+++ /dev/null
-.. Copyright © 2018 ANSSI.
- CLIP OS is a trademark of the French Republic.
- Content licensed under the Open License version 2.0 as published by Etalab
- (French task force for Open Data).
-
-.. _kernel:
-
-Kernel
-======
-
-The CLIP OS kernel is based on Linux. It also integrates:
-
-* existing hardening patches that are not upstream yet and that we consider
- relevant to our security model;
-* developments made for previous CLIP OS versions that we have not upstreamed
- yet (or that cannot be);
-* entirely new functionalities that have not been upstreamed yet (or that
- cannot be).
-
-Objectives
-----------
-
-As the core of a hardened operating system, the CLIP OS kernel is particularly
-responsible for:
-
-* providing **robust security mechanisms** to higher levels of the operating
- system, such as reliable isolation primitives;
-* maintaining maximal **trust in hardware resources**;
-* guaranteeing its **own protection** against various threats.
-
-Configuration
--------------
-
-In this section we discuss our security-relevant configuration choices for
-the CLIP OS kernel. Before starting, it is worth mentioning that:
-
-* We do our best to **limit the number of kernel modules**.
-
- In other words, as many modules as possible should be built-in. Modules are
- only used when needed either for the initramfs or to ease the automation of
- the deployment of CLIP OS on multiple different machines (for the moment, we
- only target a QEMU-KVM guest). This is particularly important as module
- loading is disabled after CLIP OS startup.
-
-* We **focus on a secure configuration**. The remaining of the configuration
- is minimal and it is your job to tune it for your machines and use cases.
-
-* CLIP OS only supports the x86-64 architecture for now.
-
-* Running 32-bit programs is voluntarily unsupported. Should you change that
- in your custom kernel, keep in mind that it requires further attention when
- configuring it (e.g., ensure that ``CONFIG_COMPAT_VDSO=n``).
-
-* Many options that are not useful to us are disabled in order to cut attack
- surface. As they are not all detailed below, please see
- ``src/portage/clip/sys-kernel/clipos-kernel/files/config.d/blacklist`` for an
- exhaustive list of the ones we **explicitly** disable.
-
-General setup
-~~~~~~~~~~~~~
-
-.. describe:: CONFIG_AUDIT=y
-
- CLIP OS will need the auditing infrastructure.
-
-.. describe:: CONFIG_IKCONFIG=n
-
- We do not need ``.config`` to be available at runtime.
-
-.. describe:: CONFIG_KALLSYMS=n
-
- Symbols are only useful for debug and attack purposes.
-
-.. describe:: CONFIG_EXPERT=y
-
- This unlocks additional configuration options we need.
-
-.. ---
-
-.. describe:: CONFIG_USER_NS=n
-
- User namespaces can be useful for some use cases but even more to an
- attacker. We choose to disable them for the moment, but we could also enable
- them and use the ``kernel.unprivileged_userns_clone`` sysctl provided by
- linux-hardened to disable their unprivileged use.
-
-.. ---
-
-.. describe:: CONFIG_SLUB_DEBUG=y
-
- Allow allocator validation checking to be enabled.
-
-.. describe:: CONFIG_SLAB_MERGE_DEFAULT=n
-
- Merging SLAB caches can make heap exploitation easier.
-
-.. describe:: CONFIG_SLAB_FREELIST_RANDOM=y
-
- Randomize allocator freelists
-
-.. describe:: CONFIG_SLAB_FREELIST_HARDENED=y
-
- Harden slab metadata
-
-.. describe:: CONFIG_SLAB_HARDENED=y
-
- Add various little checks to harden the slab allocator. [linux-hardened]_
-
-.. describe:: CONFIG_SLAB_CANARY=y
-
- Place canaries at the end of slab allocations. [linux-hardened]_
-
-.. describe:: CONFIG_SLAB_SANITIZE=y
-
- Zero-fill slab allocations on free to reduce risks of information leaks and
- help mitigate use-after-free vulnerabilities. [linux-hardened]_
-
- .. describe:: CONFIG_SLAB_SANITIZE_VERIFY=y
-
- Verify that newly allocated slab allocations are zeroed to detect
- write-after-free bugs. [linux-hardened]_
-
-
-.. ---
-
-.. describe:: CONFIG_COMPAT_BRK=n
-
- Enabling this would disable brk ASLR.
-
-.. ---
-
-.. describe:: CONFIG_GCC_PLUGINS=y
-
- Enable GCC plugins, some of which are security-relevant; GCC 4.7 at least is
- required.
-
- .. describe:: CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
-
- Instrument some kernel code to gather additional (but not
- cryptographically secure) entropy at boot time.
-
- .. describe:: CONFIG_GCC_PLUGIN_STRUCTLEAK=y
-
- Prevent potential information leakage by forcing initialization of
- structures containing userspace addresses. This is particularly
- important to prevent trivial bypassing of KASLR.
-
- .. describe:: CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y
-
- Extend forced initialization to all local structures that have their
- address taken at any point.
-
- .. describe:: CONFIG_GCC_PLUGIN_RANDSTRUCT=y
-
- Randomize layout of sensitive kernel structures. Exploits targeting such
- structures then require an additional information leak vulnerability.
-
- .. describe:: CONFIG_GCC_PLUGIN_RANDSTRUCT_PERFORMANCE=n
-
- Do not weaken structure randomization
-
-.. ---
-
-.. describe:: CONFIG_ARCH_MMAP_RND_BITS=32
-
- Use maximum number of randomized bits for the mmap base address on x86_64.
- Note that thanks to a linux-hardened patch, this also impacts the number of
- randomized bits for the stack base address.
-
-.. ---
-
-.. describe:: CONFIG_STACKPROTECTOR=y
- CONFIG_STACKPROTECTOR_STRONG=y
-
- Use ``-fstack-protector-strong`` for best stack canary coverage; GCC 4.9 at
- least is required.
-
-.. describe:: CONFIG_VMAP_STACK=y
-
- Virtually-mapped stacks benefit from guard pages, thus making kernel stack
- overflows harder to exploit.
-
-.. describe:: CONFIG_REFCOUNT_FULL=y
-
- Do extensive checks on reference counting to prevent use-after-free
- conditions. Without this option, on x86, there already is a fast
- assembly-based protection based on the PaX implementation but it does not
- cover all cases.
-
-.. ---
-
-.. describe:: CONFIG_STRICT_MODULE_RWX=y
-
- Enforce strict memory mappings permissions for loadable kernel modules.
-
-.. ---
-
-Although CLIP OS stores kernel modules in a read-only rootfs whose integrity is
-guaranteed by dm-verity, we still enable and enforce module signing as an
-additional layer of security:
-
- .. describe:: CONFIG_MODULE_SIG=y
- CONFIG_MODULE_SIG_FORCE=y
- CONFIG_MODULE_SIG_ALL=y
- CONFIG_MODULE_SIG_SHA512=y
- CONFIG_MODULE_SIG_HASH="sha512"
-
-.. ---
-
-.. describe:: CONFIG_LOCAL_INIT=n
-
- This option requires compiler support for ``-fsanitize=local-init``, which
- is only available in Clang. [linux-hardened]_
-
-Processor type and features
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_RETPOLINE=y
-
- Retpolines are needed to protect against Spectre v2. GCC 7.3.0 or higher is
- required.
-
-.. describe:: CONFIG_LEGACY_VSYSCALL_NONE=y
-
- The vsyscall table is not required anymore by libc and is a fixed-position
- potential source of ROP gadgets.
-
-.. describe:: CONFIG_X86_VSYSCALL_EMULATION=n
-
- See above.
-
-.. describe:: CONFIG_MICROCODE=y
-
- Needed to benefit from microcode updates and thus security fixes (e.g.,
- additional Intel pseudo-MSRs to be used by the kernel as a mitigation for
- various speculative execution vulnerabilities).
-
-.. describe:: CONFIG_X86_MSR=y
-
- See above explanation about ``CONFIG_MICROCODE``.
-
-.. describe:: CONFIG_KSM=n
-
- Enabling this feature can make cache side-channel attacks such as
- FLUSH+RELOAD much easier to carry out.
-
-.. ---
-
-.. describe:: CONFIG_DEFAULT_MMAP_MIN_ADDR=65536
-
- This should in particular be non-zero to prevent the exploitation of kernel
- NULL pointer bugs.
-
-.. describe:: CONFIG_MTRR=y
-
- Memory Type Range Registers can make speculative execution bugs a bit harder
- to exploit.
-
-.. describe:: CONFIG_X86_PAT=y
-
- Page Attribute Tables are the modern equivalents of MTRRs, which we
- described above.
-
-.. describe:: CONFIG_ARCH_RANDOM=y
-
- Enable the RDRAND instruction to benefit from a secure hardware RNG if
- supported. See ``CONFIG_RANDOM_TRUST_CPU`` for warnings about that.
-
-.. describe:: CONFIG_X86_SMAP=y
-
- Enable Supervisor Mode Access Prevention to prevent ret2usr exploitation
- techniques.
-
-.. describe:: CONFIG_X86_INTEL_UMIP=y
-
- Enable User Mode Instruction Prevention. Note that hardware supporting this
- feature is not common yet.
-
-.. describe:: CONFIG_X86_INTEL_MPX=n
-
- Intel Memory Protection Extensions add hardware assistance to memory
- protection. Compiler support is required but is deprecated in GCC 8 and will
- probably be dropped in GCC 9.
-
-.. describe:: CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS=n
-
- Memory Protection Keys are a promising feature but they are still not
- supported on current hardware.
-
-.. ---
-
-Enable the **seccomp** BPF userspace API for syscall attack surface reduction:
-
- .. describe:: CONFIG_SECCOMP=y
- CONFIG_SECCOMP_FILTER=y
-
-.. ---
-
-.. describe:: CONFIG_RANDOMIZE_BASE=y
-
- While this may be seen as a `controversial
- <https://grsecurity.net/kaslr_an_exercise_in_cargo_cult_security.php>`_
- feature, it makes sense for CLIP OS. Indeed, KASLR may be defeated thanks to
- the kernel interfaces that are available to an attacker, or through attacks
- leveraging hardware vulnerabilities such as speculative and out-of-order
- execution ones. However, CLIP OS follows the *defense in depth* principle
- and an attack surface reduction approach. Thus, the following points make
- KASLR relevant in the CLIP OS kernel:
-
- * KASLR was initially designed to counter remote attacks but the strong
- security model of CLIP OS (e.g., no sysfs mounts in most containers,
- minimal procfs, no arbitrary code execution) makes a local attack
- more complex to carry out.
- * STRUCTLEAK, STACKLEAK, kptr_restrict and
- ``CONFIG_SECURITY_DMESG_RESTRICT`` are enabled in CLIP OS.
- * The CLIP OS kernel is custom-compiled (at least for a given deployment),
- its image is unreadable to all users including privileged ones and updates
- are end-to-end encrypted. This makes both the content and addresses of the
- kernel image secret. Note that, however, the production kernel image is
- currently part of an EFI binary and is not encrypted, causing it to be
- accessible to a physical attacker. This will change in the future as we
- will only use the kernel included in the EFI binary to boot and then
- *kexec* to the real production kernel whose image will be located on an
- encrypted disk partition.
- * We enable ``CONFIG_PANIC_ON_OOPS`` by default so that the kernel
- cannot recover from failed exploit attempts, thus preventing any brute
- forcing.
- * We enable Kernel Page Table Isolation, mitigating Meltdown and potential
- other hardware information leakage. Variante 3a (Rogue System Register
- Read) however remains an important threat to KASLR.
-
-.. ---
-
-.. describe:: CONFIG_RANDOMIZE_MEMORY=y
-
- Most of the above explanations stand for that feature.
-
-.. describe:: CONFIG_KEXEC=n
- CONFIG_KEXEC_FILE=n
-
- Disable the ``kexec()`` system call to prevent an already-root attacker from
- rebooting on an untrusted kernel.
-
-.. describe:: CONFIG_CRASH_DUMP=n
-
- A crash dump can potentially provide an attacker with useful information.
- However we disabled ``kexec()`` syscalls above thus this configuration
- option should have no impact anyway.
-
-.. ---
-
-.. describe:: CONFIG_MODIFY_LDT_SYSCALL=n
-
- This is not supposed to be needed by userspace applications and only
- increases the kernel attack surface.
-
-Power management and ACPI options
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_HIBERNATION=n
-
- The CLIP OS swap partition is encrypted with an ephemeral key and thus
- cannot support suspend to disk.
-
-Firmware Drivers
-~~~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_RESET_ATTACK_MITIGATION=n
-
- In order to work properly, this mitigation requires userspace support that
- is currently not available in CLIP OS. Moreover, due to our use of Secure
- Boot, Trusted Boot and the fact that machines running CLIP OS are expected
- to lock their BIOS with a password, the type of *cold boot attacks* this
- mitigation is supposed to thwart should not be an issue.
-
-Executable file formats / Emulations
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_BINFMT_MISC=n
-
- We do not want our kernel to support miscellaneous binary classes. ELF
- binaries and interpreted scripts starting with a shebang are enough.
-
-.. describe:: CONFIG_COREDUMP=n
-
- Core dumps can provide an attacker with useful information.
-
-Networking support
-~~~~~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_SYN_COOKIES=y
-
- Enable TCP syncookies.
-
-Device Drivers
-~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_TCG_TPM=n
-
- TPM use is not supported by CLIP OS yet.
-
-.. describe:: CONFIG_DEVMEM=n
-
- The ``/dev/mem`` device should not be required by any user application
- nowadays.
-
- .. note::
-
- If you must enable it, at least enable ``CONFIG_STRICT_DEVMEM`` and
- ``CONFIG_IO_STRICT_DEVMEM`` to restrict at best access to this device.
-
-.. describe:: CONFIG_DEVKMEM=n
-
- This virtual device is only useful for debug purposes and is very dangerous
- as it allows direct kernel memory writing (particularly useful for
- rootkits).
-
-.. describe:: CONFIG_LEGACY_PTYS=n
-
- Use the modern PTY interface only.
-
-.. describe:: CONFIG_DEVPORT=n
-
- The ``/dev/port`` device should not be used anymore by userspace, and it
- could increase the kernel attack surface.
-
-.. describe:: CONFIG_RANDOM_TRUST_CPU=n
-
- Do not rely exclusively on the hardware RNG provided by the CPU manufacturer
- to initialize Linux's CRNG, as we do not mind blocking a bit more at boot
- time while additional entropy sources are mixed in.
-
-The IOMMU allows for protecting the system's main memory from arbitrary
-accesses from devices (e.g., DMA attacks). Note that this is related to
-hardware features. On a recent Intel machine, we enable the following:
-
- .. describe:: CONFIG_IOMMU_SUPPORT=y
- CONFIG_INTEL_IOMMU=y
- CONFIG_INTEL_IOMMU_SVM=y
- CONFIG_INTEL_IOMMU_DEFAULT_ON=y
-
-File systems
-~~~~~~~~~~~~
-
-.. describe:: CONFIG_PROC_KCORE=n
-
- Enabling this would provide an attacker with precious information on the
- running kernel.
-
-Kernel hacking
-~~~~~~~~~~~~~~
-
-.. describe:: CONFIG_MAGIC_SYSRQ=n
-
- This should only be needed for debugging.
-
-.. describe:: CONFIG_DEBUG_KERNEL=y
-
- This is useful even in a production kernel to enable further configuration
- options that have security benefits.
-
-.. describe:: CONFIG_DEBUG_VIRTUAL=y
-
- Enable sanity checks in virtual to page code.
-
-.. describe:: CONFIG_STRICT_KERNEL_RWX=y
-
- Ensure kernel page tables have strict permissions.
-
-.. describe:: CONFIG_DEBUG_WX=y
-
- Check and report any dangerous memory mapping permissions, i.e., both
- writable and executable kernel pages.
-
-.. describe:: CONFIG_DEBUG_FS=n
-
- The debugfs virtual file system is only useful for debugging and protecting
- it would require additional work.
-
-.. describe:: CONFIG_SLUB_DEBUG_ON=n
-
- Using the ``slub_debug`` command line parameter provides more fine grained
- control.
-
-.. describe:: CONFIG_PANIC_ON_OOPS=y
- CONFIG_PANIC_TIMEOUT=-1
-
- Prevent potential further exploitation of a bug by immediately panicking the
- kernel.
-
-The following options add additional checks and validation for various
-commonly targeted kernel structures:
-
- .. describe:: CONFIG_DEBUG_CREDENTIALS=y
- CONFIG_DEBUG_NOTIFIERS=y
- CONFIG_DEBUG_LIST=y
- CONFIG_DEBUG_SG=y
- .. describe:: CONFIG_BUG_ON_DATA_CORRUPTION=y
-
- Note that linux-hardened patches add more places where this configuration
- option has an impact.
-
- .. describe:: CONFIG_SCHED_STACK_END_CHECK=y
- .. describe:: CONFIG_PAGE_POISONING=n
-
- We choose to poison pages with zeroes and thus prefer using the simpler
- PaX-based implementation provided by linux-hardened (see
- ``CONFIG_PAGE_SANITIZE`` below).
-
-Security
-~~~~~~~~
-
-.. describe:: CONFIG_SECURITY_DMESG_RESTRICT=y
-
- Prevent unprivileged users from gathering information from the kernel log
- buffer via ``dmesg(8)``. Note that this still can be overridden through the
- ``kernel.dmesg_restrict`` sysctl.
-
-.. describe:: CONFIG_PAGE_TABLE_ISOLATION=y
-
- Enable KPTI to prevent Meltdown attacks and, more generally, reduce the
- number of hardware side channels.
-
-.. ---
-
-.. describe:: CONFIG_INTEL_TXT=n
-
- CLIP OS does not use Intel Trusted Execution Technology.
-
-.. ---
-
-.. describe:: CONFIG_HARDENED_USERCOPY=y
-
- Harden data copies between kernel and user spaces, preventing classes of
- heap overflow exploits and information leaks.
-
-.. describe:: CONFIG_HARDENED_USERCOPY_FALLBACK=n
-
- Use strict whitelisting mode, i.e., do not ``WARN()``.
-
-.. describe:: CONFIG_FORTIFY_SOURCE=y
-
- Leverage compiler to detect buffer overflows.
-
-.. describe:: CONFIG_FORTIFY_SOURCE_STRICT_STRING=n
-
- This extends ``FORTIFY_SOURCE`` to intra-object overflow checking. It is
- useful to find bugs but not recommended for a production kernel yet.
- [linux-hardened]_
-
-.. describe:: CONFIG_STATIC_USERMODEHELPER=y
-
- This makes the kernel route all usermode helper calls to a single binary
- that cannot have its name changed. Without this, the kernel can be tricked
- into calling an attacker-controlled binary (e.g. to bypass SMAP, cf.
- `exploitation <https://seclists.org/oss-sec/2016/q4/621>`_ of
- CVE-2016-8655).
-
- .. describe:: CONFIG_STATIC_USERMODEHELPER_PATH=""
-
- Currently, we have no need for usermode helpers therefore we simply
- disable them. If we ever need some, this path will need to be set to a
- custom trusted binary in charge of filtering and choosing what real
- helpers should then be called.
-
-.. ---
-
-.. describe:: CONFIG_SECURITY=y
-
- Enable us to choose different security modules.
-
-.. describe:: CONFIG_SECURITY_SELINUX=y
-
- CLIP OS intends to leverage SELinux in its security model.
-
-.. describe:: CONFIG_SECURITY_SELINUX_BOOTPARAM=n
-
- We do not need SELinux to be disableable.
-
-.. describe:: CONFIG_SECURITY_SELINUX_DISABLE=n
-
- We do not want SELinux to be disabled. In addition, this would prevent LSM
- structures such as security hooks from being marked as read-only.
-
-.. describe:: CONFIG_SECURITY_SELINUX_DEVELOP=y
-
- For now, but will eventually be ``n``.
-
-.. ---
-
-.. describe:: DEFAULT_SECURITY_DAC=y
-
- The default security module will be changed to SELinux once CLIP OS fully
- uses it.
-
-.. ---
-
-.. describe:: CONFIG_SECURITY_YAMA=y
-
- The Yama LSM currently provides ptrace scope restriction (which might be
- redundant with CLIP-LSM in the future).
-
-.. ---
-
-.. describe:: CONFIG_INTEGRITY=n
-
- The integrity subsystem provides several components, the security benefits
- of which are already enforced by CLIP OS (e.g., read-only mounts for all
- parts of the system containing executable programs).
-
-.. ---
-
-.. describe:: CONFIG_SECURITY_PERF_EVENTS_RESTRICT=y
-
- See documentation about the ``kernel.perf_event_paranoid`` sysctl below.
- [linux-hardened]_
-
-.. ---
-
-.. describe:: CONFIG_PAGE_SANITIZE=y
-
- Zero-fill page allocations on free to reduce risks of information leaks and
- help mitigate a subset of use-after-free vulnerabilities. This is a simpler
- equivalent to upstream's ``CONFIG_PAGE_POISONING_ZERO``. [linux-hardened]_
-
-.. describe:: CONFIG_PAGE_SANITIZE_VERIFY=y
-
- Verify that newly allocated pages are zeroed to detect write-after-free
- bugs. [linux-hardened]_
-
-.. ---
-
-.. describe:: CONFIG_SECURITY_TIOCSTI_RESTRICT=y
-
- This prevents unprivileged users from using the TIOCSTI ioctl to inject
- commands into other processes which share a tty session. [linux-hardened]_
-
-We incorporated most of the *Lockdown* patch series into the CLIP OS kernel,
-though it may be merged into the mainline kernel in the near future.
-Basically, *Lockdown* tries to disable many mechanisms that could allow the
-superuser to eventually run untrusted code in kernel mode (note that a
-significant portion of them are already disabled in the CLIP OS kernel due to
-our custom configuration). This is an interesting work for CLIP OS as we want
-to avoid persistence on a compromised machine even in the case of an
-already-root attacker. Among the several configuration options brought by
-*Lockdown*, we enable the following ones:
-
- .. describe:: CONFIG_LOCK_DOWN_KERNEL=y
- CONFIG_LOCK_DOWN_MANDATORY=y
-
-Similarly, we incorporated the *STACKLEAK* feature ported from grsecurity/PaX
-by Alexander Popov and which should be merged upstream ultimately. *STACKLEAK*
-erases the kernel stack before returning from system calls in order to reduce
-the information which kernel stack leak bugs can reveal. It also blocks kernel
-stack depth overflows caused by ``alloca()``, such as Stack Clash attacks.
-
- .. describe:: CONFIG_GCC_PLUGIN_STACKLEAK=y
- CONFIG_STACKLEAK_TRACK_MIN_SIZE=100
- CONFIG_STACKLEAK_METRICS=n
- CONFIG_STACKLEAK_RUNTIME_DISABLE=n
-
-
-Compilation
------------
-
-GCC version 7.3.0 or higher is required to fully benefit from retpolines
-(``-mindirect-branch=thunk-extern``).
-
-
-Sysctl Security Tuning
-----------------------
-
-Many sysctls are not security-relevant or only play a role if some kernel
-configuration options are enabled/disabled. In other words, the following is
-tightly related to the CLIP OS kernel configuration detailed above.
-
-.. describe:: kernel.kptr_restrict = 2
-
- Hide kernel addresses in ``/proc`` and other interfaces, even to privileged
- users.
-
-.. describe:: kernel.yama.ptrace_scope = 3
-
- Enable the strictest ptrace scope restriction provided by the Yama LSM.
-
-.. describe:: kernel.perf_event_paranoid = 3
-
- This completely disallows unprivileged access to the ``perf_event_open()``
- system call. Note that this requires a patch included in linux-hardened (see
- `here <https://lwn.net/Articles/696216/>`_ for the reason why it is not
- upstream), otherwise it is the same as setting this sysctl to ``2``. This is
- actually not needed as we already enable
- ``CONFIG_SECURITY_PERF_EVENTS_RESTRICT``.
-
-.. describe:: kernel.tiocsti_restrict = 1
-
- This is already forced by the ``CONFIG_SECURITY_TIOCSTI_RESTRICT`` kernel
- configuration option that we enable.
-
-The following two sysctls help mitigating TOCTOU vulnerabilities by preventing
-users from creating symbolic or hard links to files they do not own or have
-read/write access to:
-
- .. describe:: fs.protected_symlinks = 1
- fs.protected_hardlinks = 1
-
-In addition, the following other two sysctls impose restrictions on the
-opening of FIFOs and regular files in order to make similar spoofing attacks
-harder:
-
- .. describe:: fs.protected_fifos = 2
- fs.protected_regular = 2
-
-We do not simply disable the BPF Just in Time compiler as CLIP OS plans on
-using it:
-
- .. describe:: kernel.unprivileged_bpf_disabled = 1
-
- Prevent unprivileged users from using BPF.
-
- .. describe:: net.core.bpf_jit_harden = 2
-
- Trades off performance but helps mitigate JIT spraying.
-
-.. describe:: kernel.deny_new_usb = 0
-
- The management of USB devices is handled at a higher level by CLIP OS.
- [linux-hardened]_
-
-.. describe:: kernel.device_sidechannel_restrict = 1
-
- Restrict device timing side channels. [linux-hardened]_
-
-.. describe:: fs.suid_dumpable = 0
-
- Do not create core dumps of setuid executables. Note that we already
- disable all core dumps by setting ``CONFIG_COREDUMP=n``.
-
-.. describe:: kernel.pid_max = 65536
-
- Increase the space for PID values.
-
-.. describe:: kernel.modules_disabled = 1
-
- Disable module loading once systemd has loaded the ones required for the
- running machine according to a profile (i.e., a predefined and
- hardware-specific list of modules).
-
-Pure network sysctls (``net.ipv4.*`` and ``net.ipv6.*``) will be detailed in a
-separate place.
-
-
-Command line parameters
------------------------
-
-We pass the following command line parameters to the kernel:
-
-.. describe:: extra_latent_entropy
-
- This parameter provided by a linux-hardened patch (based on the PaX
- implementation) enables a very simple form of latent entropy extracted
- during system start-up and added to the entropy obtained with
- ``GCC_PLUGIN_LATENT_ENTROPY``.
-
-.. describe:: pti=on
-
- This force-enables KPTI even on CPUs claiming to be safe from Meltdown.
-
-.. describe:: spectre_v2=on
-
- Same reasoning as above but for the Spectre v2 vulnerability. Note that this
- implies ``spectre_v2_user=on``, which enables the mitigation against user
- space to user space task attacks (namely IBPB and STIBP when available and
- relevant).
-
-.. describe:: spec_store_bypass_disable=seccomp
-
- Same reasoning as above but for the Spectre v4 vulnerability. Note that this
- mitigation requires updated microcode for Intel processors.
-
-.. describe:: iommu=force
-
- Even if we correctly enable the IOMMU in the kernel configuration, the
- kernel can still decide for various reasons to not initialize it at boot.
- Therefore, we force it with this parameter. Note that with some Intel
- chipsets, you may need to add ``intel_iommu=igfx_off`` to allow your GPU to
- access the physical memory directly without going through the DMA Remapping.
-
-.. describe:: slub_debug=F
-
- The ``F`` option adds many sanity checks to various slab operations. Other
- interesting options that we considered but eventually chose to not use are:
-
- * The ``P`` option, which enables poisoning on slab cache allocations,
- disables the ``SLAB_SANITIZE`` and ``SLAB_SANITIZE_VERIFY`` features from
- linux-hardened. As they respectively poison with zeroes on object freeing
- and check the zeroing on object allocations, we prefer enabling them
- instead of using ``slub_debug=P``.
- * The ``Z`` option enables red zoning, i.e., it adds extra areas around
- slab objects that detect when one is overwritten past its real size.
- This can help detect overflows but we already rely on ``SLAB_CANARY``
- provided by linux-hardened. A canary is much better than a simple red
- zone as it is supposed to be random.
-
-Also, note that:
-
-* ``slub_nomerge`` is not used as we already set
- ``CONFIG_SLAB_MERGE_DEFAULT=n`` in the kernel configuration.
-* ``page_poison`` is not needed by the page poisoning implementation provided
- by linux-hardened patches.
-* ``l1tf``: The built-in PTE Inversion mitigation is sufficient to mitigate
- the L1TF vulnerability as long as CLIP OS is not used as an hypervisor with
- untrusted guest VMs. If it were to be someday, ``l1tf=full,force`` should be
- used to force-enable VMX unconditional cache flushes and force-disable SMT
- (note that an Intel microcode update is not required for this mitigation to
- work but improves performance by providing a way to invalidate caches with a
- finer granularity).
-
-.. rubric:: Citations and origin of some items
-
-.. [linux-hardened]
- This item is provided by the ``linux-hardened`` patches.
-
-.. vim: set tw=79 ts=2 sts=2 sw=2 et:
-
projects / kconfig-hardened-check.git / blobdiff