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14.4. Introdução ao AppArmor

14.4.1. Princípios

AppArmor é um sistema de Controle de Acesso Mandatório (MAC - Mandatory Access Control) construído sobre a interface LSM (Linux Security Modules) do Linux. Na prática, o kernel consulta o AppArmor antes de cada chamada do sistema para saber se o processo está autorizado a fazer a operação dada. Através desse mecanismo, o AppArmor confina programas a um conjunto limitado de recursos.
AppArmor applies a set of rules (known as “profile”) on each program. The profile applied by the kernel depends on the installation path of the program being executed. Contrary to SELinux (discussed in Seção 14.5, “Introdução ao SELinux”), the rules applied do not depend on the user. All users face the same set of rules when they are executing the same program (but traditional user permissions still apply and might result in different behavior!).
Os perfis AppArmor são armazenados em /etc/apparmor.d/ e eles contém uma lista de regras de controle de acesso em recursos que cada programa pode fazer uso. Os perfis são compilados e carregados no núcleo pelo comando apparmor_parser. Cada perfil pode ser carregado tanto em modo de aplicação ("enforcing") quanto em modo de registro ("complaining"). O primeiro aplica a política e reporta as tentativas de violação, enquanto que o último não aplica a política mas mantém os registros de chamadas de sistema que deveriam ter sido negadas.

14.4.2. Habilitando o AppArmor e gerenciando os perfis AppArmor

AppArmor support is built into the standard kernels provided by Debian. Enabling AppArmor is thus just a matter of installing some packages by executing apt install apparmor apparmor-profiles apparmor-utils with root privileges.
AppArmor is functional after the installation, and aa-status will confirm it quickly:
# aa-status
apparmor module is loaded.
40 profiles are loaded.
23 profiles are in enforce mode.
   /usr/bin/evince
   /usr/bin/evince-previewer
[...]
17 profiles are in complain mode.
   /usr/sbin/dnsmasq
[...]
14 processes have profiles defined.
12 processes are in enforce mode.
   /usr/bin/evince (3462)
[...]
2 processes are in complain mode.
   /usr/sbin/avahi-daemon (429) avahi-daemon
   /usr/sbin/avahi-daemon (511) avahi-daemon
0 processes are unconfined but have a profile defined.
The state of each profile can be switched between enforcing and complaining with calls to aa-enforce and aa-complain giving as parameter either the path of the executable or the path to the policy file. Additionally a profile can be entirely disabled with aa-disable or put in audit mode (to log accepted system calls too) with aa-audit.
# aa-enforce /usr/bin/pidgin
Setting /usr/bin/pidgin to enforce mode.
# aa-complain /usr/sbin/dnsmasq
Setting /usr/sbin/dnsmasq to complain mode.

14.4.3. Criando um novo perfil

Mesmo sendo bem fácil criar um perfil AppArmor, a maioria dos programas não tem um. Essa seção irá mostrar a você como criar um novo perfil a partir do zero apenas usando o programa alvo e deixando o AppArmor monitorar a chamada de sistema que ele faz e os recursos que ele acessa.
Os programas mais importantes que precisam ser confinados são os programas voltados para a rede, pois são mais atrativos à ataques remotos. É por isso que o AppArmor convenientemente fornece o comando aa-unconfined para listar os programas que não tem perfil associado e que expôem um soquete de rede aberto. Com a opção --paranoid você tem todos os processos não confinados que tem ao menos uma conexão de rede ativa.
# aa-unconfined
801 /sbin/dhclient not confined
409 /usr/sbin/NetworkManager not confined
411 /usr/sbin/cupsd confined by '/usr/sbin/cupsd (enforce)'
429 /usr/sbin/avahi-daemon confined by 'avahi-daemon (enforce)'
516 /usr/sbin/cups-browsed confined by '/usr/sbin/cups-browsed (enforce)'
538 /usr/sbin/zebra not confined
591 /usr/sbin/named not confined
847 /usr/sbin/mysqld not confined
849 /usr/sbin/sshd not confined
1013 /usr/sbin/dhclient (/sbin/dhclient) not confined
1276 /usr/sbin/apache2 not confined
1322 /usr/sbin/apache2 not confined
1323 /usr/sbin/apache2 not confined
1324 /usr/sbin/apache2 not confined
1325 /usr/sbin/apache2 not confined
1327 /usr/sbin/apache2 not confined
1829 /usr/lib/ipsec/charon confined by '/usr/lib/ipsec/charon (enforce)'
2132 /usr/sbin/exim4 not confined
12865 /usr/bin/python3.7 (/usr/bin/python3) not confined
12873 /usr/bin/python3.7 (/usr/bin/python3) not confined
In the following example, we will thus try to create a profile for /sbin/dhclient. For this we will use aa-genprof dhclient. In Debian Buster there is a known bug[6] that makes the previous command fail with the following error: ERROR: Include file /etc/apparmor.d/local/usr.lib.dovecot.deliver not found. To fix it create the missing files with touch file. It will invite you to use the application in another window and when done to come back to aa-genprof to scan for AppArmor events in the system logs and convert those logs into access rules. For each logged event, it will make one or more rule suggestions that you can either approve or further edit in multiple ways:
# aa-genprof dhclient
Writing updated profile for /usr/sbin/dhclient.
Setting /usr/sbin/dhclient to complain mode.

Before you begin, you may wish to check if a
profile already exists for the application you
wish to confine. See the following wiki page for
more information:
https://gitlab.com/apparmor/apparmor/wikis/Profiles

Profiling: /usr/sbin/dhclient

Please start the application to be profiled in
another window and exercise its functionality now.

Once completed, select the "Scan" option below in
order to scan the system logs for AppArmor events.

For each AppArmor event, you will be given the
opportunity to choose whether the access should be
allowed or denied.

[(S)can system log for AppArmor events] / (F)inish
Reading log entries from /var/log/syslog.
Updating AppArmor profiles in /etc/apparmor.d.

Profile:  /usr/sbin/dhclient 1
Execute:  /usr/sbin/dhclient-script
Severity: unknown

(I)nherit / (C)hild / (P)rofile / (N)amed / (U)nconfined / (X) ix On / (D)eny / Abo(r)t / (F)inish
P
Should AppArmor sanitise the environment when
switching profiles?

Sanitising environment is more secure,
but some applications depend on the presence
of LD_PRELOAD or LD_LIBRARY_PATH.

(Y)es / [(N)o]
Y
Writing updated profile for /usr/sbin/dhclient-script.
Complain-mode changes:

Profile:    /usr/sbin/dhclient 2
Capability: net_raw
Severity:   8

 [1 - capability net_raw,]
[(A)llow] / (D)eny / (I)gnore / Audi(t) / Abo(r)t / (F)inish
A
Adding capability net_raw to profile.

Profile:  /sbin/dhclient
Capability: net_bind_service
Severity:   8

 [1 - #include <abstractions/nis> ]
  2 - capability net_bind_service,
(A)llow / [(D)eny] / (I)gnore / Audi(t) / Abo(r)t / (F)inish
A
Adding #include <abstractions/nis> to profile.

Profile:  /usr/sbin/dhclient 3
Path:     /etc/ssl/openssl.cnf
New Mode: owner r
Severity: 2

 [1 - #include <abstractions/lightdm>]
  2 - #include <abstractions/openssl>
  3 - #include <abstractions/ssl_keys>
  4 - owner /etc/ssl/openssl.cnf r,
(A)llow / [(D)eny] / (I)gnore / (G)lob / Glob with (E)xtension / (N)ew / Audi(t) / (O)wner permissions off / Abo(r)t / (F)inish
2

Profile:  /usr/sbin/dhclient
Path:     /etc/ssl/openssl.cnf
New Mode: owner r
Severity: 2

  1 - #include <abstractions/lightdm>
 [2 - #include <abstractions/openssl>]
  3 - #include <abstractions/ssl_keys>
  4 - owner /etc/ssl/openssl.cnf r,
[(A)llow] / (D)eny / (I)gnore / (G)lob / Glob with (E)xtension / (N)ew / Abo(r)t / (F)inish / (M)ore
A
[...]
Profile:  /usr/sbin/dhclient-script 4
Path:     /usr/bin/dash
New Mode: owner r
Severity: unknown

 [1 - #include <abstractions/lightdm>]
  2 - #include <abstractions/ubuntu-browsers.d/plugins-common>
  3 - owner /usr/bin/dash r,
(A)llow / [(D)eny] / (I)gnore / (G)lob / Glob with (E)xtension / (N)ew / Audi(t) / (O)wner permissions off / Abo(r)t / (F)inish
A
Adding #include <abstractions/lightdm> to profile.
Deleted 2 previous matching profile entries.

= Changed Local Profiles =

The following local profiles were changed. Would you like to save them?

 [1 - /usr/sbin/dhclient]
  2 - /usr/sbin/dhclient-script
(S)ave Changes / Save Selec(t)ed Profile / [(V)iew Changes] / View Changes b/w (C)lean profiles / Abo(r)t
S
Writing updated profile for /usr/sbin/dhclient.
Writing updated profile for /usr/sbin/dhclient-script.

Profiling: /usr/sbin/dhclient

Please start the application to be profiled in
another window and exercise its functionality now.

Once completed, select the "Scan" option below in
order to scan the system logs for AppArmor events.

For each AppArmor event, you will be given the
opportunity to choose whether the access should be
allowed or denied.

[(S)can system log for AppArmor events] / (F)inish
F
Reloaded AppArmor profiles in enforce mode.

Please consider contributing your new profile!
See the following wiki page for more information:
https://gitlab.com/apparmor/apparmor/wikis/Profiles

Finished generating profile for /usr/sbin/dhclient.
Note que o programa não exibe de volta os caracteres de controle que você digitou, mas para clarear a explicação eu inclui eles na transcrição prévia.

1

O primeiro evento detectado é a execução de outro programa. Neste caso, você tem múltiplas escolhas: você pode rodar o programa com o perfil do processo pai (a escolha “Inherit”), você pode rodá-lo com seu próprio e dedicado perfil (as escolhas “Profile” e “Named”, diferindo apenas pela possibilidade de usar um nome de perfil arbitrário), você pode rodá-lo com um sub-perfil do processo pai (a escolha “Child”), você pode rodá-lo sem qualquer perfil (a escolha “Unconfined”) ou você pode decidir não rodá-lo de forma alguma (a escolha “Deny”).
Note que quando você opta por rodá-lo sob um perfil dedicado que não existe ainda, a ferramenta irá criar o perfil em falta para você e irá fazer sugestões de regras para esse perfil nesta mesma execução.

2

A nível do kernel, os poderes especiais do usuário root foram divididos em "recursos" ("capabilities"). Quando uma chamada de sistema requer um recurso específico, o AppArmor irá verificar se o perfil permite ao programa fazer uso desse recurso.

3

Here the program seeks read permissions for /etc/ssl/openssl.cnf. aa-genprof detected that this permission was also granted by multiple “abstractions” and offers them as alternative choices. An abstraction provides a reusable set of access rules grouping together multiple resources that are commonly used together. In this specific case, the file is generally accessed through the nameservice related functions of the C library and we type “2” to first select the “#include <abstractions/openssl>” choice and then “A” to allow it.

4

Notice that this access request is not part of the dhclient profile but of the new profile that we created when we allowed /usr/sbin/dhclient-script to run with its own profile.
Após termos passado por todos os eventos registrados, o programa se oferece para salvar todos os perfis que foram criados durante sua execução. Neste caso, nós temos dois perfis que nós salvamos ao mesmo tempo com "Salvar" (“Save”) (mas você pode salvá-los individualmente também) antes de sair do programa com "Terminar" (“Finish”).
O aa-genprof é na realidade apenas um envoltório inteligente em volta do aa-logprof: ele cria um perfil vazio, carrega-o em modo de registro ("complain mode") e então roda o aa-logprof que é uma ferramenta para atualizar um perfil com base nas violações de perfil que foram registradas. Então você pode rodar novamente essa ferramenta mais tarde para aprimorar o perfil que você acabou de criar.
If you want the generated profile to be complete, you should use the program in all the ways that it is legitimately used. In the case of dhclient, it means running it via Network Manager, running it via ifupdown, running it manually, etc. In the end, you might get a /etc/apparmor.d/usr.sbin.dhclient close to this:
# Last Modified: Fri Jul  5 00:51:02 2019
#include <tunables/global>

/usr/sbin/dhclient {
  #include <abstractions/base>
  #include <abstractions/nameservice>

  capability net_bind_service,
  capability net_raw,

  /bin/dash r,
  /etc/dhcp/* r,
  /etc/dhcp/dhclient-enter-hooks.d/* r,
  /etc/dhcp/dhclient-exit-hooks.d/* r,
  /etc/resolv.conf.* w,
  /etc/samba/dhcp.conf.* w,
  /proc/*/net/dev r,
  /proc/filesystems r,
  /run/dhclient*.pid w,
  /sbin/dhclient mr,
  /sbin/dhclient-script rCx,
  /usr/lib/NetworkManager/nm-dhcp-helper Px,
  /var/lib/NetworkManager/* r,
  /var/lib/NetworkManager/*.lease rw,
  /var/lib/dhcp/*.leases rw,

  owner /etc/** mrwk,
  owner /var/** mrwk,
  owner /{,var/}run/** mrwk,
}
And /etc/apparmor.d/usr.sbin.dhclient-script might be similar to this:
# Last Modified: Fri Jul  5 00:51:55 2019
#include <tunables/global>

/usr/sbin/dhclient-script {
  #include <abstractions/base>
  #include <abstractions/bash>
  #include <abstractions/lightdm>
}