--- /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
+ CONFIG_IKHEADERS=n
+
+ We do not need ``.config`` to be available at runtime, neither do we need
+ access to kernel headers through *sysfs*.
+
+.. describe:: CONFIG_KALLSYMS=n
+
+ Symbols are only useful for debug and attack purposes.
+
+.. describe:: CONFIG_USERFAULTFD=n
+
+ The ``userfaultfd()`` system call adds attack surface and can `make heap
+ sprays easier <https://duasynt.com/blog/linux-kernel-heap-spray>`_. Note
+ that the ``vm.unprivileged_userfaultfd`` sysctl can also be used to restrict
+ the use of this system call to privileged users.
+
+.. 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_CANARY=y
+
+ Place canaries at the end of slab allocations. [linux-hardened]_
+
+.. ---
+
+.. describe:: CONFIG_SHUFFLE_PAGE_ALLOCATOR=y
+
+ Page allocator randomization is primarily a performance improvement for
+ direct-mapped memory-side-cache utilization, but it does reduce the
+ predictability of page allocations and thus complements
+ ``SLAB_FREELIST_RANDOM``. The ``page_alloc.shuffle=1`` parameter needs to be
+ added to the kernel command line.
+
+.. ---
+
+.. 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
+ CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y
+
+ Prevent potential information leakage by forcing zero-initialization of:
+
+ - structures on the stack containing userspace addresses;
+ - any stack variable (thus including structures) that may be passed by
+ reference and has not already been explicitly initialized.
+
+ This is particularly important to prevent trivial bypassing of KASLR.
+
+ .. 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_INIT_STACK_ALL=n
+
+ This option requires compiler support that is currently only available in
+ Clang.
+
+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
+ CONFIG_LEGACY_VSYSCALL_EMULATE=n
+ CONFIG_LEGACY_VSYSCALL_XONLY=n
+ CONFIG_X86_VSYSCALL_EMULATION=n
+
+ The vsyscall table is not required anymore by libc and is a fixed-position
+ potential source of ROP gadgets.
+
+.. 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=n
+ CONFIG_X86_CPUID=n
+
+ Enabling those features would only present userspace with more attack
+ surface.
+
+.. 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 also ``CONFIG_RANDOM_TRUST_CPU``.
+
+.. 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 (MPX) add hardware assistance to memory
+ protection. Compiler support is required but was deprecated in GCC 8 and
+ removed from GCC 9. Moreover, MPX kernel support is `being dropped
+ <MPX_dropped_>`_.
+
+ .. _MPX_dropped: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f240652b6032b48ad7fa35c5e701cc4c8d697c0b
+
+.. describe:: CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS=n
+
+ Memory Protection Keys are a promising feature but they are still not
+ supported on current hardware.
+
+.. describe:: CONFIG_X86_INTEL_TSX_MODE_OFF=y
+
+ Set the default value of the ``tsx`` kernel parameter to ``off``.
+
+.. ---
+
+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_HW_RANDOM_TPM=y
+
+ Expose the TPM's Random Number Generator (RNG) as a Hardware RNG (HWRNG)
+ device, allowing the kernel to collect randomness from it. See documentation
+ of ``CONFIG_RANDOM_TRUST_CPU`` and the ``rng_core.default_quality`` command
+ line parameter for supplementary information.
+
+.. describe:: CONFIG_TCG_TPM=y
+
+ CLIP OS leverages the TPM to ensure :ref:`boot integrity <trusted_boot>`.
+
+.. 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_LDISC_AUTOLOAD=n
+
+ Do not automatically load any line discipline that is in a kernel module
+ when an unprivileged user asks for it.
+
+.. 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
+ CONFIG_RANDOM_TRUST_BOOLOADER=n
+
+ Do not **credit** entropy generated by the CPU manufacturer's HWRNG nor
+ provided by the booloader, and included in Linux's entropy pool. Fast and
+ robust initialization of Linux's CSPRNG is instead achieved thanks to the
+ TPM's HWRNG (see documentation of ``CONFIG_HW_RANDOM_TPM`` and the
+ ``rng_core.default_quality`` command line parameter).
+
+.. describe:: CONFIG_STAGING=n
+
+ *Staging* drivers are typically of lower quality and under heavy
+ development. They are thus more likely to contain bugs, including security
+ vulnerabilities, and should be avoided.
+
+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
+ ``init_on_free`` in combination with linux-hardened's
+ ``PAGE_SANITIZE_VERIFY``.
+
+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:: CONFIG_SECURITY_LOCKDOWN_LSM=y
+ CONFIG_SECURITY_LOCKDOWN_LSM_EARLY=y
+ CONFIG_LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY=y
+
+ Basically, the *lockdown* LSM tries to strengthen the boundary between the
+ superuser and the kernel. The *integrity* mode thus restricts access to
+ features that would allow userland to modify the running kernel, and the
+ *confidentiality* mode extends these restrictions to features that would
+ allow userland to extract confidential information held inside the kernel.
+ Note that a significant portion of such features is already disabled in the
+ CLIP OS kernel due to our custom configuration. The *lockdown* functionality
+ is important for CLIP OS as we want to prevent an attacker, be he highly
+ privileged, from persisting on a compromised machine.
+
+.. ---
+
+.. describe:: CONFIG_LSM="yama"
+
+ SELinux shall be stacked too once CLIP OS 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_SECURITY_TIOCSTI_RESTRICT=y
+
+ This prevents unprivileged users from using the TIOCSTI ioctl to inject
+ commands into other processes that share a tty session. [linux-hardened]_
+
+.. ---
+
+.. describe:: CONFIG_GCC_PLUGIN_STACKLEAK=y
+ CONFIG_STACKLEAK_TRACK_MIN_SIZE=100
+ CONFIG_STACKLEAK_METRICS=n
+ CONFIG_STACKLEAK_RUNTIME_DISABLE=n
+
+``STACKLEAK`` erases the kernel stack before returning from system calls,
+leaving it initialized to a poison value. This both reduces the information
+that kernel stack leak bugs can reveal and the exploitability of uninitialized
+stack variables. However, it does not cover functions reaching the same stack
+depth as prior functions during the same system call.
+
+It used to also block kernel stack depth overflows caused by ``alloca()``, such
+as Stack Clash attacks. We maintained this functionality for our kernel for a
+while but eventually `dropped it
+<https://github.com/clipos/src_external_linux/commit/3e5f9114fc2f70f6d2ae5d10db10869e0564eb03>`_.
+
+.. describe:: CONFIG_INIT_ON_FREE_DEFAULT_ON=y
+ CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y
+
+ These set ``init_on_free=1`` and ``init_on_alloc=1`` on the kernel command
+ line. See the documentation of these kernel parameters for details.
+
+.. describe:: CONFIG_PAGE_SANITIZE_VERIFY=y
+ CONFIG_SLAB_SANITIZE_VERIFY=y
+
+ Verify that newly allocated pages and slab allocations are zeroed to detect
+ write-after-free bugs. This works in concert with ``init_on_free`` and is
+ adjusted to not be redundant with ``init_on_alloc``.
+ [linux-hardened]_
+
+.. ---
+
+
+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:: dev.tty.ldisc_autoload = 0
+
+ See ``CONFIG_LDISC_AUTOLOAD`` above, which serves as a default value for
+ this sysctl.
+
+.. 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. This is actually not needed as we already enable
+ ``CONFIG_SECURITY_PERF_EVENTS_RESTRICT``. [linux-hardened]_
+
+ 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).
+ Indeed, on a mainline kernel without such a patch, the above is equivalent
+ to setting this sysctl to ``2``, which would still allow the profiling of
+ user processes.
+
+.. describe:: kernel.tiocsti_restrict = 1
+
+ This is already forced by the ``CONFIG_SECURITY_TIOCSTI_RESTRICT`` kernel
+ configuration option that we enable. [linux-hardened]_
+
+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
+(note however that `these restrictions currently do not apply to networked
+filesystems, among others <sysctl_protected_limitations_>`_):
+
+ .. describe:: fs.protected_fifos = 2
+ fs.protected_regular = 2
+
+.. _sysctl_protected_limitations: https://www.openwall.com/lists/oss-security/2020/01/28/2
+
+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``. [linux-hardened]_
+
+.. 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:: mds=full,nosmt
+
+ This parameter controls optional mitigations for the Microarchitectural Data
+ Sampling (MDS) class of Intel CPU vulnerabilities. Not specifying this
+ parameter is equivalent to setting ``mds=full``, which leaves SMT enabled
+ and therefore is not a complete mitigation. Note that this mitigation
+ requires an Intel microcode update and also addresses the TSX Asynchronous
+ Abort (TAA) Intel CPU vulnerability on systems that are affected by MDS.
+
+.. 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 ``init_on_free`` and ``SLAB_SANITIZE_VERIFY`` features. 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.
+
+.. describe:: page_alloc.shuffle=1
+
+ See ``CONFIG_SHUFFLE_PAGE_ALLOCATOR``.
+
+.. describe:: rng_core.default_quality=512
+
+ Increase trust in the TPM's HWRNG to robustly and fastly initialize Linux's
+ CSPRNG by **crediting** half of the entropy it provides.
+
+Also, note that:
+
+* ``slub_nomerge`` is not used as we already set
+ ``CONFIG_SLAB_MERGE_DEFAULT=n`` in the kernel configuration.
+* ``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).
+* ``tsx=off``: This parameter is already set by default thanks to
+ ``CONFIG_X86_INTEL_TSX_MODE_OFF``. It deactivates the Intel TSX feature on
+ CPUs that support TSX control (i.e. are recent enough or received a microcode
+ update) and that are not already vulnerable to MDS, therefore mitigating the
+ TSX Asynchronous Abort (TAA) Intel CPU vulnerability.
+* ``tsx_async_abort``: This parameter controls optional mitigations for the TSX
+ Asynchronous Abort (TAA) Intel CPU vulnerability. Due to our use of
+ ``mds=full,nosmt`` in addition to ``CONFIG_X86_INTEL_TSX_MODE_OFF``, CLIP OS
+ is already protected against this vulnerability as long as the CPU microcode
+ has been updated, whether or not the CPU is affected by MDS. For the record,
+ if we wanted to keep TSX activated, we could specify
+ ``tsx_async_abort=full,nosmt``. Not specifying this parameter is equivalent
+ to setting ``tsx_async_abort=full``, which leaves SMT enabled and therefore
+ is not a complete mitigation. Note that this mitigation requires an Intel
+ microcode update and has no effect on systems that are already affected by
+ MDS and enable mitigations against it, nor on systems that disable TSX.
+* ``kvm.nx_huge_pages``: This parameter allows to control the KVM hypervisor
+ iTLB multihit mitigations. Such mitigations are not needed as long as CLIP OS
+ is not used as an hypervisor with untrusted guest VMs. If it were to be
+ someday, ``kvm.nx_huge_pages=force`` should be used to ensure that guests
+ cannot exploit the iTLB multihit erratum to crash the host.
+* ``mitigations``: This parameter controls optional mitigations for CPU
+ vulnerabilities in an arch-independent and more coarse-grained way. For now,
+ we keep using arch-specific options for the sake of explicitness. Not setting
+ this parameter equals setting it to ``auto``, which itself does not update
+ anything.
+* ``init_on_free=1`` is automatically set due to ``INIT_ON_FREE_DEFAULT_ON``. It
+ zero-fills page and slab allocations on free to reduce risks of information
+ leaks and help mitigate a subset of use-after-free vulnerabilities.
+* ``init_on_alloc=1`` is automatically set due to ``INIT_ON_ALLOC_DEFAULT_ON``.
+ The purpose of this functionality is to eliminate several kinds of
+ *uninitialized heap memory* flaws by zero-filling:
+
+ * all page allocator and slab allocator memory when allocated: this is
+ already guaranteed by our use of ``init_on_free`` in combination with
+ ``PAGE_SANITIZE_VERIFY`` and ``SLAB_SANITIZE_VERIFY`` from linux-hardened,
+ and thus has no effect;
+ * a few more *special* objects when allocated: these are the ones for which
+ we enable ``init_on_alloc`` as they are not covered by the aforementioned
+ combination of ``init_on_free`` and ``SANITIZE_VERIFY`` features.
+
+.. 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