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.

• Disables SysRq completely.

• 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.

• Enforces the logging of martian packets, those with a source address which is blatantly wrong.

• Set coredump file name based on core_pattern value instead of the default of naming it 'core'.

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
• https://github.com/dell/dkms/issues/359

See:

• /etc/default/grub.d/40_only_allow_signed_modules.cfg

Disables the loading of new modules to the kernel after the fact

A systemd service dynamically sets the kernel parameter modules_disabled to 1, preventing new modules from being loaded. Since this isn't configured directly within systemctl, it does not break the loading of legitimate and necessary modules for the user, like drivers etc., given they are plugged in on startup.

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.

• 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) and the OS.

• Incorporates much of Ubuntu's 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 & Bluetooth 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.

• Unlike version 4, IPv6 addresses can provide information not only about the originating network, but also the originating device. We prevent this from happening by enabling the respective privacy extensions for IPv6.

• In addition, we deny the capability to track the originating device in the network at all, by using randomized MAC addresses per connection per default.

• Bluetooth is left enabled but users are highly discouraged from ever turning it on, due to its history of numerous security vulnurabilities. Unlike the default settings, we start the system with bluetooth turned off. We also enforce private addresses and strict timeout settings for discoverability and visibility.

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

A systemd service is triggered on boot to remount all sensitive partitions and directories with significantly more secure hardened mount options. Since this would require manual tuning for a given specific system, we handle it by creating a very solid configuration file for that very system on package install.

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

SUID / SGID removal

A systemd service removes SUID / SGID bits from non-essential binaries as these are often used in privilege escalation attacks.

File permission hardening

Various file permissions are reset with more secure and hardened defaults. These include but are not limited to:

• Limiting /home and /root to the root only.
• Limiting crontab to root as well as all the configuration files for cron.
• Limiting the configuration for cups and ssh.
• Protecting the information of sudoers from others.
• Protecting various system relevant files and modules.

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.
• Thunderbird is hardened with the following options:
  • Displays domain names in punycode to prevent IDN homograph attacks (a form of phishing).
  • Strips email client information for sent email headers.
  • Stripts user time information from sent email headers by replacing the originating time zone with UTC and rounding the timestamp to the nearest minute.
  • Disables scripting when viewing pdf files.
  • Disables implicit outgoing connections.
  • Disables all and any kind of telemetry.
• 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, OR
• B) including verifying software signatures

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