# Enhances miscellaneous security settings ## Kernel hardening This section is inspired by the Kernel Self Protection Project (KSPP). It implements all recommended Linux kernel settings by the KSPP and many more. * https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project ### sysctl sysctl settings are configured via the `/etc/sysctl.d/30_security-misc.conf` configuration file. * A kernel pointer points to a specific location in kernel memory. These can be very useful in exploiting the kernel so they are restricted to `CAP_SYSLOG`. * The kernel logs are restricted to `CAP_SYSLOG` as they can often leak sensitive information such as kernel pointers. * The `ptrace()` system call is restricted to `CAP_SYS_PTRACE`. * eBPF is restricted to `CAP_BPF` (`CAP_SYS_ADMIN` on kernel versions prior to 5.8) and JIT hardening techniques such as constant blinding are enabled. * Restricts performance events to `CAP_PERFMON` (`CAP_SYS_ADMIN` on kernel versions prior to 5.8). * Restricts loading line disciplines to `CAP_SYS_MODULE` to prevent unprivileged attackers from loading vulnerable line disciplines with the `TIOCSETD` ioctl which has been abused in a number of exploits before. * Restricts the `userfaultfd()` syscall to `CAP_SYS_PTRACE` as `userfaultfd()` is often abused to exploit use-after-free flaws. * Kexec is disabled as it can be used to load a malicious kernel and gain arbitrary code execution in kernel mode. * The bits of entropy used for mmap ASLR are increased, therefore improving its effectiveness. * Prevents unintentional writes to attacker-controlled files. * Prevents common symlink and hardlink TOCTOU races. * Restricts the SysRq key so it can only be used for shutdowns and the Secure Attention Key. * The kernel is only allowed to swap if it is absolutely necessary. This prevents writing potentially sensitive contents of memory to disk. * TCP timestamps are disabled as it can allow detecting the system time. ### Boot parameters Boot parameters are configured via the `/etc/modprobe.d/30_security-misc.conf` configuration file. * Slab merging is disabled which significantly increases the difficulty of heap exploitation by preventing overwriting objects from merged caches and by making it harder to influence slab cache layout. * Sanity checks are enabled which add various checks to prevent corruption in certain slab operations. * Redzoning is enabled which adds extra areas around slabs that detect when a slab is overwritten past its real size which can help detect overflows. * Memory zeroing at allocation and free time is enabled to mitigate some use-after-free vulnerabilities and erase sensitive information in memory. * Page allocator freelist randomization is enabled. * The machine check tolerance level is decreased which makes the kernel panic on uncorrectable errors in ECC memory that could be exploited. * Kernel Page Table Isolation is enabled to mitigate Meltdown and increase KASLR effectiveness. * vsyscalls are disabled as they are obsolete, are at fixed addresses and thus, are a potential target for ROP. * The kernel panics on oopses to thwart certain kernel exploits. * All mitigations for known CPU vulnerabilities are enabled and SMT is disabled. * IOMMU is enabled to prevent DMA attacks. ### Blacklisted kernel modules Certain kernel modules are blacklisted to reduce attack surface via the `/etc/modprobe.d/30_security-misc.conf` configuration file. * Deactivates Netfilter's connection tracking helper - this module increases kernel attack surface by enabling superfluous functionality such as IRC parsing in the kernel. Hence, this feature is disabled. * Uncommon network protocols are blacklisted. This includes: DCCP - Datagram Congestion Control Protocol SCTP - Stream Control Transmission Protocol RDS - Reliable Datagram Sockets TIPC - Transparent Inter-process Communication HDLC - High-Level Data Link Control AX25 - Amateur X.25 NetRom X25 ROSE DECnet Econet af_802154 - IEEE 802.15.4 IPX - Internetwork Packet Exchange AppleTalk PSNAP - Subnetwork Access Protocol p8023 - Novell raw IEEE 802.3 p8022 - IEEE 802.2 CAN - Controller Area Network ATM * Bluetooth is also blacklisted to reduce attack surface. Bluetooth has a history of security concerns. * The Thunderbolt and FireWire kernel modules are blacklisted as they are often vulnerable to DMA attacks. * The vivid kernel module is only required for testing and has been the cause of multiple vulnerabilities so it is blacklisted. * The MSR kernel module is blacklisted to prevent CPU MSRs from being abused to write to arbitrary memory. * Disables a large array of uncommon file systems and network file systems that reduces the attack surface especially against legacy approaches. * Disables the use of CD-ROM devices by default. * Provides some blocking of the interface between the [Intel Management Engine (ME)](https://www.kernel.org/doc/html/latest/driver-api/mei/mei.html) and the OS. ### Other * A systemd service clears the System.map file on boot as these contain kernel pointers. The file is completely overwritten with zeroes to ensure it cannot be recovered. See: `/etc/kernel/postinst.d/30_remove-system-map` `/lib/systemd/system/remove-system-map.service` `/usr/libexec/security-misc/remove-system.map` * Coredumps are disabled as they may contain important information such as encryption keys or passwords. See: `/etc/security/limits.d/30_security-misc.conf` `/etc/sysctl.d/30_security-misc.conf` `/lib/systemd/coredump.conf.d/30_security-misc.conf` * An initramfs hook sets the sysctl values in `/etc/sysctl.conf` and `/etc/sysctl.d` before init is executed so sysctl hardening is enabled as early as possible. This is implemented for `initramfs-tools` only because this is not needed for `dracut` because `dracut` does that by default, at least on `systemd` enabled systems. Not researched for non-`systemd` systems by the author of this part of the readme. ## Network hardening * TCP syncookies are enabled to prevent SYN flood attacks. * ICMP redirect acceptance, ICMP redirect sending, source routing and IPv6 router advertisements are disabled to prevent man-in-the-middle attacks. * The kernel is configured to ignore all ICMP requests to avoid Smurf attacks, make the device more difficult to enumerate on the network and prevent clock fingerprinting through ICMP timestamps. * RFC1337 is enabled to protect against time-wait assassination attacks by dropping RST packets for sockets in the time-wait state. * Reverse path filtering is enabled to prevent IP spoofing and mitigate vulnerabilities such as CVE-2019-14899. ## Entropy collection improvements * The `jitterentropy_rng` kernel module is loaded as early as possible during boot to gather more entropy via the `/usr/lib/modules-load.d/30_security-misc.conf` configuration file. * Distrusts the CPU for initial entropy at boot as it is not possible to audit, may contain weaknesses or a backdoor. For references, see: `/etc/default/grub.d/40_distrust_cpu.cfg` * Gathers more entropy during boot if using the linux-hardened kernel patch. ## Restrictive mount options Not enabled by default yet. In development. Help welcome. https://forums.whonix.org/t/re-mount-home-and-other-with-noexec-and-nosuid-among-other-useful-mount-options-for-better-security/ `/home`, `/tmp`, `/dev/shm` and `/run` are remounted with the `nosuid` and `nodev` mount options to prevent execution of setuid or setgid binaries and creation of devices on those filesystems. Optionally, they can also be mounted with `noexec` to prevent execution of any binary. To opt-in to applying `noexec`, execute `touch /etc/noexec` as root and reboot. To disable this, execute `touch /etc/remount-disable` as root. Alternatively, file `/usr/local/etc/remount-disable` or `/usr/local/etc/noexec` could be used. ## Root access restrictions * `su` is restricted to only users within the group `sudo` which prevents users from using `su` to gain root access or to switch user accounts - `/usr/share/pam-configs/wheel-security-misc` (which results in a change in file `/etc/pam.d/common-auth`). * Add user `root` to group `sudo`. This is required due to the above restriction so that logging in from a virtual console is still possible - `debian/security-misc.postinst` * Abort login for users with locked passwords - `/usr/libexec/security-misc/pam-abort-on-locked-password`. * Logging into the root account from a virtual, serial, whatnot console is prevented by shipping an existing and empty `/etc/securetty` file (deletion of `/etc/securetty` has a different effect). This package does not yet automatically lock the root account password. It is not clear if this would be sane in such a package although, it is recommended to lock and expire the root account. In new Kicksecure builds, root account will be locked by package dist-base-files. See: * https://www.kicksecure.com/wiki/Root * https://www.kicksecure.com/wiki/Dev/Permissions * https://forums.whonix.org/t/restrict-root-access/7658 However, a locked root password will break rescue and emergency shell. Therefore, this package enables passwordless rescue and emergency shell. This is the same solution that Debian will likely adapt for Debian installer: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802211 See: * `/etc/systemd/system/emergency.service.d/override.conf` * `/etc/systemd/system/rescue.service.d/override.conf` Adverse security effects can be prevented by setting up BIOS password protection, GRUB password protection and/or full disk encryption. ## Console lockdown This uses pam_access to allow members of group `console` to use console but restrict everyone else (except members of group `console-unrestricted`) from using console with ancient, unpopular login methods such as `/bin/login` over networks as this might be exploitable. (CVE-2001-0797) This is not enabled by default in this package since this package does not know which users shall be added to group 'console' and thus, would break console. See: * `/usr/share/pam-configs/console-lockdown-security-misc` * `/etc/security/access-security-misc.conf` ## Brute force attack protection User accounts are locked after 50 failed login attempts using `pam_faillock`. Informational output during Linux PAM: * Show failed and remaining password attempts. * Document unlock procedure if Linux user account got locked. * Point out that there is no password feedback for `su`. * Explain locked root account if locked. See: * `/usr/share/pam-configs/tally2-security-misc` * `/usr/libexec/security-misc/pam-info` * `/usr/libexec/security-misc/pam-abort-on-locked-password` ## Access rights restrictions ### Strong user account separation Read, write and execute access for "others" are removed during package installation, upgrade or PAM `mkhomedir` for all users who have home folders in `/home` by running, for example: ``` chmod o-rwx /home/user ``` This will be done only once per folder in `/home` so users who wish to relax file permissions are free to do so. This is to protect files in a home folder that were previously created with lax file permissions prior to the installation of this package. See: * `debian/security-misc.postinst` * `/usr/libexec/security-misc/permission-lockdown` * `/usr/share/pam-configs/mkhomedir-security-misc` ### SUID / SGID removal and permission hardening Not enabled by default yet. A systemd service removes SUID / SGID bits from non-essential binaries as these are often used in privilege escalation attacks. It is disabled by default for now during testing and can optionally be enabled by running `systemctl enable permission-hardening.service` as root. See: * `/usr/libexec/security-misc/permission-hardening` * `/lib/systemd/system/permission-hardening.service` * `/etc/permission-hardening.d` * https://forums.whonix.org/t/disable-suid-binaries/7706 * https://www.kicksecure.com/wiki/SUID_Disabler_and_Permission_Hardener ### Access rights relaxations This is not enabled yet because hidepid is not enabled by default. Calls to `pkexec` are redirected to `lxqt-sudo` because `pkexec` is incompatible with `hidepid=2`. See: * `/usr/bin/pkexec.security-misc` * https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=860040 * https://forums.whonix.org/t/cannot-use-pkexec/8129 ## Application-specific hardening * Enables "`apt-get --error-on=any`" which makes apt exit non-zero for transient failures. - `/etc/apt/apt.conf.d/40error-on-any`. * Enables APT seccomp-BPF sandboxing - `/etc/apt/apt.conf.d/40sandbox`. * Deactivates previews in Dolphin. * Deactivates previews in Nautilus - `/usr/share/glib-2.0/schemas/30_security-misc.gschema.override`. * Deactivates thumbnails in Thunar. * Displays domain names in punycode (`network.IDN_show_punycode`) in Thunderbird to prevent IDN homograph attacks (a form of phishing). * Security and privacy enhancements for gnupg's config file `/etc/skel/.gnupg/gpg.conf`. See also: https://raw.github.com/ioerror/torbirdy/master/gpg.conf https://github.com/ioerror/torbirdy/pull/11 ## Opt-in hardening Some hardening is opt-in as it causes too much breakage to be enabled by default. * TCP SACK can be disabled as it is commonly exploited and is rarely used by uncommenting settings in the `/etc/sysctl.d/30_security-misc.conf` configuration file. * An optional systemd service mounts `/proc` with `hidepid=2` at boot to prevent users from seeing another user's processes. This is disabled by default because it is incompatible with `pkexec`. It can be enabled by executing `systemctl enable proc-hidepid.service` as root. * A systemd service restricts `/proc/cpuinfo`, `/proc/bus`, `/proc/scsi` and `/sys` to the root user. This hides a lot of hardware identifiers from unprivileged users and increases security as `/sys` exposes a lot of information that shouldn't be accessible to unprivileged users. As this will break many things, it is disabled by default and can optionally be enabled by executing `systemctl enable hide-hardware-info.service` as root. ## Cold Boot Attack Defense Wiping RAM at shutdown to defeat cold boot attacks. Implemented as `dracut` module `cold-boot-attack-defense`. Requires `dracut`. In other words, RAM wipe is incompatible with systems using `initramfs-tools`. To switch to, install dracut: sudo apt update sudo apt install --no-install-recommends dracut `dracut` is intentionally not declared as a dependency of `security-misc` to avoid making all of `security-misc` dependent on `dracut` only for the sake of the wipe RAM at shutdown feature. Linux distribution such as Kicksecure are advised to (and Kicksecure is planning to) install `dracut` instead of `initramfs-tools` by default. Only tested on `systemd` enabled systems. User documentation: https://www.kicksecure.com/wiki/Cold_Boot_Attack_Defense Design documentation: https://www.kicksecure.com/wiki/Dev/RAM_Wipe Source code: * `/usr/lib/dracut/modules.d/40cold-boot-attack-defense` * `/etc/default/grub.d/40_cold_boot_attack_defense.cfg` ## miscellaneous * hardened malloc compatibility for haveged workaround `/lib/systemd/system/haveged.service.d/30_security-misc.conf` * set `dracut` `reproducible=yes` setting ## Related * Linux Kernel Runtime Guard (LKRG) * tirdad - TCP ISN CPU Information Leak Protection. * Kicksecure (TM) - a security-hardened Linux Distribution * And more. * https://www.kicksecure.com/wiki/Linux_Kernel_Runtime_Guard_LKRG * https://github.com/Kicksecure/tirdad * https://www.kicksecure.com * https://github.com/Kicksecure ## Discussion Happening primarily in forums. https://forums.whonix.org/t/kernel-hardening/7296 ## How to install `security-misc` See https://www.kicksecure.com/wiki/Security-misc#install ## How to Build deb Package from Source Code Can be build using standard Debian package build tools such as: ``` dpkg-buildpackage -b ``` See instructions. (Replace `generic-package` with the actual name of this package `security-misc`.) * **A)** [easy](https://www.kicksecure.com/wiki/Dev/Build_Documentation/generic-package/easy), _OR_ * **B)** [including verifying software signatures](https://www.kicksecure.com/wiki/Dev/Build_Documentation/generic-package) ## Contact * [Free Forum Support](https://forums.kicksecure.com) * [Professional Support](https://www.kicksecure.com/wiki/Professional_Support) ## Donate `security-misc` requires [donations](https://www.kicksecure.com/wiki/Donate) to stay alive!