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.

• Provides some blocking of the interface between the Intel Management Engine (ME) and the OS.

• Not enabled by default yet, comment only: Disables the use of CD-ROM devices by default.

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, OR
• B) including verifying software signatures

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