General configuration

Mandatory Access Control

Kernel should controls access with labels and policy.

Domain Config name Value
Kernel-General-MAC-6 CONFIG_SECURITY y

Please also refer to the Mandatory Access Control documentation in Platform part. You can also find useful documentation and links on wikipedia about MAC and about SMACK.

Disable kexec

Kexec is a system call that enables you to load and boot into another kernel from the currently running kernel. This feature is not required in a production environment.

Domain Config name Value
Kernel-General-kexec-1 CONFIG_KEXEC n

kexec can load arbitrary kernels but signing of new kernel can be enforced like it is can be enforced for new modules.

Disable kernel IP auto-configuration

It is preferable to have an 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.

Domain Config name Value
Kernel-General-IPAutoConf-1 CONFIG_IP_PNP n

Disable Sysctl syscall support

Enabling this will result in code being included that is hard to maintain and not well tested.

Domain Config name Value
Kernel-General-SysCtl_SysCall-1 CONFIG_SYSCTL_SYSCALL n

Disable Legacy Linux Support

There are some Kernel Configs which are present only to support legacy binaries. See also “Consoles” part in order to 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.

Domain Config name Value
Kernel-General-LegacyLinux-1 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, can 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 that is supported in Linux 3.9 and greater.

Domain Config name Value
Kernel-General-FirmHelper-1 CONFIG_FW_LOADER_USER_HELPER n

It doesn’t strictly need to be setuid, there is an option of shipping firmware builtin into kernel without initrd/filesystem.

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.

Domain Config name Value
Kernel-General-PanicOnOOPS-1 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.

Domain Config name Value
Kernel-General-SocketMon-1 CONFIG_PACKET_DIAG n
Kernel-General-SocketMon-2 CONFIG_UNIX_DIAG n

Disable BPF JIT

The BPF JIT can be used to create kernel-payloads from firewall table rules.

This configuration for is supported in Linux 3.16 and greater and thus should only be disabled for such versions.

Domain Config name Value
Kernel-General-BPF_JIT-1 CONFIG_BPF_JIT n

Enable Enforced Module Signing

The kernel should never allow an unprivileged user the ability to load specific kernel modules, since that would provide a facility to unexpectedly extend the available attack surface.

To protect against even privileged users, systems may need to either disable module loading entirely, or provide signed modules (e.g. CONFIG_MODULE_SIG_FORCE, or dm-crypt with LoadPin), to keep from having root load arbitrary kernel code via the module loader interface.

This configuration is supported in Linux 3.7 and greater and thus should only be enabled for such versions.

Domain Config name Value
Kernel-General-ModuleSigning-1 CONFIG_MODULE_SIG_FORCE y

It is also possible to block the loading of modules after startup with “kernel.modules_disabled”.

Domain Variable name Value
Kernel-General-ModuleSigning-2 kernel.modules_disabled 1

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.

Domain Object State
Kernel-General-Drivers-1 USB Disabled
Kernel-General-Drivers-2 PCMCIA Disabled
Kernel-General-Drivers-3 Other hotplug bus Disabled

Position Independent Executables

Domain Improvement
Kernel-General-IndependentExec-1 Kernel or/and platform part ?
Domain compiler and linker options State
Kernel-General-IndependentExec-1 -pie -fpic Enable

Produce a position independent executable on targets which supports it.

Prevent Overwrite Attacks

-z,relro linking option helps during program load, several ELF memory sections need to be written by the linker, but can be turned read-only before turning over control to the program. This prevents some Global Offset Table GOT overwrite attacks, or in the dtors section of the ELF binary.

Domain compiler and linker options State
Kernel-General-OverwriteAttacks-1 -z,relro Enable
Kernel-General-OverwriteAttacks-2 -z,now Enable

During program load, all dynamic symbols are resolved, allowing for the complete GOT to be marked read-only (due to -z relro above). This prevents GOT overwrite attacks. For very large application, this can incur some performance loss during initial load while symbols are resolved, but this shouldn’t be an issue for daemons.

Library linking

Domain Improvement
Kernel-General-LibraryLinking-1 Keep this part?

It is recommended that dynamic linking should generally not be allowed. This will avoid the user from replacing a library with malicious library.

Domain Object Recommendations
Kernel-General-LibraryLinking-1 Dynamic linking Should generally not be allowed.

Linking everything statically doesn’t change anything wrt security as binaries will live under same user:group as libraries and setuid executables ignore LD_PRELOAD/LD_LIBRARY_PATH. It also increases RSS footprint and creates problems with upgrading.