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Diffstat (limited to 'security-blueprint/part-5')
| -rw-r--r-- | security-blueprint/part-5/0_Abstract.md | 55 | ||||
| -rw-r--r-- | security-blueprint/part-5/1-MAC.md | 136 | ||||
| -rw-r--r-- | security-blueprint/part-5/2-SystemD.md | 60 | ||||
| -rw-r--r-- | security-blueprint/part-5/3-SystemBus.md | 24 | ||||
| -rw-r--r-- | security-blueprint/part-5/4-Services.md | 37 | ||||
| -rw-r--r-- | security-blueprint/part-5/5-AppFw.md | 80 | ||||
| -rw-r--r-- | security-blueprint/part-5/6-Utilities.md | 78 | ||||
| -rw-r--r-- | security-blueprint/part-5/7-Users.md | 77 |
8 files changed, 547 insertions, 0 deletions
diff --git a/security-blueprint/part-5/0_Abstract.md b/security-blueprint/part-5/0_Abstract.md new file mode 100644 index 0000000..4cbd17a --- /dev/null +++ b/security-blueprint/part-5/0_Abstract.md @@ -0,0 +1,55 @@ +# Part 5 - Platform + +## Abstract + +This part focuses on the AGL platform including all tools and techniques used to +upgrade the security and downgrade the danger. It must be possible to apply the +two fundamental principles written at the very beginning of the document. First +of all, security management must remain simple. You must also prohibit +everything by default, and then define a set of authorization rules. As cases +to deal with, we must: + +- Implement a **MAC** for processes and files. +- Limit communication between applications (_SystemBus_ and _SystemD_ part). +- Prohibit all tools used during development mode (_Utilities_ and _Services_ part). +- Manage user capabilities (_Users_ part). +- Manage application permissions and policies (_AGLFw_ part). + +<!-- note --> + +The tools and concepts used to meet these needs are only examples. Any other +tool that meets the need can be used. + +<!-- endnote --> + +In AGL, as in many other embedded systems, different security mechanisms settle +in the core layers to ensure isolation and data privacy. While the Mandatory +Access Control layer (**SMACK**) provides global security and isolation, other +mechanisms like **Cynara** are required to check application's permissions at +runtime. Applicative permissions (also called "_privileges_") may vary depending +on the user and the application being run: an application should have access to +a given service only if it is run by the proper user and if the appropriate +permissions are granted. + +-------------------------------------------------------------------------------- + +<!-- pagebreak --> + +## Acronyms and Abbreviations + +The following table lists the terms utilized within this part of the document. + +Acronyms or Abbreviations | Description +------------------------- | -------------------------------------------------------------- +_ACL_ | **A**ccess **C**ontrol **L**ists +_alsa_ | **A**dvanced **L**inux **S**ound **A**rchitecture +_API_ | **A**pplication **P**rogramming **I**nterface +_AppFw_ | **App**lication **F**rame**w**ork +_Cap_ | **Cap**abilities +_DAC_ | **D**iscretionary **A**ccess **C**ontrol +_DDOS_ | **D**istributed **D**enial **O**f **S**ervice +_DOS_ | **D**enial **O**f **S**ervice +_IPC_ | **I**nter-**P**rocess **C**ommunication +_MAC_ | **M**andatory **A**ccess **C**ontrol +_PAM_ | **P**luggable **A**uthentication **M**odules +_SMACK_ | **S**implified **M**andatory **A**ccess **C**ontrol **K**ernel diff --git a/security-blueprint/part-5/1-MAC.md b/security-blueprint/part-5/1-MAC.md new file mode 100644 index 0000000..9cfc150 --- /dev/null +++ b/security-blueprint/part-5/1-MAC.md @@ -0,0 +1,136 @@ +# Mandatory Access Control + +<!-- note --> + +We decided to put the **MAC** protection on the platform part despite the fact +that it applies to the kernel too, since its use will be mainly at the platform +level (except floor part). + +<!-- endnote --> + +**M**andatory **A**ccess **C**ontrol (**MAC**) is a protection provided by the +Linux kernel that requires a **L**inux **S**ecurity **M**odule (**LSM**). AGL +uses an **LSM** called **S**implified **M**andatory **A**ccess **C**ontrol +**K**ernel (**SMACK**). This protection involves the creation of **SMACK** +labels as part of the extended attributes **SMACK** labels to the file extended +attributes. And a policy is also created to define the behaviour of each label. + +The kernel access controls is based on these labels and this policy. If there +is no rule, no access will be granted and as a consequence, what is not +explicitly authorized is forbidden. + +There are two types of **SMACK** labels: + +- **Execution SMACK** (Attached to the process): Defines how files are + _accessed_ and _created_ by that process. +- **File Access SMACK** (Written to the extended attribute of the file): Defines + _which_ process can access the file. + +By default a process executes with its File Access **SMACK** label unless an +Execution **SMACK** label is defined. + +AGL's **SMACK** scheme is based on the _Tizen 3 Q2/2015_. It divides the System +into the following domains: + +- Floor. +- System. +- Applications, Services and User. + +See [AGL security framework review](http://iot.bzh/download/public/2017/AMMQ1Tokyo/AGL-security-framework-review.pdf) and [Smack White Paper](http://schaufler-ca.com/yahoo_site_admin/assets/docs/SmackWhitePaper.257153003.pdf) +for more information. + +-------------------------------------------------------------------------------- + +<!-- pagebreak --> + +## Floor + +The _floor_ domain includes the base system services and any associated data and +libraries. This data remains unchanged at runtime. Writing to floor files or +directories is allowed only in development mode or during software installation +or upgrade. + +The following table details the _floor_ domain: + +Label | Name | Execution **SMACK** | File Access **SMACK** +----- | ----- | ------------------- | --------------------------------------- +`-` | Floor | `r-x` for all | Only kernel and internal kernel thread. +`^` | Hat | `---` for all | `rx` on all domains. +`*` | Star | `rwx` for all | None + +<!-- note --> + +- The Hat label is Only for privileged system services (currently only + systemd-journal). Useful for backup or virus scans. No file with this label + should exist except in the debug log. + +- The Star label is used for device files or `/tmp` Access restriction managed + via **DAC**. Individual files remain protected by their **SMACK** label. + +<!-- endnote --> <!-- config --> + +Domain | `Label` name | Recommendations +------------------ | ------------ | ----------------------------------------------------------- +Kernel-MAC-Floor-1 | `^` | Only for privileged system services. +Kernel-MAC-Floor-2 | `*` | Used for device files or `/tmp` Access restriction via DAC. + +<!-- endconfig --> + +-------------------------------------------------------------------------------- + +<!-- pagebreak --> + +## System + +The _system_ domain includes a reduced set of core system services of the OS and +any associated data. This data may change at runtime. + +The following table details the _system_ domain: + +Label | Name | Execution **SMACK** | File Access **SMACK** +---------------- | --------- | ----------------------------------------------- | --------------------- +`System` | System | None | Privileged processes +`System::Run` | Run | `rwxatl` for User and System label | None +`System::Shared` | Shared | `rwxatl` for system domain `r-x` for User label | None +`System::Log` | Log | `rwa` for System label `xa` for user label | None +`System::Sub` | SubSystem | Subsystem Config files | SubSystem only + +<!-- config --> + +Domain | `Label` name | Recommendations +------------------- | ---------------- | ------------------------------------------------------------------------------------------------------------- +Kernel-MAC-System-1 | `System` | Process should write only to file with transmute attribute. +Kernel-MAC-System-2 | `System::run` | Files are created with the directory label from user and system domain (transmute) Lock is implicit with `w`. +Kernel-MAC-System-3 | `System::Shared` | Files are created with the directory label from system domain (transmute) User domain has locked privilege. +Kernel-MAC-System-4 | `System::Log` | Some limitation may impose to add `w` to enable append. +Kernel-MAC-System-5 | `System::Sub` | Isolation of risky Subsystem. + +<!-- endconfig --> + +-------------------------------------------------------------------------------- + +<!-- pagebreak --> + +## Applications, Services and User + +The _application_, _services_ and _user_ domain includes code that provides +services to the system and user, as well as any associated data. All code +running on this domain is under _Cynara_ control. + +The following table details the _application_, _services_ and _user_ domain: + +Label | Name | Execution **SMACK** | File Access **SMACK** +------------------- | ------ | --------------------------------------------------------------------------- | --------------------------- +`User::Pkg::$AppID` | AppID | `rwx` (for files created by the App). `rx` for files installed by **AppFw** | $App runtime executing $App +`User::Home` | Home | `rwx-t` from System label `r-x-l` from App | None +`User::App-Shared` | Shared | `rwxat` from System and User domains label of $User | None + +<!-- config --> + +Domain | `Label` name | Recommendations +------------------- | ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------- +Kernel-MAC-System-1 | `User::Pkg::$AppID` | Only one Label is allowed per App. A data directory is created by the AppFw in `rwx` mode. +Kernel-MAC-System-2 | `User::Home` | AppFw needs to create a directory in `/home/$USER/App-Shared` at first launch if not present with label app-data access is `User::App-Shared` without transmute. +Kernel-MAC-System-3 | `User::App-Shared` | Shared space between all App running for a given user. + +<!-- endconfig --> diff --git a/security-blueprint/part-5/2-SystemD.md b/security-blueprint/part-5/2-SystemD.md new file mode 100644 index 0000000..903df11 --- /dev/null +++ b/security-blueprint/part-5/2-SystemD.md @@ -0,0 +1,60 @@ +# SystemD + +`afm-system-daemon` is used to: + +- Manage users and user sessions. +- Setup applications and services (_CGroups_, _namespaces_, autostart, permissions). +- Use of `libsystemd` for its programs (event management, **D-Bus** interface). + +<!-- config --> + +Domain | Object | Recommendations +------------------ | -------------- | ------------------------------------ +Platform-SystemD-1 | Security model | Use Namespaces for containerization. +Platform-SystemD-2 | Security model | Use CGroups to organise processes. + +<!-- endconfig --> + +See [systemd integration and user management](http://iot.bzh/download/public/2017/AMM-Dresden/AGL-systemd.pdf) for more information. + +## Benefits + +- Removal of one privileged process: **afm-user-daemon** +- Access and use of high level features: + + - Socket activation. + - Management of users and integration of **PAM**. + - Dependency resolution to services. + - `Cgroups` and resource control. + - `Namespaces` containerization. + - Autostart of required API. + - Permissions and security settings. + - Network management. + +<!-- pagebreak --> + +## CGroups + +Control Groups offer a lot of features, with the most useful ones you can +control: Memory usage, how much CPU time is allocated, how much device I/O is +allowed or which devices can be accessed. **SystemD** uses _CGroups_ to organise +processes (each service is a _CGroups_, and all processes started by that +service use that _CGroups_). By default, **SystemD** automatically creates a +hierarchy of slice, scope and service units to provide a unified structure for +the _CGroups_ tree. With the `systemctl` command, you can further modify this +structure by creating custom slices. Currently, in AGL, there are 2 slices +(**user.slice** and **system.slice**). + +## Namespaces + +### User side + +There are several ways of authenticating users (Key Radio Frequency, Phone, +Gesture, ...). Each authentication provides dynamic allocation of **uids** to +authenticated users. **Uids** is used to ensure privacy of users and **SMACK** +for applications privacy. + +First, the user initiates authentication with **PAM** activation. **PAM** +Standard offers highly configurable authentication with modular design like +face recognition, Voice identification or with a password. Then users should +access identity services with services and applications. diff --git a/security-blueprint/part-5/3-SystemBus.md b/security-blueprint/part-5/3-SystemBus.md new file mode 100644 index 0000000..2a98124 --- /dev/null +++ b/security-blueprint/part-5/3-SystemBus.md @@ -0,0 +1,24 @@ +# D-Bus + +D-Bus is a well-known **IPC** (Inter-Process Communication) protocol (and +daemon) that helps applications to talk to each other. The use of D-Bus is great +because it allows to implement discovery and signaling. + +The D-Bus session is by default addressed by environment variable +`DBUS_SESSION_BUS_ADDRESS`. Using **systemd** variable `DBUS_SESSION_BUS_ADDRESS` +is automatically set for user sessions. D-Bus usage is linked to permissions. + +D-Bus has already had several [security issues](https://www.cvedetails.com/vulnerability-list/vendor_id-13442/D-bus-Project.html) +(mostly **DoS** issues), to allow applications to keep talking to each other. +It is important to protect against this type of attack to keep the system more +stable. + + +<!-- config --> + +Domain | Object | Recommendations +--------------- | -------------- | ------------------------------------ +Platform-DBus-1 | Security model | Use D-Bus as IPC. +Platform-DBus-2 | Security model | Apply D-BUS security patches: [D-Bus CVE](https://www.cvedetails.com/vulnerability-list/vendor_id-13442/D-bus-Project.html) + +<!-- endconfig -->
\ No newline at end of file diff --git a/security-blueprint/part-5/4-Services.md b/security-blueprint/part-5/4-Services.md new file mode 100644 index 0000000..18f56ec --- /dev/null +++ b/security-blueprint/part-5/4-Services.md @@ -0,0 +1,37 @@ +# System services and daemons + +<!-- todo --> + +Domain | Improvement +------------------- | ----------- +Platform-Services-1 | SystemD ? +Platform-Services-2 | Secure daemon ? + +<!-- endtodo --> + +## Tools + +- **connman**: An internet connection manager designed to be slim and to use as + few resources as possible. It is a fully modular system that can be extended, + through plug-ins, to support all kinds of wired or wireless technologies. +- **bluez** is a Bluetooth stack. Its goal is to program an implementation of + the Bluetooth wireless standards specifications. In addition to the basic stack, + the `bluez-utils` and `bluez-firmware` packages contain low level utilities such + as `dfutool` which can interrogate the Bluetooth adapter chipset in order to + determine whether its firmware can be upgraded. +- **gstreamer** is a pipeline-based multimedia framework. It can be used to build + a system that reads files in one format, processes them, and exports them in + another format. +- **alsa** is a software framework and part of the Linux kernel that provides an + **API** for sound card device drivers. + +<!-- config --> + +Domain | `Tool` name | _State_ +-------------------- | ----------- | ------- +Platform-Utilities-1 | `connman` | _Used_ as a connection manager. +Platform-Utilities-2 | `bluez` | _Used_ as a Bluetooth manager. +Platform-Utilities-3 | `gstreamer` | _Used_ to manage multimedia file format. +Platform-Utilities-4 | `alsa` | _Used_ to provides an API for sound card device drivers. + +<!-- endconfig --> diff --git a/security-blueprint/part-5/5-AppFw.md b/security-blueprint/part-5/5-AppFw.md new file mode 100644 index 0000000..9f67b16 --- /dev/null +++ b/security-blueprint/part-5/5-AppFw.md @@ -0,0 +1,80 @@ +# Application framework/model (**AppFw**) + +The application framework manages: + +- The applications and services management: Installing, Uninstalling, Listing, ... +- The life cycle of applications: Start -> (Pause, Resume) -> Stop. +- Events and signals propagation. +- Privileges granting and checking. +- API for interaction with applications. + +<!-- note --> + +- The **security model** refers to the security model used to ensure security + and to the tools that are provided for implementing that model. It's an + implementation detail that should not impact the layers above the application + framework. + +- The **security model** refers to how **DAC** (Discretionary Access Control), + **MAC** (Mandatory Access Control) and Capabilities are used by the system to + ensure security and privacy. It also includes features of reporting using + audit features and by managing logs and alerts. + +<!-- endnote --> + +The **AppFw** uses the security model to ensure the security and the privacy of +the applications that it manages. It must be compliant with the underlying +security model. But it should hide it to the applications. + +<!-- config --> + +Domain | Object | Recommendations +---------------------- | -------------- | -------------------------------- +Platform-AGLFw-AppFw-1 | Security model | Use the AppFw as Security model. + +<!-- endconfig --> + +See [AGL AppFw Privileges Management](http://docs.automotivelinux.org/docs/devguides/en/dev/reference/iotbzh2016/appfw/03-AGL-AppFW-Privileges-Management.pdf) and [AGL - Application Framework Documentation](http://iot.bzh/download/public/2017/SDK/AppFw-Documentation-v3.1.pdf) for more +information. + +<!-- pagebreak --> + +## Cynara + +There's a need for another mechanism responsible for checking applicative +permissions: Currently in AGL, this task depends on a policy-checker service +(**Cynara**). + +- Stores complex policies in databases. +- "Soft" security (access is checked by the framework). + +Cynara interact with **D-Bus** in order to deliver this information. + +<!-- config --> + +Domain | Object | Recommendations +----------------------- | ----------- | ------------------------------------- +Platform-AGLFw-Cynara-1 | Permissions | Use Cynara as policy-checker service. + +<!-- endconfig --> + +### Policies + +- Policy rules: + + - Are simple - for pair [application context, privilege] there is straight + answer (single Policy Type): [ALLOW / DENY / ...]. + - No code is executed (no script). + - Can be easily cached and managed. + +- Application context (describes id of the user and the application credentials) + It is build of: + + - UID of the user that runs the application. + - **SMACK** label of application. + +## Holding policies + +Policies are kept in buckets. Buckets are set of policies which have additional +a property of default answer, the default answer is yielded if no policy matches +searched key. Buckets have names which might be used in policies (for directions). diff --git a/security-blueprint/part-5/6-Utilities.md b/security-blueprint/part-5/6-Utilities.md new file mode 100644 index 0000000..d723c10 --- /dev/null +++ b/security-blueprint/part-5/6-Utilities.md @@ -0,0 +1,78 @@ +# Utilities + +- **busybox**: Software that provides several stripped-down Unix tools in a + single executable file. Of course, it will be necessary to use a "production" + version of **busybox** in order to avoid all the tools useful only in + development mode. + +<!-- config --> + +Domain | `Tool` name | _State_ +-------------------- | ----------- | ---------------------------------------------------------------------- +Platform-Utilities-1 | `busybox` | _Used_ to provide a number of tools. Do not compile development tools. + +<!-- endconfig --> + +## Functionalities to exclude in production mode + +In production mode, a number of tools must be disabled to prevent an attacker +from finding logs for example. This is useful to limit the visible surface and +thus complicate the fault finding process. The tools used only in development +mode are marked by an '**agl-devel**' feature. When building in production mode, +these tools will not be compiled. + +<!-- config --> + +Domain | `Utility` name and normal `path` | _State_ +--------------------- | ---------------------------------------------------- | ---------- +Platform-Utilities-1 | `chgrp` in `/bin/chgrp` | _Disabled_ +Platform-Utilities-2 | `chmod` in `/bin/chmod` | _Disabled_ +Platform-Utilities-3 | `chown` in `/bin/chown` | _Disabled_ +Platform-Utilities-4 | `dmesg` in `/bin/dmesg` | _Disabled_ +Platform-Utilities-5 | `Dnsdomainname` in `/bin/dnsdomainname` | _Disabled_ +Platform-Utilities-6 | `dropbear`, Remove "dropbear" from `/etc/init.d/rcs` | _Disabled_ +Platform-Utilities-7 | `Editors` in (vi) `/bin/vi` | _Disabled_ +Platform-Utilities-8 | `find` in `/bin/find` | _Disabled_ +Platform-Utilities-9 | `gdbserver` in `/bin/gdbserver` | _Disabled_ +Platform-Utilities-10 | `hexdump` in `/bin/hexdump` | _Disabled_ +Platform-Utilities-11 | `hostname` in `/bin/hostname` | _Disabled_ +Platform-Utilities-12 | `install` in `/bin/install` | _Disabled_ +Platform-Utilities-13 | `iostat` in `/bin/iostat` | _Disabled_ +Platform-Utilities-14 | `killall` in `/bin/killall` | _Disabled_ +Platform-Utilities-15 | `klogd` in `/sbin/klogd` | _Disabled_ +Platform-Utilities-16 | `logger` in `/bin/logger` | _Disabled_ +Platform-Utilities-17 | `lsmod` in `/sbin/lsmod` | _Disabled_ +Platform-Utilities-18 | `pmap` in `/bin/pmap` | _Disabled_ +Platform-Utilities-19 | `ps` in `/bin/ps` | _Disabled_ +Platform-Utilities-20 | `ps` in `/bin/ps` | _Disabled_ +Platform-Utilities-21 | `rpm` in `/bin/rpm` | _Disabled_ +Platform-Utilities-22 | `SSH` | _Disabled_ +Platform-Utilities-23 | `stbhotplug` in `/sbin/stbhotplug` | _Disabled_ +Platform-Utilities-24 | `strace` in `/bin/trace` | _Disabled_ +Platform-Utilities-25 | `su` in `/bin/su` | _Disabled_ +Platform-Utilities-26 | `syslogd` in (logger) `/bin/logger` | _Disabled_ +Platform-Utilities-27 | `top` in `/bin/top` | _Disabled_ +Platform-Utilities-28 | `UART` in `/proc/tty/driver/` | _Disabled_ +Platform-Utilities-29 | `which` in `/bin/which` | _Disabled_ +Platform-Utilities-30 | `who` and `whoami` in `/bin/whoami` | _Disabled_ +Platform-Utilities-31 | `awk` (busybox) | _Enabled_ +Platform-Utilities-32 | `cut` (busybox) | _Enabled_ +Platform-Utilities-33 | `df` (busybox) | _Enabled_ +Platform-Utilities-34 | `echo` (busybox) | _Enabled_ +Platform-Utilities-35 | `fdisk` (busybox) | _Enabled_ +Platform-Utilities-36 | `grep` (busybox) | _Enabled_ +Platform-Utilities-37 | `mkdir` (busybox) | _Enabled_ +Platform-Utilities-38 | `mount` (vfat) (busybox) | _Enabled_ +Platform-Utilities-39 | `printf` (busybox) | _Enabled_ +Platform-Utilities-40 | `sed` in `/bin/sed` (busybox) | _Enabled_ +Platform-Utilities-41 | `tail` (busybox) | _Enabled_ +Platform-Utilities-42 | `tee` (busybox) | _Enabled_ +Platform-Utilities-43 | `test` (busybox) | _Enabled_ + +<!-- endconfig --> <!-- note --> + +The _Enabled_ Unix/Linux utilities above shall be permitted as they are often +used in the start-up scripts and for USB logging. If any of these utilities are +not required by the device then those should be removed. + +<!-- endnote --> diff --git a/security-blueprint/part-5/7-Users.md b/security-blueprint/part-5/7-Users.md new file mode 100644 index 0000000..9fc7a65 --- /dev/null +++ b/security-blueprint/part-5/7-Users.md @@ -0,0 +1,77 @@ +# Users + +The user policy can group users by function within the car. For example, we can +consider a driver and his passengers. Each user is assigned to a single group to +simplify the management of space security. + +## Root Access + +The main applications, those that provide the principal functionality of the +embedded device, should not execute with root identity or any capability. + +If the main application is allowed to execute at any capability, then the entire +system is at the mercy of the said application's good behaviour. Problems arise +when an application is compromised and able to execute commands which could +consistently and persistently compromise the system by implanting rogue +applications. + +It is suggested that the middleware and the UI should run in a context on a user +with no capability and all persistent resources should be maintained without any +capability. + +One way to ensure this is by implementing a server-client paradigm. Services +provided by the system's drivers can be shared this way. The other advantage of +this approach is that multiple applications can share the same resources at the +same time. + +<!-- config --> + +Domain | Object | Recommendations +--------------------- | ---------------- | ----------------------------------------------------- +Platform-Users-root-1 | Main application | Should not execute as root. +Platform-Users-root-2 | UI | Should run in a context on a user with no capability. + +<!-- endconfig --> + +Root access should not be allowed for the following utilities: + +<!-- config --> + +Domain | `Utility` name | _State_ +--------------------- | -------------- | ------------- +Platform-Users-root-3 | `login` | _Not allowed_ +Platform-Users-root-4 | `su` | _Not allowed_ +Platform-Users-root-5 | `ssh` | _Not allowed_ +Platform-Users-root-6 | `scp` | _Not allowed_ +Platform-Users-root-7 | `sftp` | _Not allowed_ + +<!-- endconfig --> + +Root access should not be allowed for the console device. The development +environment should allow users to login with pre-created user accounts. + +Switching to elevated privileges shall be allowed in the development environment +via `sudo`. + +-------------------------------------------------------------------------------- + +<!-- pagebreak --> + +## Capabilities + +<!-- todo --> + +Domain | Improvement +----------------------------- | ------------------------ +Platform-Users-Capabilities-1 | Kernel or Platform-user? +Platform-Users-Capabilities-2 | Add config note. + +<!-- endtodo --> + +The goal is to restrict functionality that will not be useful in **AGL**. They +are integrated into the **LSM**. Each privileged transaction is associated with +a capability. These capabilities are divided into three groups: + +- e: Effective: This means the capability is “activated”. +- p: Permitted: This means the capability can be used/is allowed. +- i: Inherited: The capability is kept by child/subprocesses upon execve() for example. |