v811_spc009/compatibility/cdd/9_security-model/9_7_security-features.md

9.7. Security Features

Device implementations MUST ensure compliance with security features in both the kernel and platform as described below.

The Android Sandbox includes features that use the Security-Enhanced Linux (SELinux) mandatory access control (MAC) system, seccomp sandboxing, and other security features in the Linux kernel. Device implementations:

• [C-0-1] MUST maintain compatibility with existing applications, even when SELinux or any other security features are implemented below the Android framework.
• [C-0-2] MUST NOT have a visible user interface when a security violation is detected and successfully blocked by the security feature implemented below the Android framework, but MAY have a visible user interface when an unblocked security violation occurs resulting in a successful exploit.
• [C-0-3] MUST NOT make SELinux or any other security features implemented below the Android framework configurable to the user or app developer.
• [C-0-4] MUST NOT allow an application that can affect another application through an API (such as a Device Administration API) to configure a policy that breaks compatibility.
• [C-0-5] MUST split the media framework into multiple processes so that it is possible to more narrowly grant access for each process as described in the Android Open Source Project site.
• [C-0-6] MUST implement a kernel application sandboxing mechanism which allows filtering of system calls using a configurable policy from multithreaded programs. The upstream Android Open Source Project meets this requirement through enabling the seccomp-BPF with threadgroup synchronization (TSYNC) as described in the Kernel Configuration section of source.android.com.

Kernel integrity and self-protection features are integral to Android security. Device implementations:

• [C-0-7] MUST implement kernel stack buffer overflow protection mechanisms. Examples of such mechanisms are CC_STACKPROTECTOR_REGULAR and CONFIG_CC_STACKPROTECTOR_STRONG.

• [C-0-8] MUST implement strict kernel memory protections where executable code is read-only, read-only data is non-executable and non-writable, and writable data is non-executable (e.g. CONFIG_DEBUG_RODATA or CONFIG_STRICT_KERNEL_RWX).

• [C-0-9] MUST implement static and dynamic object size bounds checking of copies between user-space and kernel-space (e.g. CONFIG_HARDENED_USERCOPY) on devices originally shipping with API level 28 or higher.

• [C-0-10] MUST NOT execute user-space memory when executing in the kernel mode (e.g. hardware PXN, or emulated via CONFIG_CPU_SW_DOMAIN_PAN or CONFIG_ARM64_SW_TTBR0_PAN) on devices originally shipping with API level 28 or higher.

• [C-0-11] MUST NOT read or write user-space memory in the kernel outside of normal usercopy access APIs (e.g. hardware PAN, or emulated via CONFIG_CPU_SW_DOMAIN_PAN or CONFIG_ARM64_SW_TTBR0_PAN) on devices originally shipping with API level 28 or higher.

• [C-0-12] MUST implement kernel page table isolation if the hardware is vulnerable to CVE-2017-5754 on all devices originally shipping with API level 28 or higher (e.g. CONFIG_PAGE_TABLE_ISOLATION or CONFIG_UNMAP_KERNEL_AT_EL0).

• [C-0-13] MUST implement branch prediction hardening if the hardware is vulnerable to CVE-2017-5715 on all devices originally shipping with API level 28 or higher (e.g. CONFIG_HARDEN_BRANCH_PREDICTOR).

• [SR] STRONGLY RECOMMENDED to keep kernel data which is written only during initialization marked read-only after initialization (e.g. __ro_after_init).

• [C-SR] Are STRONGLY RECOMMENDED to randomize the layout of the kernel code and memory, and to avoid exposures that would compromise the randomization (e.g. CONFIG_RANDOMIZE_BASE with bootloader entropy via the /chosen/kaslr-seed Device Tree node or EFI_RNG_PROTOCOL).

• [C-SR] Are STRONGLY RECOMMENDED to enable control flow integrity (CFI) in the kernel to provide additional protection against code-reuse attacks (e.g. CONFIG_CFI_CLANG and CONFIG_SHADOW_CALL_STACK).

• [C-SR] Are STRONGLY RECOMMENDED not to disable Control-Flow Integrity (CFI), Shadow Call Stack (SCS) or Integer Overflow Sanitization (IntSan) on components that have it enabled.

• [C-SR] Are STRONGLY RECOMMENDED to enable CFI, SCS, and IntSan for any additional security-sensitive userspace components as explained in CFI and IntSan.

• [C-SR] Are STRONGLY RECOMMENDED to enable stack initialization in the kernel to prevent uses of uninitialized local variables (CONFIG_INIT_STACK_ALL or CONFIG_INIT_STACK_ALL_ZERO). Also, device implementations SHOULD NOT assume the value used by the compiler to initialize the locals.

• [C-SR] Are STRONGLY RECOMMENDED to enable heap initialization in the kernel to prevent uses of uninitialized heap allocations (CONFIG_INIT_ON_ALLOC_DEFAULT_ON) and they SHOULD NOT assume the value used by the kernel to initialize those allocations.

If device implementations use a Linux kernel, they:

• [C-1-1] MUST implement SELinux.
• [C-1-2] MUST set SELinux to global enforcing mode.
• [C-1-3] MUST configure all domains in enforcing mode. No permissive mode domains are allowed, including domains specific to a device/vendor.
• [C-1-4] MUST NOT modify, omit, or replace the neverallow rules present within the system/sepolicy folder provided in the upstream Android Open Source Project (AOSP) and the policy MUST compile with all neverallow rules present, for both AOSP SELinux domains as well as device/vendor specific domains.
• [C-1-5] MUST run third-party applications targeting API level 28 or higher in per-application SELinux sandboxes with per-app SELinux restrictions on each application's private data directory.
• SHOULD retain the default SELinux policy provided in the system/sepolicy folder of the upstream Android Open Source Project and only further add to this policy for their own device-specific configuration.

If device implementations use kernel other than Linux, they:

• [C-2-1] MUST use a mandatory access control system that is equivalent to SELinux.

Android contains multiple defense-in-depth features that are integral to device security.