/etc/pam.d/su :

#
# The PAM configuration file for the Shadow `su' service
#

# This allows root to su without passwords (normal operation)
auth sufficient pam_rootok.so

# Uncomment this to force users to be a member of group root
# before they can use `su'. You can also add "group=foo"
# to the end of this line if you want to use a group other
# than the default "root" (but this may have side effect of
# denying "root" user, unless she's a member of "foo" or explicitly
# permitted earlier by e.g. "sufficient pam_rootok.so").
# (Replaces the `SU_WHEEL_ONLY' option from login.defs)
# auth required pam_wheel.so

# Uncomment this if you want wheel members to be able to
# su without a password.
# auth sufficient pam_wheel.so trust

# Uncomment this if you want members of a specific group to not
# be allowed to use su at all.
# auth required pam_wheel.so deny group=nosu

# Uncomment and edit /etc/security/time.conf if you need to set
# time restrainst on su usage.
# (Replaces the `PORTTIME_CHECKS_ENAB' option from login.defs
# as well as /etc/porttime)
# account requisite pam_time.so

# This module parses /etc/environment (the standard for setting
# environ vars) and also allows you to use an extended config
# file /etc/security/pam_env.conf.
#
# parsing /etc/environment needs "readenv=1"
session required pam_env.so readenv=1

# The standard Unix authentication modules, used with
# NIS (man nsswitch) as well as normal /etc/passwd and
# /etc/shadow entries.

e per /etc/login.defs:

#
# /etc/login.defs - Configuration control definitions for the login package.
#
# $Id: login.defs 6 2005-03-20 15:34:28Z bubulle $
#
# Three items must be defined: MAIL_DIR, ENV_SUPATH, and ENV_PATH.
# If unspecified, some arbitrary (and possibly incorrect) value will
# be assumed. All other items are optional - if not specified then
# the described action or option will be inhibited.
#
# Comment lines (lines beginning with "#") and blank lines are ignored.
#
# Modified for Linux. --marekm

#
# *REQUIRED*
# Directory where mailboxes reside, _or_ name of file, relative to the
# home directory. If you _do_ define both, MAIL_DIR takes precedence.
# QMAIL_DIR is for Qmail (or any maildir-compatible MTA, such as Exim or
# Postfix when suitably configured).
#
# Essentially, MAIL_DIR defines the $MAIL environmental variable
# (for mbox use) by appending the username to MAIL_DIR as defined
# below. MAIL_FILE defines the $MAIL environment variable as the
# fully-qualified filename obtained by prepending the user home
# directory before $MAIL_FILE, and QMAIL_DIR defines the MAIL
# environment variable as the fully-qualified directory name
# obtained by prepending the user home directory before $QMAIL_DIR.
#
# NOTE: This is used to setup your MAIL environment variable, and also
# used by userdel to determine if any mail spools need to be removed when
# removing a user. If you change this, you should also change the
# pam_mail.so module setup in /etc/pam.d/login, which affects the "You
# have mail" message on login, and, in default setup, overrides this setting
# in determining the $MAIL environmental variable.
#
#QMAIL_DIR Maildir/
MAIL_DIR /var/mail
#MAIL_FILE .mail

#
# Delay in seconds before being allowed another attempt after a login failure
#
FAIL_DELAY 3

#
# Enable logging and display of /var/log/faillog login failure info.
#
FAILLOG_ENAB yes

#
# Enable display of unknown usernames when login failures are recorded.
#
LOG_UNKFAIL_ENAB no

#
# Enable logging of successful logins
#
LOG_OK_LOGINS no

#
# Enable setting of ulimit, umask, and niceness from passwd gecos field.
#
QUOTAS_ENAB yes

#
# Enable "syslog" logging of su activity - in addition to sulog file logging.
# SYSLOG_SG_ENAB does the same for newgrp and sg.
#
SYSLOG_SU_ENAB yes
SYSLOG_SG_ENAB yes

#
# If defined, all su activity is logged to this file.
#
#SULOG_FILE /var/log/sulog

#
# If defined, file which maps tty line to TERM environment parameter.
# Each line of the file is in a format something like "vt100 tty01".
#
#TTYTYPE_FILE /etc/ttytype

#
# If defined, login failures will be logged here in a utmp format.
# last, when invoked as lastb, will read /var/log/btmp, so...
#
FTMP_FILE /var/log/btmp

#
# If defined, the command name to display when running "su -". For
# example, if this is defined as "su" then a "ps" will display the
# command is "-su". If not defined, then "ps" would display the
# name of the shell actually being run, e.g. something like "-sh".
#
SU_NAME su

#
# If defined, file which inhibits all the usual chatter during the login
# sequence. If a full pathname, then hushed mode will be enabled if the
# user's name or shell are found in the file. If not a full pathname, then
# hushed mode will be enabled if the file exists in the user's home directory.
#
HUSHLOGIN_FILE .hushlogin
#HUSHLOGIN_FILE /etc/hushlogins

#
# If defined, the presence of this value in an /etc/passwd "shell" field will
# disable logins for that user, although "su" will still be allowed.
#
# XXX this does not seem to be implemented yet... --marekm
# no, it was implemented but I ripped it out -- jfh
NOLOGIN_STR NOLOGIN

#
# If defined, either a TZ environment parameter spec or the
# fully-rooted pathname of a file containing such a spec.
#
#ENV_TZ TZ=CST6CDT
#ENV_TZ /etc/tzname

#
# If defined, an HZ environment parameter spec.
#
# for Linux/x86
ENV_HZ HZ=100
# For Linux/Alpha...
#ENV_HZ HZ=1024

#
# *REQUIRED* The default PATH settings, for superuser and normal users.
#
# (they are minimal, add the rest in the shell startup files)
ENV_SUPATH PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/bin/X11:/usr/local/sbin:/usr/local/bin
ENV_PATH PATH=/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games

#
# Terminal permissions
#
# TTYGROUP Login tty will be assigned this group ownership.
# TTYPERM Login tty will be set to this permission.
#
# If you have a "write" program which is "setgid" to a special group
# which owns the terminals, define TTYGROUP to the group number and
# TTYPERM to 0620. Otherwise leave TTYGROUP commented out and assign
# TTYPERM to either 622 or 600.
#
TTYGROUP tty
TTYPERM 0600

#
# Login configuration initializations:
#
# ERASECHAR Terminal ERASE character ('\010' = backspace).
# KILLCHAR Terminal KILL character ('\025' = CTRL/U).
# UMASK Default "umask" value.
# ULIMIT Default "ulimit" value.
#
# The ERASECHAR and KILLCHAR are used only on System V machines.
# The ULIMIT is used only if the system supports it.
# (now it works with setrlimit too; ulimit is in 512-byte units)
#
# Prefix these values with "0" to get octal, "0x" to get hexadecimal.
#
ERASECHAR 0177
KILLCHAR 025
UMASK 022
#ULIMIT 2097152

#
# Password aging controls:
#
# PASS_MAX_DAYS Maximum number of days a password may be used.
# PASS_MIN_DAYS Minimum number of days allowed between password changes.
# PASS_WARN_AGE Number of days warning given before a password expires.
#
PASS_MAX_DAYS 99999
PASS_MIN_DAYS 0
PASS_WARN_AGE 7

#
# Min/max values for automatic uid selection in useradd
#
UID_MIN 1000
UID_MAX 60000

#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 100
GID_MAX 60000

#
# Max number of login retries if password is bad. This will most likely be
# overriden by PAM, since the default pam_unix module has it's own built
# in of 3 retries However, this is a safe fallback in case you are using
# and authentication module that does not enforce PAM_MAXTRIES.
#
LOGIN_RETRIES 5

#
# Max time in seconds for login
#
LOGIN_TIMEOUT 60

#
# Number of significant characters in the password for crypt().
# Default is 8, don't change unless your crypt() is better.
# If using MD5 in your PAM configuration, set this higher.
#
PASS_MAX_LEN 8

#
# Require password before chfn/chsh can make any changes.
#
CHFN_AUTH yes

#
# Which fields may be changed by regular users using chfn - use
# any combination of letters "frwh" (full name, room number, work
# phone, home phone). If not defined, no changes are allowed.
# For backward compatibility, "yes" = "rwh" and "no" = "frwh".
#
CHFN_RESTRICT rwh

#
# Password prompt (%s will be replaced by user name).
#
# XXX - it doesn't work correctly yet, for now leave it commented out
# to use the default which is just "Password: ".
#LOGIN_STRING "%s's Password: "

#
# Should login be allowed if we can't cd to the home directory?
# Default in no.
#
DEFAULT_HOME yes

#
# If defined, this command is run when removing a user.
# It should remove any at/cron/print jobs etc. owned by
# the user to be removed (passed as the first argument).
#
#USERDEL_CMD /usr/sbin/userdel_local

#
# If defined, either full pathname of a file containing device names or
# a ":" delimited list of device names. No password is required to log in
# as a non-root user on these devices.
#
#NO_PASSWORD_CONSOLE tty1:tty2:tty3:tty4:tty5:tty6

#
# When prompting for password without echo, getpass() can optionally
# display a random number (in the range 1 to GETPASS_ASTERISKS) of '*'
# characters for each character typed. This feature is designed to
# confuse people looking over your shoulder when you enter a password .
# Also, the new getpass() accepts both Backspace (8) and Delete (127)
# keys to delete previous character (to cope with different terminal
# types), Control-U to delete all characters, and beeps when there are
# no more characters to delete, or too many characters entered.
#
# Setting GETPASS_ASTERISKS to 1 results in more traditional behaviour -
# exactly one '*' displayed for each character typed.
#
# Setting GETPASS_ASTERISKS to 0 disables the '*' characters (Backspace,
# Delete, Control-U and beep continue to work as described above).
#
# Setting GETPASS_ASTERISKS to -1 reverts to the traditional getpass()
# without any new features. This is the default.
#
#GETPASS_ASTERISKS 1

#
# Enable setting of the umask group bits to be the same as owner bits
# (examples: 022 -> 002, 077 -> 007) for non-root users, if the uid is
# the same as gid, and username is the same as the primary group name.
#
# This also enables userdel to remove user groups if no members exist.
#
USERGROUPS_ENAB yes

#
# Instead of the real user shell, the program specified by this parameter
# will be launched, although its visible name (argv[0]) will be the shell's.
# The program may do whatever it wants (logging, additional authentification,
# banner, ...) before running the actual shell.
#
# FAKE_SHELL /bin/fakeshell

#
# Enable pam_close_session() calling. When using normal (pam_unix.so)
# session handling modules, this is not needed. However with modules
# (such as kerberos or other persistent session models), login and su
# need to fork and wait for the shell to exit so that sessions can be
# cleaned up.
#
CLOSE_SESSIONS no

################# OBSOLETED BY PAM ##############
# #
# These options are now handled by PAM. Please #
# edit the appropriate file in /etc/pam.d/ to #
# enable the equivelants of them.
#
###############

#MOTD_FILE
#DIALUPS_CHECK_ENAB
#LASTLOG_ENAB
#MAIL_CHECK_ENAB
#OBSCURE_CHECKS_ENAB
#PORTTIME_CHECKS_ENAB
#CONSOLE
#SU_WHEEL_ONLY
#CRACKLIB_DICTPATH
#PASS_CHANGE_TRIES
#PASS_ALWAYS_WARN
#MD5_CRYPT_ENAB
#CONSOLE_GROUPS
#ENVIRON_FILE
#NOLOGINS_FILE
#ISSUE_FILE
#PASS_MIN_LEN

e se provo ad eseguirli mi scrive:

bash: /etc/pam.d/su: Permission denied

ho sbagliato qualche aggiornamento???