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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd">
<refentry id="pam_fail_delay">
<refmeta>
<refentrytitle>pam_fail_delay</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo class='setdesc'>Linux-PAM Manual</refmiscinfo>
</refmeta>
<refnamediv id="pam_fail_delay-name">
<refname>pam_fail_delay</refname>
<refpurpose>request a delay on failure</refpurpose>
</refnamediv>
<!-- body begins here -->
<refsynopsisdiv>
<funcsynopsis id="pam_fail_delay-synopsis">
<funcsynopsisinfo>#include <security/pam_appl.h></funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>pam_fail_delay</function></funcdef>
<paramdef>pam_handle_t *<parameter>pamh</parameter></paramdef>
<paramdef>unsigned int <parameter>usec</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 id='pam_fail_delay-description'>
<title>DESCRIPTION</title>
<para>
The <function>pam_fail_delay</function> function provides a
mechanism by which an application or module can suggest a minimum
delay of <emphasis>usec</emphasis> micro-seconds. The
function keeps a record of the longest time requested with this
function. Should
<citerefentry>
<refentrytitle>pam_authenticate</refentrytitle><manvolnum>3</manvolnum>
</citerefentry> fail, the failing return to the application is
delayed by an amount of time randomly distributed (by up to 50%)
about this longest value.
</para>
<para>
Independent of success, the delay time is reset to its zero
default value when the PAM service module returns control to
the application. The delay occurs <emphasis>after</emphasis> all
authentication modules have been called, but <emphasis>before</emphasis>
control is returned to the service application.
</para>
<para>
When using this function the programmer should check if it is
available with:
</para>
<programlisting>
#ifdef HAVE_PAM_FAIL_DELAY
....
#endif /* HAVE_PAM_FAIL_DELAY */
</programlisting>
<para>
For applications written with a single thread that are event
driven in nature, generating this delay may be undesirable.
Instead, the application may want to register the delay in some
other way. For example, in a single threaded server that serves
multiple authentication requests from a single event loop, the
application might want to simply mark a given connection as
blocked until an application timer expires. For this reason
the delay function can be changed with the
<emphasis>PAM_FAIL_DELAY</emphasis> item. It can be queried and
set with
<citerefentry>
<refentrytitle>pam_get_item</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>
and
<citerefentry>
<refentrytitle>pam_set_item</refentrytitle><manvolnum>3</manvolnum>
</citerefentry> respectively. The value used to set it should be
a function pointer of the following prototype:
<programlisting>
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);
</programlisting>
The arguments being the <emphasis>retval</emphasis> return code
of the module stack, the <emphasis>usec_delay</emphasis>
micro-second delay that libpam is requesting and the
<emphasis>appdata_ptr</emphasis> that the application has associated
with the current <emphasis>pamh</emphasis>. This last value was set
by the application when it called
<citerefentry>
<refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum>
</citerefentry> or explicitly with
<citerefentry>
<refentrytitle>pam_set_item</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>.
</para>
<para>
Note that the PAM_FAIL_DELAY item is set to NULL by default. This
indicates that PAM should perform a random delay as described
above when authentication fails and a delay has been suggested.
If an application does not want the PAM library to perform any
delay on authentication failure, then the application must define
a custom delay function that executes no statements and set
the PAM_FAIL_DELAY item to point to this function.
</para>
</refsect1>
<refsect1 id='pam_fail_delay-rationale'>
<title>RATIONALE</title>
<para>
It is often possible to attack an authentication scheme by exploiting
the time it takes the scheme to deny access to an applicant user. In
cases of <emphasis>short</emphasis> timeouts, it may prove possible
to attempt a <emphasis>brute force</emphasis> dictionary attack --
with an automated process, the attacker tries all possible passwords
to gain access to the system. In other cases, where individual
failures can take measurable amounts of time (indicating the nature
of the failure), an attacker can obtain useful information about the
authentication process. These latter attacks make use of procedural
delays that constitute a <emphasis>covert channel</emphasis>
of useful information.
</para>
<para>
To minimize the effectiveness of such attacks, it is desirable to
introduce a random delay in a failed authentication process.
Preferable this value should be set by the application or a special
PAM module. Standard PAM modules should not modify the delay
unconditional.
</para>
</refsect1>
<refsect1 id='pam_fail_delay-example'>
<title>EXAMPLE</title>
<para>
For example, a login application may require a failure delay of
roughly 3 seconds. It will contain the following code:
</para>
<programlisting>
pam_fail_delay (pamh, 3000000 /* micro-seconds */ );
pam_authenticate (pamh, 0);
</programlisting>
<para>
if the modules do not request a delay, the failure delay will be
between 1.5 and 4.5 seconds.
</para>
<para>
However, the modules, invoked in the authentication process, may
also request delays:
</para>
<programlisting>
module #1: pam_fail_delay (pamh, 2000000);
module #2: pam_fail_delay (pamh, 4000000);
</programlisting>
<para>
in this case, it is the largest requested value that is used to
compute the actual failed delay: here between 2 and 6 seconds.
</para>
</refsect1>
<refsect1 id='pam_fail_delay-return_values'>
<title>RETURN VALUES</title>
<variablelist>
<varlistentry>
<term>PAM_SUCCESS</term>
<listitem>
<para>
Delay was successful adjusted.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>PAM_SYSTEM_ERR</term>
<listitem>
<para>
A NULL pointer was submitted as PAM handle.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id='pam_fail_delay-see_also'>
<title>SEE ALSO</title>
<para>
<citerefentry>
<refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>,
<citerefentry>
<refentrytitle>pam_get_item</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>,
<citerefentry>
<refentrytitle>pam_strerror</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>
</para>
</refsect1>
<refsect1 id='pam_fail_delay-standards'>
<title>STANDARDS</title>
<para>
The <function>pam_fail_delay</function> function is an
Linux-PAM extension.
</para>
</refsect1>
</refentry>
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