apparmor.d/docs/development/guidelines.md

title
Guidelines

Common structure

AppArmor profiles can be written without any specific guidelines. However, when you work with over 1500 profiles, you need a common structure among all the profiles.

The logic behind it is that if a rule is present in a profile, it should only be in one place, making it easier to review profiles.

For example, if a program needs to run executable binaries then the rules allowing it can only be in a specific rule block (just after the @{exec_path} mr, rule). It is therefore easy to ensure some profile features such as:

• A profile has access to a given resource
• A profile enforces a strict write xor execute (W^X) policy.

It also improves compatibilities and makes personalization easier thanks to the use of more variables.

Guidelines

!!! note

This profile guideline is still evolving, feel free to propose improvements
as long as they do not vary too much from the existing rules.

In order to ensure a common structure across the profiles, all new profile must follow the guidelines presented here.

The rules in the profile should be sorted in the rule block as follows:

Order	Name	Example
1	include	:octicons-link-external-24:
2	set rlimit	:octicons-link-external-24:
3	userns	:octicons-link-external-24:
4	capability	:octicons-link-external-24:
5	network	:octicons-link-external-24:
6	mount	:octicons-link-external-24:
7	remount	:octicons-link-external-24:
8	umount	:octicons-link-external-24:
9	pivot_root	:octicons-link-external-24:
10	change_profile	:octicons-link-external-24:
11	mqueue	:octicons-link-external-24:
12	signal	:octicons-link-external-24:
13	ptrace	:octicons-link-external-24:
14	unix	:octicons-link-external-24:
15	io_uring	:octicons-link-external-24:
16	dbus	:octicons-link-external-24:
17	file	:octicons-link-external-24:
18	local include	:octicons-link-external-24:

This rule order is taken from AppArmor with minor changes as we tend to:

• Divide the file block into multiple subcategories
• Put the block with the longer rules (files, dbus) after the other blocks

The file block

The file block should be sorted as follows:

Order	Description	Example	Link
1	The entry point of the profile	@{exec_path} mr,	:octicons-link-external-24:
2	The binaries and library required	@{bin}/, @{lib}/, /opt/. It is the only place where you can have mr, rix, rPx, rUx, rPUX rules.	:octicons-link-external-24:
3	The shared resources	/usr/share	:octicons-link-external-24:
4	The system configuration	/etc	:octicons-link-external-24:
5	The system data	/, /var, /boot	:octicons-link-external-24:
6	The user data	owner @{HOME}/	:octicons-link-external-24:
7	The user configuration, cache and dotfiles	@{user_cache_dirs}, @{user_config_dirs}, @{user_share_dirs}	:octicons-link-external-24:
8	Temporary and runtime data	/tmp/, @{run}/, /dev/shm/	:octicons-link-external-24:
9	Sys files	@{sys}/	:octicons-link-external-24:
10	Proc files	@{PROC}/	:octicons-link-external-24:
11	Dev files	/dev/	:octicons-link-external-24:
12	Deny rules	deny	:octicons-link-external-24:

The dbus block

The dbus block should be sorted as follows:

• The system bus should be sorted before the session bus
• The bind rules should be sorted after send & receive rules

For DBus, try to determine peer's label when possible. E.g.:

dbus send bus=session path=/org/freedesktop/DBus
     interface=org.freedesktop.DBus
     member={RequestName,ReleaseName}
     peer=(name=org.freedesktop.DBus, label=dbus-session),

If there is no predictable label it can be omitted.

Profile rules

:material-numeric-1-circle: Variables

: Always use the apparmor.d variables. Example:

- `/usr/lib` or `/usr/bin` become `@{bin}` or `@{lib}`
- `/usr/sbin` or `/sbin` become `@{bin}`.

:material-numeric-2-circle: Sort

: In a rule block, all rules must be alphabetically sorted.

:material-numeric-3-circle: Sub-profiles

: Sub-profiles should come at the end of a profile.

:material-numeric-4-circle: Similar purpose

: When some rules share similar purposes, they may be sorted together. E.g.: /etc/machine-id r, /var/lib/dbus/machine-id r,

:material-numeric-5-circle: Limit the use of deny

: The use of deny should be limited to the minimum:

- In MAC policies, we only allow access ([Rule :material-numeric-1-circle:](index.md#rule-mandatory-access-control "Mandatory Access Control"))
- `deny` rules are enforced even in complain mode,
- If it works on your machine does not mean it will work on others ([Rule :material-numeric-4-circle:](index.md#rule-distribution-and-devices-agnostic "Distribution and devices agnostic")).

:material-numeric-6-circle: Comments

: Ensure you only have useful comments. E.g.: # Config files for foo owner @{user_config_dirs}/foo/{,**} r, Does not help, and if generalized it would add a lot of complexity to any profiles.