SELinux - soluzioni

Esercitazione 1

Visualizzazione dello stato di SELinux

  - Visualizzazione dello stato di SELinux <code>
[root@statichostname ~]# sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Max kernel policy version:      31
[root@statichostname ~]# getenforce
Enforcing

Visualizzazione del context dei processi di systemd e cron

[root@statichostname ~]# ps -e -Z | grep -e systemd -e cron
system_u:system_r:init_t:s0         1 ?        00:00:01 systemd
system_u:system_r:udev_t:s0-s0:c0.c1023 496 ?  00:00:00 systemd-udevd
system_u:system_r:systemd_logind_t:s0 768 ?    00:00:00 systemd-logind
system_u:system_r:crond_t:s0-s0:c0.c1023 1285 ? 00:00:00 atd
system_u:system_r:crond_t:s0-s0:c0.c1023 1287 ? 00:00:00 crond
system_u:system_r:syslogd_t:s0   1928 ?        00:00:00 systemd-journal

Visualizzazione del context dei file /etc/passwd, /etc/shadow, /etc/cron.d (verificare la presenza degli extended attributes corrispondenti)

[root@statichostname ~]# ls -Z -d /etc/passwd /etc/shadow /etc/cron.d /etc/cron.d/sysstat 
drwxr-xr-x. root root system_u:object_r:system_cron_spool_t:s0 /etc/cron.d
-rw-------. root root system_u:object_r:system_cron_spool_t:s0 /etc/cron.d/sysstat
-rw-r--r--. root root system_u:object_r:passwd_file_t:s0 /etc/passwd
----------. root root system_u:object_r:shadow_t:s0    /etc/shadow

[root@statichostname ~]# getfattr -d -m ".*" /etc/passwd /etc/shadow
getfattr: Removing leading '/' from absolute path names
# file: etc/passwd
security.selinux="system_u:object_r:passwd_file_t:s0"

# file: etc/shadow
security.selinux="system_u:object_r:shadow_t:s0"

Visualizzazione degli SELinux user e associazione con role e level

[root@statichostname ~]# semanage user -l

                Labeling   MLS/       MLS/                          
SELinux User    Prefix     MCS Level  MCS Range                      SELinux Roles

guest_u         user       s0         s0                             guest_r
root            user       s0         s0-s0:c0.c1023                 staff_r sysadm_r system_r unconfined_r
staff_u         user       s0         s0-s0:c0.c1023                 staff_r sysadm_r system_r unconfined_r
sysadm_u        user       s0         s0-s0:c0.c1023                 sysadm_r
system_u        user       s0         s0-s0:c0.c1023                 system_r unconfined_r
unconfined_u    user       s0         s0-s0:c0.c1023                 system_r unconfined_r
user_u          user       s0         s0                             user_r
xguest_u        user       s0         s0                             xguest_r

Visualizzazione della associazione degli user agli SELinux user

[root@statichostname ~]# semanage login -l

Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *
system_u             system_u             s0-s0:c0.c1023       *

Visualizzare i context di httpd e della sua DocumentRoot ed identificare la rule che permette l'accesso
Utilizzare sesearch (man sesearch) per trovare la rule)

# Far partire httpd e visualizzare il context del processo

[root@statichostname ~]# systemctl start httpd
[root@statichostname ~]# ps -e -Z | grep httpd
system_u:system_r:httpd_t:s0     4021 ?        00:00:00 httpd
system_u:system_r:httpd_t:s0     4022 ?        00:00:00 httpd
...

# visualizzare il context della attuale document root (creare anche una index.html vuota)

[root@statichostname ~]# ls -lZd /var/www/html/
drwxr-xr-x. root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
[root@statichostname ~]# touch /var/www/html/index.html
[root@statichostname ~]# ls -lZd /var/www/html/index.html 
-rw-r--r--. root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/index.html

# cercare con sesearch la regola che permette a httpd di leggere i file della DocumentRoot

[root@statichostname ~]# sesearch --allow --source httpd_t --target httpd_sys_content_t --class file
Found 5 semantic av rules:
   allow httpd_t httpd_content_type : file { ioctl read getattr lock open } ; 
->   allow httpd_t httpd_sys_content_t : file { ioctl read getattr lock open } ; 
   allow httpd_t httpdcontent : file { ioctl read write create getattr setattr lock append unlink link rename open } ; 
   allow httpd_t httpdcontent : file { read getattr execute open } ; 
   allow httpd_t httpd_content_type : file { ioctl read getattr lock open } ;

Verificare che cambiando il context type di /var/www/html/index.html in var_t l'accesso a httpd e' negato, quindi restorare il context del file e verificare che l'accesso e' consentito

[root@statichostname ~]# chcon -t var_t /var/www/html/index.html 

[root@statichostname ~]# ls -Z /var/www/html/index.html 
-rw-r--r--. root root unconfined_u:object_r:var_t:s0   /var/www/html/index.html

[root@statichostname ~]# wget http://127.0.0.1:80/index.html
--2018-11-28 05:49:52--  http://127.0.0.1/index.html
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2018-11-28 05:49:52 ERROR 403: Forbidden.

[root@statichostname ~]# restorecon /var/www/html/index.html 
[root@statichostname ~]# ls -Z /var/www/html/index.html 
-rw-r--r--. root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/index.html
[root@statichostname ~]# wget http://127.0.0.1:80/index.html
--2018-11-28 06:00:52--  http://127.0.0.1/index.html
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/html]
Saving to: ‘index.html’

    [ <=>                                                                                                                                      ] 0           --.-K/s   in 0s      

2018-11-28 06:00:52 (0.00 B/s) - ‘index.html’ saved [0/0]

Spostare la DocumentRoot di httpd in /www/html
Per fare questo si deve: modificare la configurazione di httpd, creare la nuova document root, verificare che non funziona, assegnare il default context (semanage fcontext), e modificare il context (restorecon)

# creare le directory

[root@statichostname ~]# mkdir -p /www/html
[root@statichostname ~]# ls -lZd /www/html/
drwxr-xr-x. root root unconfined_u:object_r:default_t:s0 /www/html/

# default_t e' il default type se non ci sono configurazioni apposite

[root@statichostname ~]# cp /var/www/html/index.html /www/html/
[root@statichostname ~]# ls -Z /www/html/index.html 
-rw-r--r--. root root unconfined_u:object_r:default_t:s0 /www/html/index.html

# modificare la configurazione di httpd

[root@statichostname ~]# vi /etc/httpd/conf/httpd.conf 
[root@statichostname ~]# grep ^DocumentRoot -A15 /etc/httpd/conf/httpd.conf 
DocumentRoot "/www/html"

#
# Relax access to content within /var/www.
#
#<Directory "/var/www">
#    AllowOverride None
#    # Allow open access:
#    Require all granted
#</Directory>
<Directory "/www">
    AllowOverride None
    # Allow open access:
    Require all granted
</Directory>

[root@statichostname ~]# systemctl reload httpd

# verificare che l'accesso e' negato

[root@statichostname ~]# wget http://127.0.0.1:80/index.html
--2018-11-28 06:14:37--  http://127.0.0.1/index.html
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2018-11-28 06:14:37 ERROR 403: Forbidden.

# identificare tramite matchpathconf quale type devono avere i file della DocumentRoot

[root@statichostname ~]# matchpathcon -m file /var/www/html/x.html
/var/www/html/x.html	system_u:object_r:httpd_sys_content_t:s0

# utilizzare semanage per modificare il context di configurazione della nuova document root

[root@statichostname ~]# semanage fcontext --add --type httpd_sys_content_t "/www(/.*)?"
[root@statichostname ~]# semanage fcontext --add --type httpd_sys_content_t "/www/html(/.*)?"

# verificare che le entry aggiunte compaiono nel file di configuraizone (locale) file_context.local

[root@statichostname ~]# cat /etc/selinux/targeted/contexts/files/file_contexts.local
# This file is auto-generated by libsemanage
# Do not edit directly.

/www(/.*)?    system_u:object_r:httpd_sys_content_t:s0
/www/html(/.*)?    system_u:object_r:httpd_sys_content_t:s0

# restorare il context della nuova documento root al type della configurazione

[root@statichostname ~]# restorecon -Rv /www
restorecon reset /www context unconfined_u:object_r:default_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
restorecon reset /www/html context unconfined_u:object_r:default_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
restorecon reset /www/html/index.html context unconfined_u:object_r:default_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0

[root@statichostname ~]# ls -Z /www/html/index.html 
-rw-r--r--. root root unconfined_u:object_r:httpd_sys_content_t:s0 /www/html/index.html

# verificare che l'accesso e' garantito

[root@statichostname ~]# wget http://127.0.0.1:80/index.html
--2018-11-28 06:42:31--  http://127.0.0.1/index.html
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/html]
Saving to: ‘index.html.1’

    [ <=>                                                                                                                                      ] 0           --.-K/s   in 0s      

2018-11-28 06:42:31 (0.00 B/s) - ‘index.html.1’ saved [0/0]

Esercitazione 2

Utilizzare seinfo per visualizzare l'esistenza di un type init_t, l'elenco degli attributi associati a questo type e verificare che domain sia tra questi

[root@statichostname ~]# seinfo -t | grep init_t
   cloud_init_tmp_t
   run_init_t
   cloud_init_t
   init_tmp_t
->   init_t
   namespace_init_t
[root@statichostname ~]# seinfo -tinit_t -x
   init_t
...
      domain
...

Utilizzare seinfo per visualizzare l'esistenza dell'attribute domain, l'elenco di type che hanno domain tra i loro attributi e verificare che init_t sia tra questi

[root@statichostname ~]# seinfo -adomain
   domain
[root@statichostname ~]# seinfo -adomain -x | grep init_t
      cloud_init_t
->      init_t
      namespace_init_t
      run_init_t

Domain transition: identificare le policy che permettono la domain transition da systemd a httpd

Visualizzare domain e type di systemd, httpd, /usr/sbin/httpd

root@statichostname ~]# ps -eZ | grep systemd$
system_u:system_r:init_t:s0         1 ?        00:00:03 systemd
[root@statichostname ~]# ls -Z /usr/sbin/httpd 
-rwxr-xr-x. root root system_u:object_r:httpd_exec_t:s0 /usr/sbin/httpd
[root@statichostname ~]# ps -eZ | grep httpd | head -n 1
system_u:system_r:httpd_t:s0     4021 ?        00:00:01 httpd

Verificare l'esistenza di una rule che permetta a systemd di eseguire /usr/sbin/httpd

[root@statichostname ~]# sesearch -s init_t -t httpd_exec_t -c file -A
Found 3 semantic av rules:
...
   allow initrc_domain direct_init_entry : file { read getattr execute open } ; 
...

# La regola e' permessa perche' init_t ha l'attributo initrc_domain:

[root@statichostname ~]# seinfo -ainitrc_domain -x | grep init_t
      init_t
e perche' httpd_exec_t ha l'attributo direct_init_entry
[root@statichostname ~]# seinfo -adirect_init_entry -x | grep http
      httpd_exec_t
      httpd_rotatelogs_exec_t

Verificare che esista una rule che definisca che il type dell'eseguibile sia entrypoint del dominio di destinazione

[root@statichostname ~]# sesearch -s httpd_t -p entrypoint -A
Found 3 semantic av rules:
   allow sepgsql_client_type sepgsql_temp_object_t : db_procedure { create drop getattr setattr execute entrypoint install } ; 
   allow sepgsql_client_type sepgsql_trusted_procedure_type : db_procedure { getattr execute entrypoint } ; 
->   allow httpd_t httpd_exec_t : file { ioctl read getattr lock execute execute_no_trans entrypoint open } ;

Verificare che esista una rule che permette al parent domain (init_t) una transizione verso il destination domain (httpd_t)

[root@statichostname ~]# sesearch -s init_t -t httpd_t -p transition -A
Found 1 semantic av rules:
   allow initrc_domain daemon : process transition ; 

# initrc_domain e' attributo di init_t (gia' visto), e daemon e' attributo di httpd_t.

[root@statichostname ~]# seinfo -adaemon -x | grep httpd_t
      httpd_t

Verificare l'esistenza di una transition rule che definisca il default destination domain conseguente alla esecuzione di un eseguibile di type httpd_exec_t da parte di un processo di domain init_t
```
[root@statichostname ~]# sesearch -T -s init_t -t httpd_exec_t
Found 1 semantic te rules:
   type_transition init_t httpd_exec_t : process httpd_t; 
```

Esercitazione 3

Confinamento di un utente: confinare user1 a user_u (verifica impossibilita' di fare su)

Visualizzazione context del processo di login dell'utente user1 e verifica che l'utente user1 possa eseguire su e diventare user2

[user1@statichostname ~]$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[user1@statichostname ~]$ su - user2
Password: 
Attempting to create directory /home/user2/perl5
[user2@statichostname ~]$ logout
[user1@statichostname ~]$ logout
Connection to 192.168.56.102 closed.

Confinare il linux user user1 associandolo all'SELinux user user_u

[root@statichostname ~]# semanage login -a -s user_u user1
[root@statichostname ~]# semanage login -l

Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *
system_u             system_u             s0-s0:c0.c1023       *
user1                user_u               s0                   *

Verificare che user1, il cui dominio attuale e' user_t, non puo' fare su e diventare user2

# ssh user1@192.168.56.102
user1@192.168.56.102's password: 
Warning: No xauth data; using fake authentication data for X11 forwarding.
Last login: Wed Nov 28 18:45:06 2018 from 192.168.56.1
[user1@statichostname ~]$ id -Z
user_u:user_r:user_t:s0
[user1@statichostname ~]$ su - user2
Password: 
su: Authentication failure

Visualizzazione del policy module apache

[root@statichostname ~]# semodule -l | grep apache
apache	2.7.2
[root@statichostname ~]# semodule -lfull | grep apache
100 apache            pp         
[root@statichostname ~]# ls /etc/selinux/targeted/active/modules/100/apache/
cil  hll  lang_ext
[root@statichostname ~]# cat /etc/selinux/targeted/active/modules/100/apache/cil | bunzip2 > /tmp/apache.txt
[root@statichostname ~]# wc -l /tmp/apache.txt 
4116 /tmp/apache.txt

Abilitare tramite boolean httpd a leggere le home directory degli utenti

[root@statichostname ~]# semanage boolean -l  | grep http | grep user
httpd_read_user_content        (off  ,  off)  Allow httpd to read user content
[root@statichostname ~]# sesearch -b httpd_read_user_content -AC
Found 40 semantic av rules:
DT allow httpd_user_script_t user_home_type : dir { getattr search open } ; [ httpd_read_user_content ]
DT allow httpd_user_script_t user_home_type : dir { ioctl read getattr lock search open } ; [ httpd_read_user_content ]
...
[root@statichostname ~]# semanage boolean --modify --on httpd_read_user_content
[root@statichostname ~]# semanage boolean -l  httpd_read_user_content
list option can not be used with --boolean
[root@statichostname ~]# semanage boolean -l  | grep httpd_read_user_content
httpd_read_user_content        (on   ,   on)  Allow httpd to read user content

o alternativamente tramite getsebool e setsebool

[root@statichostname ~]# getsebool httpd_read_user_content
httpd_read_user_content --> off
[root@statichostname ~]# setsebool -P httpd_read_user_content on
[root@statichostname ~]# getsebool httpd_read_user_content
httpd_read_user_content --> on
[root@statichostname ~]# semanage boolean -l  | grep httpd_read_user_content
httpd_read_user_content        (on   ,   on)  Allow httpd to read user content
[root@statichostname ~]#

Esercitazione 4

Generare un messaggio di errore eseguendo il comando su come user user1

[root@statichostname ~]# semanage login -lC

Login Name           SELinux User         MLS/MCS Range        Service

user1                user_u               s0                   *


[user1@statichostname ~]$ su - user2
Password: 
su: Authentication failure

Analizzare l'errore e generare un modulo che contenga le rule per rendere lecita l'operazione

[root@statichostname ~]# ausearch -m avc --start recent
----
time->Thu Nov 29 14:38:06 2018
type=PROCTITLE msg=audit(1543498686.922:223): proctitle=7375002D007573657232
type=SYSCALL msg=audit(1543498686.922:223): arch=c000003e syscall=105 success=yes exit=0 a0=0 a1=0 a2=800020 a3=7f0bc56d9300 items=0 ppid=2459 pid=2460 auid=1000 uid=1000 gid=1000 euid=0 suid=0 fsuid=0 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=6 comm="su" exe="/usr/bin/su" subj=user_u:user_r:user_t:s0 key=(null)
type=AVC msg=audit(1543498686.922:223): avc:  denied  { setuid } for  pid=2460 comm="su" capability=7  scontext=user_u:user_r:user_t:s0 tcontext=user_u:user_r:user_t:s0 tclass=capability
----
time->Thu Nov 29 14:38:11 2018
type=PROCTITLE msg=audit(1543498691.746:226): proctitle=7375002D007573657232
type=SYSCALL msg=audit(1543498691.746:226): arch=c000003e syscall=2 success=no exit=-13 a0=55763c0f20d0 a1=1 a2=99b a3=5bffebc3 items=0 ppid=2305 pid=2459 auid=1000 uid=1000 gid=1000 euid=0 suid=0 fsuid=0 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=6 comm="su" exe="/usr/bin/su" subj=user_u:user_r:user_t:s0 key=(null)
type=AVC msg=audit(1543498691.746:226): avc:  denied  { write } for  pid=2459 comm="su" name="btmp" dev="dm-0" ino=33860287 scontext=user_u:user_r:user_t:s0 tcontext=system_u:object_r:faillog_t:s0 tclass=file
----
time->Thu Nov 29 14:38:09 2018
type=PROCTITLE msg=audit(1543498689.416:224): proctitle=7375002D007573657232
type=SYSCALL msg=audit(1543498689.416:224): arch=c000003e syscall=105 success=yes exit=0 a0=0 a1=55763dbd8840 a2=800020 a3=7f0bc56d9300 items=0 ppid=2459 pid=2461 auid=1000 uid=1000 gid=1000 euid=0 suid=0 fsuid=0 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=6 comm="su" exe="/usr/bin/su" subj=user_u:user_r:user_t:s0 key=(null)
type=AVC msg=audit(1543498689.416:224): avc:  denied  { setuid } for  pid=2461 comm="su" capability=7  scontext=user_u:user_r:user_t:s0 tcontext=user_u:user_r:user_t:s0 tclass=capability

[root@statichostname ~]# ausearch -m avc --start recent | audit2allow 


#============= user_t ==============
allow user_t faillog_t:file write;

#!!!! This avc can be allowed using the boolean 'selinuxuser_use_ssh_chroot'
allow user_t self:capability setuid;


[root@statichostname ~]# ausearch -m avc --start recent | audit2allow -M mylocalmodule
******************** IMPORTANT ***********************
To make this policy package active, execute:

semodule -i mylocalmodule.pp

[root@statichostname ~]# ls -l mylocalmodule.*
-rw-r--r--. 1 root root 1120 Nov 29 14:47 mylocalmodule.pp
-rw-r--r--. 1 root root  304 Nov 29 14:47 mylocalmodule.te
[root@statichostname ~]#

Caricare il modulo generato e verificare che l'operazione ora e' permessa

[root@statichostname ~]# semodule -i mylocalmodule.pp
[root@statichostname ~]# semodule -lfull | grep mylocalmodule
400 mylocalmodule     pp         
[root@statichostname ~]# ls /etc/selinux/targeted/active/modules/
100/      400/      disabled/ 
[root@statichostname ~]# ls /etc/selinux/targeted/active/modules/400/
mylocalmodule
[root@statichostname ~]# ls /etc/selinux/targeted/active/modules/400/mylocalmodule/
cil  hll  lang_ext
[root@statichostname ~]# 

[user1@statichostname ~]$ su - user2
Password: 
[user2@statichostname ~]$

Disabilitare e rimuovere il modulo, verificando che l'operazione e' nuovamente proibita

[root@statichostname ~]# semodule -d mylocalmodule
[root@statichostname ~]# semodule -lfull | grep mylocalmodule
400 mylocalmodule     pp disabled
[root@statichostname ~]# semodule -r mylocalmodule
libsemanage.semanage_direct_remove_key: Removing last mylocalmodule module (no other mylocalmodule module exists at another priority).
[root@statichostname ~]# semodule -lfull | grep mylocalmodule

[user1@statichostname ~]$ su - user2
Password: 
su: Authentication failure

INFN wiki

Table of Contents

SELinux - soluzioni

Esercitazione 1

Esercitazione 2

Esercitazione 3

Esercitazione 4