security-misc/usr/lib/sysctl.d/990-security-misc.conf

## Copyright (C) 2019 - 2024 ENCRYPTED SUPPORT LP <adrelanos@whonix.org>
## See the file COPYING for copying conditions.

## NOTE:
## This file has a special name to ensure that /usr/lib/sysctl.d/99-protect-links.conf
## is parsed first, followed by /usr/lib/sysctl.d/990-security-misc.conf.
## https://github.com/Kicksecure/security-misc/pull/135

## This configuration file is divided into 5 sections:
## 1. Kernel Space
## 2. User Space
## 3. Core Dumps
## 4. Swap Space
## 5. Networking

## For detailed explanations of most of the selected commands, refer to:
## https://www.kernel.org/doc/html/latest/admin-guide/sysctl/kernel.html
## https://www.kernel.org/doc/html/latest/admin-guide/sysctl/fs.html
## https://www.kernel.org/doc/html/latest/admin-guide/sysctl/net.html
## https://www.kernel.org/doc/html/latest/admin-guide/sysctl/vm.html
## https://www.kernel.org/doc/html/latest//networking/ip-sysctl.html

## 1. Kernel Space:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#sysctl-kernel
## https://kspp.github.io/Recommended_Settings#sysctls
## https://wiki.archlinux.org/title/Security#Kernel_hardening

## Restrict kernel address visibility via /proc and other interfaces, regardless of user privileges.
## Kernel pointers expose specific locations in kernel memory.
##
## https://kernsec.org/wiki/index.php/Bug_Classes/Kernel_pointer_leak
##
kernel.kptr_restrict=2

## Restrict access to the kernel log buffer to users with CAP_SYSLOG.
## Kernel logs often contain sensitive information such as kernel pointers.
##
kernel.dmesg_restrict=1

## Prevent kernel information leaks in the console during boot.
## Must be used in conjunction with kernel boot parameters.
## See /etc/default/grub.d/41_quiet_boot.cfg for implementation.
##
## https://www.kernel.org/doc/html/latest/core-api/printk-basics.html
##
## See /usr/lib/sysctl.d/30_silent-kernel-printk.conf for implementation.
##
#kernel.printk=3 3 3 3

## Restrict eBPF access to CAP_BPF and enable associated JIT compiler hardening.
##
## https://en.wikipedia.org/wiki/EBPF#Security
##
kernel.unprivileged_bpf_disabled=1
net.core.bpf_jit_harden=2

## Restrict loading TTY line disciplines to users with CAP_SYS_MODULE.
## Prevents unprivileged users from loading vulnerable line disciplines with the TIOCSETD ioctl.
##
## https://a13xp0p0v.github.io/2017/03/24/CVE-2017-2636.html
## https://lkml.org/lkml/2019/4/15/890
##
dev.tty.ldisc_autoload=0

## Restrict the userfaultfd() syscall to users with SYS_CAP_PTRACE.
## Reduces the likelihood of use-after-free exploits from heap sprays.
##
## https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cefdca0a86be517bc390fc4541e3674b8e7803b0
## https://duasynt.com/blog/linux-kernel-heap-spray
##
vm.unprivileged_userfaultfd=0

## Disables kexec, which can be used to replace the running kernel.
## Useful for live kernel patching without rebooting.
##
## https://en.wikipedia.org/wiki/Kexec
##
## See /usr/lib/sysctl.d/30_security-misc_kexec-disable.conf for implementation.
##
#kernel.kexec_load_disabled=1

## Disable the SysRq key to prevent leakage of kernel information.
## The Secure Attention Key (SAK) can no longer be utilized.
##
## https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html
## https://www.kicksecure.com/wiki/SysRq
## https://github.com/xairy/unlockdown
##
kernel.sysrq=0

## Restrict user namespaces to users with CAP_SYS_ADMIN.
## User namespaces aim to improve sandboxing and accessibility for unprivileged users.
## Unprivileged user namespaces pose substantial privilege escalation risks.
## Restricting may lead to breakages in numerous software packages.
## Uncomment the second sysctl to entirely disable user namespaces.
## Disabling entirely will reduce compatibility with some AppArmor profiles.
##
## https://lwn.net/Articles/673597/
## https://madaidans-insecurities.github.io/linux.html#kernel
## https://github.com/a13xp0p0v/kernel-hardening-checker#questions-and-answers
## https://github.com/NixOS/nixpkgs/pull/84522#issuecomment-614640601
## https://github.com/Kicksecure/security-misc/pull/263
##
## KSPP=partial
## KSPP sets the stricter sysctl user.max_user_namespaces=0.
##
kernel.unprivileged_userns_clone=0
#user.max_user_namespaces=0

## Restricts kernel profiling to users with CAP_PERFMON.
## The performance events system should not be accessible by unprivileged users.
## Other distributions such as Ubuntu and Fedora may permit further restricting.
##
## https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html#unprivileged-users
## https://lore.kernel.org/kernel-hardening/1469630746-32279-1-git-send-email-jeffv@google.com/
##
kernel.perf_event_paranoid=3

## Force the kernel to panic on "oopses".
## Can sometimes potentially indicate and thwart certain kernel exploitation attempts.
## Also cause panics on machine check exceptions.
## Panics may be due to false-positives such as bad drivers.
##
## https://forums.whonix.org/t/set-oops-panic-kernel-parameter-or-kernel-panic-on-oops-1-sysctl-for-better-security/7713
##
## See /usr/libexec/security-misc/panic-on-oops for implementation.
##
#kernel.panic_on_oops=1
#kernel.panic=-1

## Disable the use of legacy TIOCSTI operations which can be used to inject keypresses.
## Can lead to privilege escalation by pushing characters into a controlling TTY.
## Will break out-dated screen readers that continue to rely on this legacy functionality.
##
## https://lore.kernel.org/lkml/20221228205726.rfevry7ud6gmttg5@begin/T/
##
## TODO: Debian 13 Trixie
## This is disabled by default when using Linux kernel >= 6.2.
##
dev.tty.legacy_tiocsti=0

## Disable asynchronous I/O for all processes.
## Leading cause of numerous kernel exploits.
## Disabling will reduce the read/write performance of storage devices.
##
## https://en.wikipedia.org/wiki/Io_uring#Security
## https://lwn.net/Articles/902466/
## https://security.googleblog.com/2023/06/learnings-from-kctf-vrps-42-linux.html
## https://github.com/moby/moby/pull/46762
## https://forums.whonix.org/t/io-uring-security-vulnerabilties/16890
##
## TODO: Debian 13 Trixie
## Applicable when using Linux kernel >= 6.6 (retained here for future-proofing and completeness).
##
kernel.io_uring_disabled=2

## 2. User Space:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#sysctl-userspace

## Restrict usage of the ptrace() system call to only processes with CAP_SYS_PTRACE.
## Limit ptrace() as it enables programs to inspect and modify other active processes.
## Prevents native code debugging which some programs use as a method to detect tampering.
## May cause breakages in 'anti-cheat' software and programs running under Proton/WINE.
##
## https://www.kernel.org/doc/html/latest/admin-guide/LSM/Yama.html#ptrace-scope
## https://en.wikipedia.org/wiki/Ptrace
## https://grapheneos.org/features#attack-surface-reduction
## https://github.com/GrapheneOS/os-issue-tracker/issues/651#issuecomment-917599928
## https://github.com/netblue30/firejail/issues/2860
##
## It is possible to harden further by disabling ptrace() for all users, see documentation.
## https://github.com/Kicksecure/security-misc/pull/242
##
kernel.yama.ptrace_scope=2

## Maximize bits of entropy for improved effectiveness of mmap ASLR.
## The maximum number of bits depends on CPU architecture (the ones shown below are for x86).
## Both explicit sysctl are made redundant due to automation.
## Do NOT enable either sysctl - displaying only for clarity.
##
## https://forums.whonix.org/t/automate-mmap-randomisation-to-fix-ppc64el/16514
##
## See /usr/libexec/security-misc/mmap-rnd-bits for implementation.
##
#vm.mmap_rnd_bits=32
#vm.mmap_rnd_compat_bits=16

## Prevent hardlink creation by users who do not have read/write/ownership of source file.
## Only allow symlinks to be followed when outside of world-writable sticky directories.
## Allow symlinks when the owner and follower match or when the directory owner matches the symlink's owner.
## Hardens cross-privilege boundaries if root process follows a hardlink/symlink belonging to another user.
## This mitigates many hardlink/symlink-based TOCTOU races in world-writable directories like /tmp.
##
## https://wiki.archlinux.org/title/Security#File_systems
## https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
## https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use#Preventing_TOCTOU
##
fs.protected_hardlinks=1
fs.protected_symlinks=1

## Disallow writes to files in world-writable sticky directories unless owned by the directory owner.
## Also applies to group-writable sticky directories to make data spoofing attacks more difficult.
## Prevents unintentional writes to attacker-controlled files.
##
fs.protected_fifos=2
fs.protected_regular=2

## Enable ASLR for mmap base, stack, VDSO pages, and heap.
## Forces shared libraries to be loaded to random addresses
## Start location of PIE-linked binaries is randomized.
## Heap randomization can lead to breakages with legacy applications.
##
## https://en.wikipedia.org/wiki/Address_space_layout_randomization#Linux
##
kernel.randomize_va_space=2

## Increase the maximum number of memory map areas a process is permitted to utilize.
## Addresses performance, crash, and start-up issues for some memory-intensive applications.
## Required to accommodate the very large number of guard pages created by hardened_malloc.
## Kicksecure version 18 will deprecate hardened_malloc, so this sysctl will be applied here instead.
##
## https://archlinux.org/news/increasing-the-default-vmmax_map_count-value/
## https://github.com/GrapheneOS/hardened_malloc#traditional-linux-based-operating-systems
## https://github.com/Kicksecure/hardened_malloc/blob/master/debian/hardened_malloc.conf
## https://www.kicksecure.com/wiki/Hardened_Malloc#Deprecation_in_Kicksecure
##
vm.max_map_count=1048576

## Disable the miscellaneous binary format virtual file system to prevent unintended code execution.
## Prevents registering interpreters for various binary formats based on a magic number or their file extension.
## Otherwise arbitrary executables with recognized file formats will be passed to relevant user space applications.
## These interpreters will then run with root permissions when a setuid binary is owned by root.
## Can stop maliciously crafted files with specific file extensions from automatically executing.
## Breaks many scripts that do not have appropriate shebang interpreter directives (#!/bin/...).
##
## https://www.kernel.org/doc/html/latest/admin-guide/binfmt-misc.html
## https://salsa.debian.org/debian/binfmt-support
## https://access.redhat.com/solutions/1985633
## https://en.wikipedia.org/wiki/Binfmt_misc
## https://security.stackexchange.com/questions/271786/does-allowing-binfmt-misc-significantly-increase-the-attack-surface-for-unprivil
## https://unix.stackexchange.com/questions/439569/what-kinds-of-executable-formats-do-the-files-under-proc-sys-fs-binfmt-misc-al
## https://github.com/Kicksecure/security-misc/pull/249
##
## KSPP=yes
## KSPP does not set CONFIG_BINFMT_MISC.
##
fs.binfmt_misc.status=0

## 3. Core Dumps:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#core-dumps

## Disable core dump files by preventing any pattern names.
## This setting may be overwritten by systemd and is not comprehensive.
## Core dumps are also disabled in security-misc via other means.
##
## https://wiki.archlinux.org/title/Core_dump#Disabling_automatic_core_dumps
##
kernel.core_pattern=|/bin/false

## Prevent setuid processes or otherwise protected/tainted binaries from creating core dumps.
## Any process which has changed privilege levels or is execute-only will not be dumped.
##
fs.suid_dumpable=0

## Set core dump file name to 'core.PID' instead of 'core' as a form of defense-in-depth.
## If core dumps are permitted, only useful if PID listings are hidden from non-root users.
##
kernel.core_uses_pid=1

## 4. Swap Space:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#swap

## Limit the copying of memory to the swap device only if absolutely necessary.
## Minimizes the likelihood of writing potentially sensitive contents to disk.
## Not recommended to set to zero since this disables periodic write behavior.
##
## https://en.wikipedia.org/wiki/Memory_paging#Linux
## https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Performance_Tuning_Guide/s-memory-tunables.html
##
vm.swappiness=1

## 5. Networking:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#sysctl-network
## https://wiki.archlinux.org/title/Sysctl#TCP/IP_stack_hardening

## Enable TCP SYN cookie protection to assist against SYN flood attacks.
##
## https://en.wikipedia.org/wiki/SYN_flood
## https://cateee.net/lkddb/web-lkddb/SYN_COOKIES.html
##
net.ipv4.tcp_syncookies=1

## Protect against TCP time-wait assassination hazards.
## Drops RST packets for sockets in the time-wait state.
##
## https://tools.ietf.org/html/rfc1337
##
net.ipv4.tcp_rfc1337=1

## Enable reverse path filtering (source validation) of packets received from all interfaces.
## Prevents IP spoofing and mitigates vulnerabilities such as CVE-2019-14899.
## The second "default" command fixes a bug in the existing kernel implementation.
##
## https://en.wikipedia.org/wiki/IP_address_spoofing
## https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/security_guide/sect-security_guide-server_security-reverse_path_forwarding#sect-Security_Guide-Server_Security-Reverse_Path_Forwarding
## https://forums.whonix.org/t/enable-reverse-path-filtering/8594
## https://seclists.org/oss-sec/2019/q4/122
## https://github.com/Kicksecure/security-misc/pull/261
##
net.ipv4.conf.*.rp_filter=1
net.ipv4.conf.default.rp_filter=1

## Disable ICMP redirect acceptance and redirect sending messages.
## Prevents man-in-the-middle attacks and minimizes information disclosure.
## If ICMP redirects are permitted, accept messages only through approved gateways (kernel default).
## Approving gateways requires the managing of a default gateway list.
##
## https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/security_guide/sect-security_guide-server_security-disable-source-routing#sect-Security_Guide-Server_Security-Disable-Source-Routing
## https://www.frozentux.net/ipsysctl-tutorial/chunkyhtml/theconfvariables.html
## https://www.debian.org/doc/manuals/securing-debian-manual/network-secure.en.html
## https://askubuntu.com/questions/118273/what-are-icmp-redirects-and-should-they-be-blocked
## https://github.com/Kicksecure/security-misc/pull/248
##
net.ipv4.conf.*.accept_redirects=0
net.ipv4.conf.*.send_redirects=0
net.ipv6.conf.*.accept_redirects=0
#net.ipv4.conf.*.secure_redirects=1

## Ignore ICMP echo requests.
## Prevents clock fingerprinting through ICMP timestamps and Smurf attacks.
##
## https://en.wikipedia.org/wiki/Smurf_attack
##
net.ipv4.icmp_echo_ignore_all=1
net.ipv6.icmp.echo_ignore_all=1

## Ignore bogus ICMP error responses.
## Mitigates attacks designed to fill log files with useless error messages.
##
net.ipv4.icmp_ignore_bogus_error_responses=1

## Disable source routing which allows users to redirect network traffic.
## Prevents man-in-the-middle attacks in which the traffic is redirected.
##
## https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/security_guide/sect-security_guide-server_security-disable-source-routing
##
net.ipv4.conf.*.accept_source_route=0
net.ipv6.conf.*.accept_source_route=0

## Do not accept IPv6 router advertisements and solicitations.
##
net.ipv6.conf.*.accept_ra=0

## Disable SACK and DSACK.
## Select acknowledgements (SACKs) are a known common vector of exploitation.
## Duplicate select acknowledgements (DSACKs) are an extension of SACK.
## Disabling can cause severe connectivity issues on networks with high latency or packet loss.
## Enabling on stable high-bandwidth networks can lead to reduced efficiency of TCP connections.
##
## https://datatracker.ietf.org/doc/html/rfc2018
## https://datatracker.ietf.org/doc/html/rfc2883
## https://access.redhat.com/sites/default/files/attachments/20150325_network_performance_tuning.pdf
## https://github.com/Netflix/security-bulletins/blob/master/advisories/third-party/2019-001.md
## https://wiki.archlinux.org/title/Sysctl#TCP_Selective_Acknowledgement
## https://forums.whonix.org/t/disabling-tcp-sack-dsack-fack/8109/5
##
## SACK and DSACK are currently enabled.
##
#net.ipv4.tcp_sack=0
#net.ipv4.tcp_dsack=0

## Disable TCP timestamps to limit device fingerprinting via system time.
## Timestamps allow round-trip time measurement and protection against wrapped sequence numbers.
## Disabling timestamps on very fast links is likely to cause TCP Sequence Numbers to wrap.
## Segments with wrapped numbers will be incorrectly discarded, reducing network performance.
##
## https://datatracker.ietf.org/doc/html/rfc1323
## https://forums.whonix.org/t/do-ntp-and-tcp-timestamps-really-leak-your-local-time/7824
## https://web.archive.org/web/20170201160732/https://mailman.boum.org/pipermail/tails-dev/2013-December/004520.html
## https://access.redhat.com/sites/default/files/attachments/20150325_network_performance_tuning.pdf
##
net.ipv4.tcp_timestamps=0

## Enable logging of packets with impossible source or destination addresses.
## Martian and unroutable packets may be used for malicious purposes.
## Recommended to keep a (kernel dmesg) log of these to identify suspicious packets.
## Useful for troubleshooting and diagnostics but not necessary by default.
## Known to cause performance issues, especially on systems with multiple interfaces.
##
## https://wiki.archlinux.org/title/Sysctl#Log_martian_packets
## https://github.com/Kicksecure/security-misc/issues/214
##
## The logging of martian packets is currently disabled.
##
#net.ipv4.conf.*.log_martians=1

## Enable IPv6 Privacy Extensions to prefer temporary addresses over public addresses.
## The temporary/privacy address is used as the source for all outgoing traffic.
## Must be used in combination with /usr/lib/systemd/networkd.conf.d/80_ipv6-privacy-extensions.conf.
## Must be used in combination with /usr/lib/NetworkManager/conf.d/80_ipv6-privacy.conf.
## Should be used with MAC randomization in /usr/lib/NetworkManager/conf.d/80_randomize-mac.conf.
##
## MAC randomization breaks root server and VirtualBox DHCP, likely due to IPv6 Privacy Extensions.
##
## https://datatracker.ietf.org/doc/html/rfc4941
## https://github.com/Kicksecure/security-misc/pull/145
## https://github.com/Kicksecure/security-misc/issues/184
##
## The use of IPv6 Privacy Extensions is currently disabled due to these breakages.
##
#net.ipv6.conf.*.use_tempaddr=2