# 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. * Randomises the addresses for mmap base, heap, stack, and VDSO pages. * 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. ### mmap ASLR * The bits of entropy used for mmap ASLR are maxed out via `/usr/libexec/security-misc/mmap-rnd-bits` (set to the values of `CONFIG_ARCH_MMAP_RND_BITS_MAX` and `CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX` that the kernel was built with), therefore improving its effectiveness. ### Boot parameters Boot parameters are outlined in configuration files located in the `etc/default/grub.d/` directory. * 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. * 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. * 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. * Enables randomisation of the kernel stack offset on syscall entries. * All mitigations for known CPU vulnerabilities are enabled and SMT is disabled. * IOMMU is enabled to prevent DMA attacks along with strict enforcement of IOMMU TLB invalidation so devices will never be able to access stale data contents. * Distrust the 'randomly' generated CPU and bootloader seeds. ### Kernel Modules #### Kernel Module Signature Verification Not yet due to issues: https://forums.whonix.org/t/enforce-kernel-module-software-signature-verification-module-signing-disallow-kernel-module-loading-by-default/7880/64 See: * `/etc/default/grub.d/40_only_allow_signed_modules.cfg` #### Disables and blacklists kernel modules Certain kernel modules are disabled and blacklisted by default 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. * Bluetooth is disabled to reduce attack surface. Bluetooth has a lengthy history of security concerns. * Thunderbolt and numerous FireWire kernel modules are also disabled as they are often vulnerable to DMA attacks. * The MSR kernel module is disabled to prevent CPU MSRs from being abused to write to arbitrary memory. * 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 * Disables a large array of uncommon file systems and network file systems that reduces the attack surface especially against legacy approaches. * The vivid kernel module is only required for testing and has been the cause of multiple vulnerabilities so it is disabled. * Provides some disabling of the interface between the [Intel Management Engine (ME)](https://www.kernel.org/doc/html/latest/driver-api/mei/mei.html) and the OS. * Incorporates much of [Ubuntu's](https://git.launchpad.net/ubuntu/+source/kmod/tree/debian/modprobe.d?h=ubuntu/disco) default blacklist of modules to be blocked from automatically loading. However, they are still permitted to load. * Blocks automatic loading of the modules needed to use of CD-ROM devices by default. Not completely disabled yet. ### 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 #### Permission Lockdown 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` #### umask Default `umask` is set to `027` for files created by non-root users such as for example user `user`. This is doing using pam module `pam_mkhomedir.so umask=027`. This means, files created by non-root users cannot be read by other non-root users by default. While Permission Lockdown already protects the `/home` folder, this protects other folders such as `/tmp`. `group` read permissions are not removed. This is unnecessary due to Debian's use of User Private Groups (UPGs). See also: https://wiki.debian.org/UserPrivateGroups Default `umask` is unchanged for root, because then configuration files created in `/etc` by the system administrator would be unreadable by "others" and break applications. Examples include `/etc/firefox-esr` and `/etc/thunderbird`. See: * `/usr/share/pam-configs/umask-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. * 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. ## 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!