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systemd/man/common-variables.xml
Zbigniew Jędrzejewski-Szmek cd93478af8 pager: also check for $SUDO_UID 
This returns to the original approach proposed in
https://github.com/systemd/systemd/pull/17270. After review, the approach was
changed to use sd_pid_get_owner_uid() instead. Back then, when running in a
typical graphical session, sd_pid_get_owner_uid() would usually return the user
UID, and when running under sudo, geteuid() would return 0, so we'd trigger the
secure path.

sudo may allocate a new session if is invoked outside of a session (depending
on the PAM config). Since nowadays desktop environments usually start the user
shell through user units, the typical shell in a terminal emulator is not part
of a session, and when sudo is invoked, a new session is allocated, and
sd_pid_get_owner_uid() returns 0 too. Technically, the code still works as
documented in the man page, but in the common case, it doesn't do the expected
thing.

$ build/test-sd-login |& rg 'get_(owner_uid|cgroup|session)'
sd_pid_get_session(0) → No data available
sd_pid_get_owner_uid(0) → 1000
sd_pid_get_cgroup(0) → /user.slice/user-1000.slice/user@1000.service/app.slice/app-ghostty-transient-5088.scope/surfaces/556FAF50BA40.scope

$ sudo build/test-sd-login |& rg 'get_(owner_uid|cgroup|session)'
sd_pid_get_session(0) → c289
sd_pid_get_owner_uid(0) → 0
sd_pid_get_cgroup(0) → /user.slice/user-0.slice/session-c289.scope

I think it's worth checking for sudo because it is a common case used by users.
There obviously are other mechanims, so the man page is extended to say that
only some common mechanisms are supported, and to (again) recommend setting
SYSTEMD_LESSSECURE explicitly. The other option would be to set "secure mode"
by default. But this would create an inconvenience for users doing the right
thing, running systemctl and other tools directly, because then they can't run
privileged commands from the pager, e.g. to save the output to a file. (Or the
user would need to explicitly set SYSTEMD_LESSSECURE. One option would be to
set it always in the environment and to rely on sudo and other tools stripping
it from the environment before running privileged code. But that is also fairly
fragile and it obviously relies on the user doing a complicated setup to
support a fairly common use case. I think this decreases usability of the
system quite a bit. I don't think we should build solutions that work in
priniciple, but are painfully inconvenient in common cases.)

Fixes https://yeswehack.com/vulnerability-center/reports/346802.

Also see https://github.com/polkit-org/polkit/pull/562, which adds support for
$SUDO_UID/$SUDO_GID to pkexec.
2025-05-13 18:08:49 +02:00

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<?xml version="1.0"?>
<!DOCTYPE refsect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refsect1>
<title>Environment</title>
<variablelist class='environment-variables'>
<varlistentry id='log-level'>
<term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
<listitem><para id='log-level-body'>The maximum log level of emitted messages (messages with a higher
log level, i.e. less important ones, will be suppressed). Takes a comma-separated list of values. A
value may be either one of (in order of decreasing importance) <constant>emerg</constant>,
<constant>alert</constant>, <constant>crit</constant>, <constant>err</constant>,
<constant>warning</constant>, <constant>notice</constant>, <constant>info</constant>,
<constant>debug</constant>, or an integer in the range 0…7. See
<citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
for more information. Each value may optionally be prefixed with one of <constant>console</constant>,
<constant>syslog</constant>, <constant>kmsg</constant> or <constant>journal</constant> followed by a
colon to set the maximum log level for that specific log target (e.g.
<constant>SYSTEMD_LOG_LEVEL=debug,console:info</constant> specifies to log at debug level except when
logging to the console which should be at info level). Note that the global maximum log level takes
priority over any per target maximum log levels.</para></listitem>
</varlistentry>
<varlistentry id='log-color'>
<term><varname>$SYSTEMD_LOG_COLOR</varname></term>
<listitem><para id='log-color-body'>A boolean. If true, messages written to the tty will be colored
according to priority.</para>
<para>This setting is only useful when messages are written directly to the terminal, because
<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> and
other tools that display logs will color messages based on the log level on their own.</para>
</listitem>
</varlistentry>
<varlistentry id='log-time'>
<term><varname>$SYSTEMD_LOG_TIME</varname></term>
<listitem><para id='log-time-body'>A boolean. If true, console log messages will be prefixed with a
timestamp.</para>
<para>This setting is only useful when messages are written directly to the terminal or a file, because
<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> and
other tools that display logs will attach timestamps based on the entry metadata on their own.</para>
</listitem>
</varlistentry>
<varlistentry id='log-location'>
<term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
<listitem><para id='log-location-body'>A boolean. If true, messages will be prefixed with a filename
and line number in the source code where the message originates.</para>
<para>Note that the log location is often attached as metadata to journal entries anyway. Including it
directly in the message text can nevertheless be convenient when debugging programs.</para>
</listitem>
</varlistentry>
<varlistentry id='log-tid'>
<term><varname>$SYSTEMD_LOG_TID</varname></term>
<listitem><para id='log-tid-body'>A boolean. If true, messages will be prefixed with the current
numerical thread ID (TID).</para>
<para>Note that the this information is attached as metadata to journal entries anyway. Including it
directly in the message text can nevertheless be convenient when debugging programs.</para>
</listitem>
</varlistentry>
<varlistentry id='log-target'>
<term><varname>$SYSTEMD_LOG_TARGET</varname></term>
<listitem><para id='log-target-body'>The destination for log messages. One of
<constant>console</constant> (log to the attached tty), <constant>console-prefixed</constant> (log to
the attached tty but with prefixes encoding the log level and "facility", see <citerefentry
project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<constant>kmsg</constant> (log to the kernel circular log buffer), <constant>journal</constant> (log to
the journal), <constant>journal-or-kmsg</constant> (log to the journal if available, and to kmsg
otherwise), <constant>auto</constant> (determine the appropriate log target automatically, the default),
<constant>null</constant> (disable log output).</para>
<!-- <constant>syslog</constant>, <constant>syslog-or-kmsg</constant> are deprecated -->
</listitem>
</varlistentry>
<varlistentry id='log-ratelimit-kmsg'>
<term><varname>$SYSTEMD_LOG_RATELIMIT_KMSG</varname></term>
<listitem><para id='log-ratelimit-kmsg-body'> Whether to ratelimit kmsg or not. Takes a boolean.
Defaults to <literal>true</literal>. If disabled, systemd will not ratelimit messages written to kmsg.
</para></listitem>
</varlistentry>
<varlistentry id='pager'>
<term><varname>$SYSTEMD_PAGER</varname></term>
<term><varname>$PAGER</varname></term>
<listitem><para>Pager to use when <option>--no-pager</option> is not given.
<varname>$SYSTEMD_PAGER</varname> is used if set; otherwise <varname>$PAGER</varname> is used.
If neither <varname>$SYSTEMD_PAGER</varname> nor <varname>$PAGER</varname> are set, a set of well-known
pager implementations is tried in turn, including
<citerefentry project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry>
and
<citerefentry project='man-pages'><refentrytitle>more</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
until one is found. If no pager implementation is discovered, no pager is invoked. Setting those
environment variables to an empty string or the value <literal>cat</literal> is equivalent to passing
<option>--no-pager</option>.</para>
<para>Note: if <varname>$SYSTEMD_PAGERSECURE</varname> is not set, <varname>$SYSTEMD_PAGER</varname>
and <varname>$PAGER</varname> can only be used to disable the pager (with <literal>cat</literal> or
<literal></literal>), and are otherwise ignored.</para></listitem>
</varlistentry>
<varlistentry id='less'>
<term><varname>$SYSTEMD_LESS</varname></term>
<listitem><para>Override the options passed to <command>less</command> (by default
<literal>FRSXMK</literal>).</para>
<para>Users might want to change two options in particular:</para>
<variablelist>
<varlistentry>
<term><option>K</option></term>
<listitem>
<para>This option instructs the pager to exit immediately when
<keycombo><keycap>Ctrl</keycap><keycap>C</keycap></keycombo> is pressed. To allow
<command>less</command> to handle <keycombo><keycap>Ctrl</keycap><keycap>C</keycap></keycombo>
itself to switch back to the pager command prompt, unset this option.</para>
<para>If the value of <varname>$SYSTEMD_LESS</varname> does not include <literal>K</literal>,
and the pager that is invoked is <command>less</command>,
<keycombo><keycap>Ctrl</keycap><keycap>C</keycap></keycombo> will be ignored by the
executable, and needs to be handled by the pager.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>X</option></term>
<listitem>
<para>This option instructs the pager to not send termcap initialization and deinitialization
strings to the terminal. It is set by default to allow command output to remain visible in the
terminal even after the pager exits. Nevertheless, this prevents some pager functionality from
working, in particular paged output cannot be scrolled with the mouse.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Note that setting the regular <varname>$LESS</varname> environment variable has no effect
for <command>less</command> invocations by systemd tools.</para>
<para>See
<citerefentry project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry>
for more discussion.</para></listitem>
</varlistentry>
<varlistentry id='lesscharset'>
<term><varname>$SYSTEMD_LESSCHARSET</varname></term>
<listitem><para>Override the charset passed to <command>less</command> (by default <literal>utf-8</literal>, if
the invoking terminal is determined to be UTF-8 compatible).</para>
<para>Note that setting the regular <varname>$LESSCHARSET</varname> environment variable has no effect
for <command>less</command> invocations by systemd tools.</para></listitem>
</varlistentry>
<varlistentry id='lesssecure'>
<term><varname>$SYSTEMD_PAGERSECURE</varname></term>
<listitem>
<para>Common pager commands like <citerefentry
project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry>, in
addition to "paging", i.e. scrolling through the output, support opening of or writing to other files
and running arbitrary shell commands. 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>, the
pager becomes a security boundary. Care must be taken that only programs with strictly limited
functionality are used as pagers, and unintended interactive features like opening or creation of new
files or starting of subprocesses are not allowed. "Secure mode" for the pager may be enabled as
described below, <emphasis>if the pager supports that</emphasis> (most pagers are not written in a way
that takes this into consideration). It is recommended to either explicitly enable "secure mode" or to
completely disable the pager using <option>--no-pager</option> or <varname>PAGER=cat</varname> when
allowing untrusted users to execute commands with elevated privileges.</para>
<para>This option takes a boolean argument. When set to true, the "secure mode" of the pager is
enabled. In "secure mode", <option>LESSSECURE=1</option> will be set when invoking the pager, which
instructs the pager to disable commands that open or create new files or start new subprocesses.
Currently only <citerefentry
project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry> is known
to understand this variable and implement "secure mode".</para>
<para>When set to false, no limitation is placed on the pager. Setting
<varname>SYSTEMD_PAGERSECURE=0</varname> or not removing it from the inherited environment may allow
the user to invoke arbitrary commands.</para>
<para>When <varname>$SYSTEMD_PAGERSECURE</varname> is not set, systemd tools attempt to automatically
figure out if "secure mode" should be enabled and whether the pager supports it. "Secure mode" is
enabled if the effective UID is not the same as the owner of the login session, see
<citerefentry project='man-pages'><refentrytitle>geteuid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
and
<citerefentry><refentrytitle>sd_pid_get_owner_uid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
or when running under
<citerefentry><refentrytitle>sudo</refentrytitle><manvolnum>8</manvolnum></citerefentry> or similar
tools (<varname>$SUDO_UID</varname> is set <footnote>
<para>It is recommended for other tools to set and check <varname>$SUDO_UID</varname> as appropriate,
treating it is a common interface.</para></footnote>). In those cases,
<varname>SYSTEMD_PAGERSECURE=1</varname> will be set and pagers which are not known to implement
"secure mode" will not be used at all. Note that this autodetection only covers the most common
mechanisms to elevate privileges and is intended as convenience. It is recommended to explicitly set
<varname>$SYSTEMD_PAGERSECURE</varname> or disable the pager.</para>
<para>Note that if the <varname>$SYSTEMD_PAGER</varname> or <varname>$PAGER</varname> variables are to
be honoured, other than to disable the pager, <varname>$SYSTEMD_PAGERSECURE</varname> must be set
too.</para></listitem>
</varlistentry>
<varlistentry id='colors'>
<term><varname>$SYSTEMD_COLORS</varname></term>
<listitem><para>Takes a boolean argument. When true, <command>systemd</command> and related utilities
will use colors in their output, otherwise the output will be monochrome. Additionally, the variable can
take one of the following special values: <literal>16</literal>, <literal>256</literal> to restrict the use
of colors to the base 16 or 256 ANSI colors, respectively. This can be specified to override the automatic
decision based on <varname>$TERM</varname> and what the console is connected to.</para></listitem>
</varlistentry>
<!-- This is not documented on purpose, because it is not clear if $NO_COLOR will become supported
widely enough. So let's provide support, but without advertising this.
<varlistentry id='no-color'>
<term><varname>$NO_COLOR</varname></term>
<listitem><para>If set (to any value), and <varname>$SYSTEMD_COLORS</varname> is not set, equivalent to
<option>SYSTEMD_COLORS=0</option>. See <ulink url="https://no-color.org/">no-color.org</ulink>.</para>
</listitem>
</varlistentry>
-->
<varlistentry id='urlify'>
<term><varname>$SYSTEMD_URLIFY</varname></term>
<listitem><para>The value must be a boolean. Controls whether clickable links should be generated in
the output for terminal emulators supporting this. This can be specified to override the decision that
<command>systemd</command> makes based on <varname>$TERM</varname> and other conditions.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
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