disabled.</para></listitem>
+      project='man-pages'><refentrytitle>geteuid</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
+      <citerefentry><refentrytitle>sd_pid_get_owner_uid</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+      In secure mode, <option>LESSSECURE=1</option> will be set when invoking the pager, and the pager shall
+      disable commands that open or create new files or start new subprocesses. When
+      <varname>$SYSTEMD_PAGERSECURE</varname> is not set at all, pagers which are not known to implement
+      secure mode will not be used. (Currently only
+      <citerefentry><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry> implements
+      secure mode.)</para>
+
+      <para>Note: when commands are invoked with elevated privileges, for example under <citerefentry
+      project='man-pages'><refentrytitle>sudo</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
+      <citerefentry
+      project='die-net'><refentrytitle>pkexec</refentrytitle><manvolnum>1</manvolnum></citerefentry>, care
+      must be taken to ensure that unintended interactive features are not enabled. "Secure" mode for the
+      pager may be enabled automatically as describe above. Setting <varname>SYSTEMD_PAGERSECURE=0</varname>
+      or not removing it from the inherited environment allows the user to invoke arbitrary commands. Note
+      that if the <varname>$SYSTEMD_PAGER</varname> or <varname>$PAGER</varname> variables are to be
+      honoured, <varname>$SYSTEMD_PAGERSECURE</varname> must be set too. It might be reasonable to completly
+      disable the pager using <option>--no-pager</option> instead.</para></listitem>
     </varlistentry>
 
     <varlistentry id='colors'>

src/shared/pager.c

@@ -8,6 +8,8 @@
 #include <sys/prctl.h>
 #include <unistd.h>
 
+#include "sd-login.h"
+
 #include "copy.h"
 #include "env-util.h"
 #include "fd-util.h"
@@ -165,25 +167,42 @@ int pager_open(PagerFlags flags) {
                 }
 
                 /* People might invoke us from sudo, don't needlessly allow less to be a way to shell out
-                 * privileged stuff. */
+                 * privileged stuff. If the user set $SYSTEMD_PAGERSECURE, trust their configuration of the
-                r = getenv_bool("SYSTEMD_LESSSECURE");
+                 * pager. If they didn't, use secure mode when under euid is changed. If $SYSTEMD_PAGERSECURE
-                if (r == 0) { /* Remove env var if off */
+                 * wasn't explicitly set, and we autodetect the need for secure mode, only use the pager we
-                        if (unsetenv("LESSSECURE") < 0) {
+                 * know to be good. */
-                                log_error_errno(errno, "Failed to uset environment variable LESSSECURE: %m");
+                int use_secure_mode = getenv_bool("SYSTEMD_PAGERSECURE");
-                                _exit(EXIT_FAILURE);
+                bool trust_pager = use_secure_mode >= 0;
-                        }
+                if (use_secure_mode == -ENXIO) {
-                } else {
+                        uid_t uid;
-                        /* Set env var otherwise */
-                        if (r < 0)
-                                log_warning_errno(r, "Unable to parse $SYSTEMD_LESSSECURE, ignoring: %m");
 
-                        if (setenv("LESSSECURE", "1", 1) < 0) {
+                        r = sd_pid_get_owner_uid(0, &uid);
-                                log_error_errno(errno, "Failed to set environment variable LESSSECURE: %m");
+                        if (r < 0)
-                                _exit(EXIT_FAILURE);
+                                log_debug_errno(r, "sd_pid_get_owner_uid() failed, enabling pager secure mode: %m");
-                        }
+
+                        use_secure_mode = r < 0 || uid != geteuid();
+
+                } else if (use_secure_mode < 0) {
+                        log_warning_errno(use_secure_mode, "Unable to parse $SYSTEMD_PAGERSECURE, assuming true: %m");
+                        use_secure_mode = true;
                 }
 
-                if (pager_args) {
+                /* We generally always set variables used by less, even if we end up using a different pager.
+                 * They shouldn't hurt in any case, and ideally other pagers would look at them too. */
+                if (use_secure_mode)
+                        r = setenv("LESSSECURE", "1", 1);
+                else
+                        r = unsetenv("LESSSECURE");
+                if (r < 0) {
+                        log_error_errno(errno, "Failed to adjust environment variable LESSSECURE: %m");
+                        _exit(EXIT_FAILURE);
+                }
+
+                if (trust_pager && pager_args) { /* The pager config might be set globally, and we cannot
+                                                  * know if the user adjusted it to be appropriate for the
+                                                  * secure mode. Thus, start the pager specified through
+                                                  * envvars only when $SYSTEMD_PAGERSECURE was explicitly set
+                                                  * as well. */
                         r = loop_write(exe_name_pipe[1], pager_args[0], strlen(pager_args[0]) + 1, false);
                         if (r < 0) {
                                 log_error_errno(r, "Failed to write pager name to socket: %m");
@@ -195,13 +214,14 @@ int pager_open(PagerFlags flags) {
                                        "Failed to execute '%s', using fallback pagers: %m", pager_args[0]);
                 }
 
-                /* Debian's alternatives command for pagers is
+                /* Debian's alternatives command for pagers is called 'pager'. Note that we do not call
-                 * called 'pager'. Note that we do not call
+                 * sensible-pagers here, since that is just a shell script that implements a logic that is
-                 * sensible-pagers here, since that is just a
+                 * similar to this one anyway, but is Debian-specific. */
-                 * shell script that implements a logic that
-                 * is similar to this one anyway, but is
-                 * Debian-specific. */
                 FOREACH_STRING(exe, "pager", "less", "more") {
+                        /* Only less implements secure mode right now. */
+                        if (use_secure_mode && !streq(exe, "less"))
+                                continue;
+
                         r = loop_write(exe_name_pipe[1], exe, strlen(exe) + 1, false);
                         if (r  < 0) {
                                 log_error_errno(r, "Failed to write pager name to socket: %m");
@@ -212,6 +232,7 @@ int pager_open(PagerFlags flags) {
                                        "Failed to execute '%s', using next fallback pager: %m", exe);
                 }
 
+                /* Our builtin is also very secure. */
                 r = loop_write(exe_name_pipe[1], "(built-in)", strlen("(built-in)") + 1, false);
                 if (r < 0) {
                         log_error_errno(r, "Failed to write pager name to socket: %m");