Overview ======== Scope ----- The information contained in this document is applicable to systems based on Automotive Grade Linux. Limitations ----------- - This document is based on knowledge and research gained from looking at security desktop and server versions of Linux as well as Android exploits and hardening. - Some kernel configuration options can have an impact on performance. This will be noted where applicable.­ Document Structure ------------------ This document has been divided into three sections; REQUIREMENTS, RECOMMENDATIONS, and VALIDATION. The REQUIREMENTS section details explicit requirements that must be adhered to for the embedded device. The RECOMMENDATIONS section details best practices, and some recommended security settings for the embedded device. The third section, VALIDATION, provides reference scripts and test procedures that can be used to verify adherence with the REQUIREMENTS detailed in the first section of this guide. Hardening --------- The term *Hardening* refers to the tools, techniques and processes required in order to reduce the attack surface on an embedded system, such as an embedded control unit (ECU) or other managed device. The target for all hardening activities is to prevent the execution of invalid binaries on the device, and to prevent copying of security related data from the device. There are three main areas of focus for hardening an embedded device: Requirements ============ For the purposes of reference and explanation, we are providing guidance on how to configure an embedded device that runs with a linux 3. 10.17 Linux kernel. These requirements must still be met by manufacturers that opt to build using an alternative version of the Linux kernel. Kernel Hardening ---------------- The following sub-sections contain information on various kernel configuration options to enhance the security measures in the kernel and also for applications compiled to take advantage of these security features. Additionally, there are also configuration options that close known vulnerable configuration options. Here’s a high level summary of various kernel configurations that shall be required for deployment. ### Disable the serial console The serial console should be disabled to prevent an attacker from accessing this powerful interface. ```bash CONFIG_SERIAL_8250=n CONFIG_SERIAL_8250_CONSOLE=n CONFIG_SERIAL_CORE=n CONFIG_SERIAL_CORE_CONSOLE=n ``` ### Restrict access to kernel memory through device file The /dev/kmem file in Linux systems is directly mapped to kernel virtual memory. This can be disastrous if an attacker gains root access, as the attacker would have direct access to kernel virtual memory. To disable the /dev/kmem file, which is very infrequently used by applications, the following kernel option should be set in the compile-time kernel configuration: ``` CONFIG_DEVKMEM=n ``` In case applications in userspace need /dev/kmem support, it should be available only for authenticated applications. ### Bake-in the kernel command-line The kernel command-line is used to control many aspects of the booting kernel, and is prone to tampering as they are passed in RAM with little to no reverse validation on these parameters. To prevent this type of attack, the kernel shall be configured to ignore command line arguments, and use pre-configured (compile time) options instead. Set the kernel command line in the CONFIG\_CMDLINE KConfig item and then pass no arguments from the bootloader. ```bash CONFIG_CMDLINE_BOOL=y CONFIG_CMDLINE=”<*insert kernel command line here*>” CONFIG_CMDLINE_OVERRIDE=y ``` It is recommended that any per-device settings (eg. MAC addresses, serial numbers, etc.) be stored and accessed from read-only memory (or files), and that any such parameters be verified (signature checking) prior to their use. ### Disable kernel debug symbols Debug symbols should always be removed from production kernels as they provide a lot of information to attackers. ```bash CONFIG_DEBUG_INFO=n ``` These kernel debug symbols are enabled by other config items in the kernel. Care should be taken to disable those also. If CONFIG\_DEBUG\_INFO cannot be disabled then enabling CONFIG\_DEBUG\_INFO\_REDUCED is second best. ### Disable access to a kernel core dump This kernel configuration disables access to a kernel core dump from user space -- if enabled it gives attackers a useful view into kernel memory. ```bash CONFIG_PROC_KCORE=n ``` ### Disable KGDB The Linux kernel supports KGDB over USB and console ports. These mechanisms are controlled by the kgdbdbgp and kgdboc kernel command-line parameters. It is important to ensure that no shipping product contains a kernel with KGDB compiled-in. ```bash CONFIG_KGDB=n ``` ### Disable Kprobes Kprobes enables you to dynamically break into any kernel routine and collect debugging and performance information non-disruptively. You can trap at almost any kernel code address, specifying a handler routine to be invoked when the breakpoint is hit. ```bash CONFIG_KPROBES=n ``` ### Disable Tracing FTrace enables the kernel to trace every kernel function. Providing kernel trace functionality would assist an attacker in discovering attack vectors. ```bash CONFIG_FTRACE=n ``` ### Disable Profiling Profiling and OProfile enables profiling the whole system, include the kernel, kernel modules, libraries, and applications. Providing profiling functionality would assist an attacker in discovering attack vectors. ```bash CONFIG_OPROFILE=n CONFIG_PROFILING=n ``` ### Disable magic sysrq support On a few architectures, you can access a powerful debugger interface from the keyboard. The same powerful interface can be present on the serial console – responding to serial break – of Linux on other architectures. Disable to avoid potentially exposing this powerful backdoor. ```bash CONFIG_MAGIC_SYSRQ=n ``` ### Disable OOPS print on BUG() The output from OOPS print can be helpful in Return Oriented Programming (ROP) when trying to determine the effectives of an exploit. ```bash CONFIG_DEBUG_BUGVERBOSE=n ``` ### Disable kexec This prevents someone who gets root from supplanting the kernel. This can be used as a way to bypass signed kernels. ```bash CONFIG_KEXEC=n ``` ### Disable kernel IP autoconfiguration It is preferable to have IP configuration performed using a user-space tool as these tend to have more validation. We do not want the network interface coming up until the system has come up properly. ```bash CONFIG_IP_PNP=n ``` ### Disable /proc/config.gz It is extremely important to not expose the kernel configuration used on a production device to a potential attacker. With access to the kernel config, it could be possible for an attacker to build a custom kernel for the device that may disable critical security features. ```bash CONFIG_IKCONFIG=n ``` ### Disable swap If not disabled, attackers can enable swap at runtime, add pressure to the memory subsystem and then scour the pages written to swap for useful information. ```bash CONFIG_SWAP=n ``` ### Disable namespace support Do not allow namespace support to prevent duplicates of dev nodes, pids and mount points. This may be used in virtualization and container solutions like LXC, so in some cases it cannot be disabled. ```bash CONFIG_NAMESPACES=n ``` ### Disable NFS file system While often enabled in development, when left enabled in production builds this can be a very useful way for an attacker to get files onto and off of an STB. ```bash CONFIG_NFSD=n CONFIG_NFS_FS=n ``` ### Disable support for binary formats other than ELF This will make possible to plug wrapper-driven binary formats into the kernel. It enables support for binary formats other than ELF. Providing the ability to use alternate interpreters would assist an attacker in discovering attack vectors ```bash CONFIG_BINFMT_MISC=n ``` ### Disable “Load All Symbols” There is a /proc/kallsyms file which exposes the kernel memory space address of many kernel symbols (functions, variables, etc.). This information is useful to attackers in identifying kernel versions/configurations and in preparing payloads for exploits of kernel space. Both KALLSYMS\_ALL and KALLSYMS shall be disabled; ```bash CONFIG_KALLSYMS=n CONFIG_KALLSYMS_ALL=n ``` ### Disable Kernel Debugging There are development-only branches of code in the kernel enabled by the DEBUG\_KERNEL conf. This should be disabled to compile-out these branches. ```bash CONFIG_DEBUG_KERNEL=n ``` In some kernel versions, disabling this requires also disabling CONFIG\_EMBEDDED, and CONFIG\_EXPERT Disabling CONFIG\_EXPERT makes it impossible to disable \_COREDUMP, DEBUG\_BUGVERBOSE, \_NAMESPACES, \_KALLSYMS and \_BUG. In which case it is better to leave this enabled than enable the others. ### Disable the kernel debug filesystem The kernel debug filesystem presents a lot of useful information and means of manipulation of the kernel to an attacker. ```bash CONFIG_DEBUG_FS=n ``` ### Disable BUG() support The kernel will display backtrace and register information for BUGs and WARNs in kernel space, making it easier for attackers to develop exploits. ```bash CONFIG_BUG=n ``` ### Disable Sysctl syscall support Enabling this will result in code being included that is hard to maintain and not well tested. ```bash CONFIG_SYSCTL_SYSCALL=n ``` ### Kernel Modules ### Disable module unloading This stops an attacker unloading security focused kernel modules. It will also prevent the attacker from removing evidence of any attempted kernel tampering that may have been initiated by loading of a kernel module. ```bash CONFIG_MODULE_UNLOAD=n ``` ### Disable Forced Module Loading If enabled, then modules without version information or with mismatched version information may be forcibly loaded into the kernel. Disabling this configuration forces the attackers to build modules with matched kernel sources and configuration in order to load them. ```bash CONFIG_MODULE_FORCE_LOAD=n ``` Recommendations =============== The following sections detail best practices that should be applied in order to secure a device. Although they are not currently listed as hard requirements, they may be upgraded to requirements status in the future. Kernel Hardening ---------------- The following sub-sections contain information on various kernel configuration options that will require updating to a newer kernel version in order to enhance the security measures in the kernel and also for applications compiled to take advantage of these security features. Additionally, there are also configuration options that close known vulnerable configuration options. Here’s a high level summary of the various kernel configurations and which kernel version they pertain: | Kernel Configuration | Kernel Version | |---------------------------------------|----------------| | CONFIG\_CHECKPOINT\_RESTORE=n |3.3+ | | CONFIG\_UNIX\_DIAG=n |3.3+ | | CROSS\_MEMORY\_ATTACH=n |3.5+ | | CONFIG\_PANIC\_ON\_OOPS=y |3.5+ | | CONFIG\_COREDUMP=n |3.7+ | | CONFIG\_MODULE\_SIG\_FORCE=y |3.7+ | | CONFIG\_PACKET\_DIAG=n |3.7+ | | CONFIG\_FW\_LOADER\_USER\_HELPER=n |3.9+ | | CONFIG\_CC\_STACKPROTECTOR=y |3.11+ (MIPS) | | CONFIG\_USELIB=n |3.15+ | | BPF\_JIT=n |3.16+ (MIPS) | | CONFIG\_DEVMEM=n |4.0+ | ### Build with Stack Protection Similar to the stack protector used for ELF programs in user-space, the kernel can protect its internal stacks as well. This configuration for the MIPS architecture is supported in Linux 3.11 and greater and thus should only be enabled for such versions. This configuration also requires building the kernel with the gcc compiler 4.2 or greater. ```bash CONFIG_CC_STACKPROTECTOR=y ``` ### Disable access to /dev/mem The /dev/mem file in Linux systems is directly mapped to physical memory. This can be disastrous if an attacker gains root access, as the attacker would have direct access to physical memory through this convenient device file. It may not always be possible to disable such file, as some applications might need such support. In that case then this device file should be available only for authenticated applications. This configuration is supported in Linux 4.0 and greater and thus should only be disabled for such versions. ```bash CONFIG_DEVMEM=n ``` ### Disable cross-memory attach Disable the process\_vm\_\*v syscalls which allow one process to peek/poke the virtual memory of another. This configuration is supported in Linux 3.5 and greater and thus should only be disabled for such versions. ```bash CROSS_MEMORY_ATTACH=n ``` ### Disable core dumps Core dumps provide lot of debug information for hackers. So disabling core dumps is recommended in production builds. This configuration is supported in Linux 3.7 and greater and thus should only be disabled for such versions. ```bash CONFIG_COREDUMP=n ``` ### Disable Legacy Linux Support There are some Kernel Configs which are present only to support legacy binaries. See also section 2.2.2.18 for disabling support for legacy binary formats. The uselib system call, in particular, has no valid use in any libc6 or uclibc system in recent times. This configuration is supported in Linux 3.15 and greater and thus should only be disabled for such versions. ```bash CONFIG_USELIB=n ``` ### Disable firmware auto-loading user mode helper The firmware auto loading helper, which is a utility executed by the kernel on hotplug events requiring firmware, needs to be set setuid. As a result of this, the helper utility is an attractive target for attackers with control of physical ports on the device. Disabling this configuration is supported in Linux 3.9 and greater. ```bash CONFIG_FW_LOADER_USER_HELPER=n ``` ### Enable Kernel Panic on OOPS When fuzzing the kernel or attempting kernel exploits attackers are likely to trigger kernel OOPSes. Setting the behavior on OOPS to PANIC can impede their progress. This configuration is supported in Linux 3.5 and greater and thus should only be enabled for such versions. ```bash CONFIG_PANIC_ON_OOPS=y ``` ### Disable socket monitoring interface These monitors can be used to inspect shared file descriptors on Unix Domain sockets or traffic on ‘localhost’ which is otherwise assumed to be confidential. The **CONFIG\_PACKET\_DIAG** configuration is supported in Linux 3.7 and greater and thus should only be disabled for such versions. The **CONFIG\_UNIX\_DIAG** configuration is supported in Linux 3.3 and greater and thus should only be disabled for such versions. ```bash CONFIG_PACKET_DIAG=n CONFIG_UNIX_DIAG=n ``` ### Disable BPF JIT The BPF JIT can be used to create kernel-payloads from firewall table rules. This configuration for the MIPS architecture is supported in Linux 3.16 and greater and thus should only be disabled for such versions. ```bash BPF_JIT=n ``` ### Disable checkpoint/restore The checkpoint/restore service can take a process, freeze it and migrate it. This results in providing more info than a core dump. This configuration is supported in Linux 3.3 and greater and thus should only be disabled for such versions. ```bash CONFIG_CHECKPOINT_RESTORE=n ``` ### Enable Enforced Module Signing This configuration is supported in Linux 3.7 and greater and thus should only be enabled for such versions. ```bash CONFIG_MODULE_SIG_FORCE=y ``` ### Disable all USB, PCMCIA (and other hotplug bus) drivers that aren’t needed To reduce the attack surface, the driver enumeration, probe, and operation happen in the kernel. The driver data is parsed by the kernel, so any logic bugs in these drivers can become kernel exploits. ### Disable all file systems not needed To reduce the attack surface; file system data is parsed by the kernel so any logic bugs in file system drivers can become kernel exploits. ### Kernel Address Display Restriction When attackers try to develop "run anywhere" exploits for kernel vulnerabilities, they frequently need to know the location of internal kernel structures. By treating kernel addresses as sensitive information, those locations are not visible to regular local users. /proc/sys/kernel/kptr\_restrict is set to "1" to block the reporting of known kernel address leaks. Additionally, various files and directories should be readable only by the root user: /boot/vmlinuz\*, /boot/System.map\*, /sys/kernel/debug/, /proc/slabinfo ### DMESG Restrictions When attackers try to develop "run anywhere" exploits for vulnerabilties, they frequently will use dmesg output. By treating dmesg output as sensitive information, this output is not available to the attacker. /proc/sys/kernel/dmesg\_restrict can be set to "1" to treat dmesg output as sensitive. ### SELinux If SELinux is in use on the embedded system, the following kernel options should be enabled to prevent SELlinux from being disabled at either runtime or boot time. ```bash CONFIG_SECURITY_SELINUX_DEVELOP=n CONFIG_SECURITY_SELINUX_DISABLE=n CONFIG_SECURITY_SELINUX_BOOTPARAM=n ``` Validation ========== Image Security Analysis Framework (ISAFW) ----------------------------------------- **meta-security-isafw** is an OE layer that allows enabling the Image Security Analysis Framework (isafw) for your image builds. The primary purpose of isafw is to provide an extensible framework for analysing different security aspects of images during the build process. The isafw project itself can be found at https://github.com/01org/isafw This layer can be added to your builds to produce an analysis report, including a kernel config analysis. ### Usage In order to enable the isafw during the image build, please add the following line to your build/conf/local.conf file: ```python INHERIT += "isafw" ```