Kernel Self Protection Project/Recommended Settings
Sometimes people ask the Kernel Self Protection Project what a secure set of build CONFIGs and runtime settings are. This is a brain-dump of the various options for a particularly paranoid system.
Another place to find recommended kernel hardening settings is via the "kconfig-hardened-check" tool maintained by Alexander Popov.
# Report BUG() conditions and kill the offending process. CONFIG_BUG=y # Make sure kernel page tables have safe permissions. CONFIG_DEBUG_KERNEL=y (prior to v4.11, needed to select CONFIG_DEBUG_RODATA below) CONFIG_DEBUG_RODATA=y (prior to v4.11) CONFIG_STRICT_KERNEL_RWX=y (since v4.11) # Report any dangerous memory permissions (not available on all archs). CONFIG_DEBUG_WX=y # Use -fstack-protector-strong (gcc 4.9+) for best stack canary coverage. # Prior to v4.18, these are: # CONFIG_CC_STACKPROTECTOR=y # CONFIG_CC_STACKPROTECTOR_STRONG=y CONFIG_STACKPROTECTOR=y CONFIG_STACKPROTECTOR_STRONG=y # Do not allow direct physical memory access (but if you must have it, at least enable STRICT mode...) # CONFIG_DEVMEM is not set CONFIG_STRICT_DEVMEM=y CONFIG_IO_STRICT_DEVMEM=y # Provides some protections against SYN flooding. CONFIG_SYN_COOKIES=y # Perform additional validation of various commonly targeted structures. CONFIG_DEBUG_CREDENTIALS=y CONFIG_DEBUG_NOTIFIERS=y CONFIG_DEBUG_LIST=y CONFIG_DEBUG_SG=y CONFIG_BUG_ON_DATA_CORRUPTION=y CONFIG_SCHED_STACK_END_CHECK=y # Provide userspace with seccomp BPF API for syscall attack surface reduction. CONFIG_SECCOMP=y CONFIG_SECCOMP_FILTER=y # Provide userspace with ptrace ancestry protections. CONFIG_SECURITY=y CONFIG_SECURITY_YAMA=y # Perform usercopy bounds checking. (And disable fallback to gain full whitelist enforcement.) CONFIG_HARDENED_USERCOPY=y # CONFIG_HARDENED_USERCOPY_FALLBACK is not set # CONFIG_HARDENED_USERCOPY_PAGESPAN is not set # Randomize allocator freelists, harden metadata. CONFIG_SLAB_FREELIST_RANDOM=y CONFIG_SLAB_FREELIST_HARDENED=y # Randomize high-order page allocation freelist. CONFIG_SHUFFLE_PAGE_ALLOCATOR=y # Allow allocator validation checking to be enabled (see "slub_debug=P" below). CONFIG_SLUB_DEBUG=y # Wipe higher-level memory allocations when they are freed (needs "page_poison=1" command line below). # (If you can afford even more performance penalty, leave CONFIG_PAGE_POISONING_NO_SANITY=n) CONFIG_PAGE_POISONING=y CONFIG_PAGE_POISONING_NO_SANITY=y CONFIG_PAGE_POISONING_ZERO=y # Wipe slab and page allocations (since v5.3) # Instead of "slub_debug=P" and "page_poison=1", a single place can control memory allocation wiping now. # The init_on_free is only needed if there is concern about minimizing stale data lifetime. CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y CONFIG_INIT_ON_FREE_DEFAULT_ON=y # Initialize all stack variables on function entry. (Clang and GCC 12+ builds only. For earlier GCC, see CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y below) CONFIG_INIT_STACK_ALL_ZERO=y # Adds guard pages to kernel stacks (not all architectures support this yet). CONFIG_VMAP_STACK=y # Perform extensive checks on reference counting. CONFIG_REFCOUNT_FULL=y # Check for memory copies that might overflow a structure in str*() and mem*() functions both at build-time and run-time. CONFIG_FORTIFY_SOURCE=y # Avoid kernel memory address exposures via dmesg (sets sysctl kernel.dmesg_restrict initial value to 1) CONFIG_SECURITY_DMESG_RESTRICT=y # Randomize kernel stack offset on syscall entry (since v5.13). CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT=y # Enable sampling-based overflow detection. This is similar to KASAN coverage, but with almost zero runtime overhead. CONFIG_KFENCE=y # Do not ignore compile-time warnings (since v5.15) CONFIG_WERROR=y # Force IOMMU TLB invalidation so devices will never be able to access stale data contents (or set "iommu.passthrough=0 iommu.strict=1" at boot) CONFIG_IOMMU_DEFAULT_DMA_STRICT=y # Wipe all caller-used registers on exit from the function (reduces available ROP gadgets and minimizes stale data in registers) CONFIG_ZERO_CALL_USED_REGS=y # Dangerous; enabling this allows direct physical memory writing. # CONFIG_ACPI_CUSTOM_METHOD is not set # Dangerous; enabling this disables brk ASLR. # CONFIG_COMPAT_BRK is not set # Dangerous; enabling this allows direct kernel memory writing. # CONFIG_DEVKMEM is not set # Dangerous; exposes kernel text image layout. # CONFIG_PROC_KCORE is not set # Dangerous; enabling this disables VDSO ASLR. # CONFIG_COMPAT_VDSO is not set # Dangerous; enabling this allows replacement of running kernel. # CONFIG_KEXEC is not set # Dangerous; enabling this allows replacement of running kernel. # CONFIG_HIBERNATION is not set # Prior to v4.1, assists heap memory attacks; best to keep interface disabled. # CONFIG_INET_DIAG is not set # Easily confused by misconfigured userspace, keep off. # CONFIG_BINFMT_MISC is not set # Use the modern PTY interface (devpts) only. # CONFIG_LEGACY_PTYS is not set # If SELinux can be disabled at runtime, the LSM structures cannot be read-only; keep off. # CONFIG_SECURITY_SELINUX_DISABLE is not set # Reboot devices immediately if kernel experiences an Oops. CONFIG_PANIC_ON_OOPS=y CONFIG_PANIC_TIMEOUT=-1 # Keep root from altering kernel memory via loadable modules. # CONFIG_MODULES is not set # But if CONFIG_MODULE=y is needed, at least they must be signed with a per-build key. CONFIG_DEBUG_SET_MODULE_RONX=y (prior to v4.11) CONFIG_STRICT_MODULE_RWX=y (since v4.11) CONFIG_MODULE_SIG=y CONFIG_MODULE_SIG_FORCE=y CONFIG_MODULE_SIG_ALL=y CONFIG_MODULE_SIG_SHA512=y CONFIG_MODULE_SIG_HASH="sha512" CONFIG_MODULE_SIG_KEY="certs/signing_key.pem"
# Enable GCC Plugins CONFIG_GCC_PLUGINS=y # Gather additional entropy at boot time for systems that may not have appropriate entropy sources. CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y # Force all structures to be initialized before they are passed to other functions. # When building with GCC: CONFIG_GCC_PLUGIN_STRUCTLEAK=y CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y # Wipe stack contents on syscall exit (reduces stale data lifetime in stack) CONFIG_GCC_PLUGIN_STACKLEAK=y # Randomize the layout of system structures. This may have dramatic performance impact, so # use with caution or also use CONFIG_GCC_PLUGIN_RANDSTRUCT_PERFORMANCE=y CONFIG_GCC_PLUGIN_RANDSTRUCT=y
# Full 64-bit means PAE and NX bit. CONFIG_X86_64=y # Disallow allocating the first 64k of memory. CONFIG_DEFAULT_MMAP_MIN_ADDR=65536 # Disable Model-Specific Register writes. # CONFIG_X86_MSR is not set # Randomize position of kernel and memory. CONFIG_RANDOMIZE_BASE=y CONFIG_RANDOMIZE_MEMORY=y # Modern libc no longer needs a fixed-position mapping in userspace, remove it as a possible target. CONFIG_LEGACY_VSYSCALL_NONE=y # Enable Kernel Page Table Isolation to remove an entire class of cache timing side-channels. CONFIG_PAGE_TABLE_ISOLATION=y # Remove additional attack surface, unless you really need them. # CONFIG_IA32_EMULATION is not set # CONFIG_X86_X32 is not set # CONFIG_MODIFY_LDT_SYSCALL is not set
# Disallow allocating the first 32k of memory (cannot be 64k due to ARM loader). CONFIG_DEFAULT_MMAP_MIN_ADDR=32768 # Randomize position of kernel (requires UEFI RNG or bootloader support for /chosen/kaslr-seed DT property). CONFIG_RANDOMIZE_BASE=y # Make sure PAN emulation is enabled. CONFIG_ARM64_SW_TTBR0_PAN=y # Enable Kernel Page Table Isolation to remove an entire class of cache timing side-channels. CONFIG_UNMAP_KERNEL_AT_EL0=y
# On 32-bit kernels, require PAE for NX bit support. # CONFIG_M486 is not set # CONFIG_HIGHMEM4G is not set CONFIG_HIGHMEM64G=y CONFIG_X86_PAE=y # Disallow allocating the first 64k of memory. CONFIG_DEFAULT_MMAP_MIN_ADDR=65536 # Disable Model-Specific Register writes. # CONFIG_X86_MSR is not set # Randomize position of kernel. CONFIG_RANDOMIZE_BASE=y # Enable Kernel Page Table Isolation to remove an entire class of cache timing side-channels. CONFIG_PAGE_TABLE_ISOLATION=y # Don't allow for 16-bit program emulation and associated LDT tricks. # CONFIG_MODIFY_LDT_SYSCALL is not set
# Disallow allocating the first 32k of memory (cannot be 64k due to ARM loader). CONFIG_DEFAULT_MMAP_MIN_ADDR=32768 # For maximal userspace memory area (and maximum ASLR). CONFIG_VMSPLIT_3G=y # If building an old out-of-tree Qualcomm kernel, this is similar to CONFIG_STRICT_KERNEL_RWX. CONFIG_STRICT_MEMORY_RWX=y # Make sure PXN/PAN emulation is enabled. CONFIG_CPU_SW_DOMAIN_PAN=y # Dangerous; old interfaces and needless additional attack surface. # CONFIG_OABI_COMPAT is not set
kernel command line options
# Wipe slab and page allocations (Since v5.3; supersedes "slub_debug=P" and "page_poison=1" below) # See CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y and CONFIG_INIT_ON_FREE_DEFAULT_ON=y above. init_on_alloc=1 init_on_free=1 # Randomize kernel stack offset on syscall entry (since v5.13). # See CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT above. randomize_kstack_offset=on # Disable slab merging (makes many heap overflow attacks more difficult). slab_nomerge # Always enable Kernel Page Table Isolation, even if the CPU claims it is safe from Meltdown. pti=on # To prevent against L1TF, at the cost of losing hyper threading (slow). nosmt # Enable SLUB redzoning and sanity checking (slow; requires CONFIG_SLUB_DEBUG=y above). slub_debug=ZF # (Before v5.3 without "init_on_free=1") Enable slub/slab allocator free poisoning (requires CONFIG_SLUB_DEBUG=y above). slub_debug=P # (Before v5.3 without "init_on_free=1") Enable buddy allocator free poisoning (requires CONFIG_PAGE_POISONING=y above). page_poison=1 # Force IOMMU TLB invalidation so devices will never be able to access stale data contents (see CONFIG_IOMMU_DEFAULT_DMA_STRICT=y above). iommu.passthrough=0 iommu.strict=1
# Remove vsyscall entirely to avoid it being a fixed-position ROP target of any kind. # (Same as CONFIG_LEGACY_VSYSCALL_NONE=y above.) vsyscall=none
# Try to keep kernel address exposures out of various /proc files (kallsyms, modules, etc). (There is no CONFIG for the changing the initial value.) kernel.kptr_restrict = 1 # Avoid kernel memory address exposures via dmesg (this value can also be set by CONFIG_SECURITY_DMESG_RESTRICT). kernel.dmesg_restrict = 1 # Block non-uid-0 profiling (needs distro patch, otherwise this is the same as "= 2") kernel.perf_event_paranoid = 3 # Turn off kexec, even if it's built in. kernel.kexec_load_disabled = 1 # Avoid non-ancestor ptrace access to running processes and their credentials. kernel.yama.ptrace_scope = 1 # Disable User Namespaces, as it opens up a large attack surface to unprivileged users. user.max_user_namespaces = 0 # Turn off unprivileged eBPF access. kernel.unprivileged_bpf_disabled = 1 # Turn on BPF JIT hardening, if the JIT is enabled. net.core.bpf_jit_harden = 2