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diff --git a/docs/security-blueprint/part-5/1-MAC.md b/docs/security-blueprint/part-5/1-MAC.md new file mode 100644 index 0000000..73543e9 --- /dev/null +++ b/docs/security-blueprint/part-5/1-MAC.md @@ -0,0 +1,165 @@ +# Mandatory Access Control + +<!-- section-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). + +<!-- end-section-note --> + +**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 + +<!-- section-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. + +<!-- end-section-note --> <!-- section-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. + +<!-- end-section-config --> + +-------------------------------------------------------------------------------- + +<!-- 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 + +<!-- section-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. + +<!-- end-section-config --> + +-------------------------------------------------------------------------------- + +<!-- 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 $User | None + +<!-- section-config --> + +Domain | `Label` name | Recommendations +------------------- | ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------- +Kernel-MAC-System-1 | `User::Pkg::$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. + +<!-- end-section-config --> + +## Attack Vectors + +There are 4 major components to the system: + +- The LSM kernel module. +- The `smackfs` filesystem. +- Basic utilities for policy management and checking. +- The policy/configuration data. + +As with any mandatory access system, the policy management needs to be carefully separated +from the checking, as the management utilities can become a convenient point of attack. +Dynamic additions to the policy system need to be carefully verified, as the ability to +update the policies is often needed, but introduces a possible threat. Finally, +even if the policy management is well secured, the policy checking and failure response +to that checking is also of vital importance to the smooth operation of the system. + +While **MAC** is a certainly a step up in security when compared to DAC, there are still +many ways to compromise a SMACK-enabled Linux system. Some of these ways are as follows: + +- Disabling SMACK at invocation of the kernel (with command-line: security=none). +- Disabling SMACK in the kernel build and redeploying the kernel. +- Changing a SMACK attribute of a file or directory at install time. +- Tampering with a process with the CAP_MAC_ADMIN privilege. +- Setting/Re-setting the SMACK label of a file. +- Tampering with the default domains (i.e. /etc/smack/accesses.d/default-access-domains). +- Disabling or tampering with the SMACK filesystem (i.e. /smackfs). +- Adding policies with `smackload` (adding the utility if not present). +- Changing labels with `chsmack` (adding the utility if not present). |