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Patrick Schleizer cb51847085
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enhances misc security settings

Inspired by Kernel Self Protection Project (KSPP)

kernel hardening:

  • deactivates Netfilter's connection tracking helper Netfilter's connection tracking helper module increases kernel attack surface by enabling superfluous functionality such as IRC parsing in the kernel. (!) Hence, this package disables this feature by shipping the /etc/modprobe.d/30_security-misc.conf configuration file.

  • Kernel symbols in various files in /proc are hidden as they can be very useful for kernel exploits.

  • Kexec is disabled as it can be used to load a malicious kernel. /etc/modprobe.d/30_security-misc.conf

  • ASLR effectiveness for mmap is increased.

  • The TCP/IP stack is hardened by disabling ICMP redirect acceptance, ICMP redirect sending and source routing to prevent man-in-the-middle attacks, ignoring all ICMP requests, enabling TCP syncookies to prevent SYN flood attacks, enabling RFC1337 to protect against time-wait assassination attacks and enabling reverse path filtering to prevent IP spoofing and mitigate vulnerabilities such as CVE-2019-14899.

  • Avoids unintentional writes to attacker-controlled files.

  • Prevents symlink/hardlink TOCTOU races.

  • SACK can be disabled as it is commonly exploited and is rarely used by uncommenting settings in file /etc/sysctl.d/30_security-misc.conf.

  • Slab merging is disabled as sometimes a slab can be used in a vulnerable way which an attacker can exploit.

  • Sanity checks and redzoning are enabled.

  • Memory zeroing at allocation and free time 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.

  • Enables all mitigations for CPU vulnerabilities and disables SMT.

  • A systemd service clears System.map on boot as these contain kernel symbols that could be useful to an attacker. /etc/kernel/postinst.d/30_remove-system-map /lib/systemd/system/remove-system-map.service /usr/lib/security-misc/remove-system.map

  • Coredumps are disabled as they may contain important information such as encryption keys or passwords. /etc/security/limits.d/30_security-misc.conf /etc/sysctl.d/30_security-misc.conf /lib/systemd/coredump.conf.d/30_security-misc.conf

  • The thunderbolt and firewire kernel modules are blacklisted as they can be used for DMA (Direct Memory Access) attacks.

  • IOMMU is enabled with a boot parameter to prevent DMA attacks.

  • Bluetooth is blacklisted to reduce attack surface. Bluetooth also has a history of security concerns. https://en.wikipedia.org/wiki/Bluetooth#History_of_security_concerns /etc/modprobe.d/30_security-misc.conf

  • A systemd service restricts /proc/cpuinfo, /proc/bus, /proc/scsi and /sys to the root user only. 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 running systemctl enable hide-hardware-info.service as root. /usr/lib/security-misc/hide-hardware-info /lib/systemd/system/hide-hardware-info.service /lib/systemd/system/user@.service.d/sysfs.conf /etc/hide-hardware-info.d/30_default.conf

  • The MSR kernel module is blacklisted to prevent CPU MSRs from being abused to write to arbitrary memory.

  • Vsyscalls are disabled as they are obsolete, are at fixed addresses and are a target for ROP.

  • Page allocator freelist randomization is enabled.

  • The vivid kernel module is blacklisted as it's only required for testing and has been the cause of multiple vulnerabilities.

  • 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.

  • The kernel panics on oopses to prevent it from continuing to run a flawed process and to deter brute forcing.

  • Restricts the SysRq key so it can only be used for shutdowns and the Secure Attention Key.

  • Restricts loading line disciplines to CAP_SYS_MODULE.

  • Restricts the userfaultfd() syscall to root.

Improve Entropy Collection

Uncommon network protocols are blacklisted: These are rarely used and may have unknown vulnerabilities. /etc/modprobe.d/30_security-misc.conf The network protocols that are blacklisted are:

  • 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

user restrictions:

  • remount /home, /tmp, /dev/shm and /run with nosuid,nodev (default) and noexec (opt-in). To disable this, run sudo touch /etc/remount-disable. To opt-in noexec, run sudo touch /etc/noexec and reboot (easiest). Alternatively file /usr/local/etc/remount-disable or file /usr/local/etc/noexec could be used. /lib/systemd/system/remount-secure.service /usr/lib/security-misc/remount-secure

  • An optional systemd service mounts /proc with hidepid=2 at boot to prevent users from seeing each other's processes. Not enabled because not compatible with pkexec.

  • The kernel logs are restricted to root only.

  • The BPF JIT compiler is restricted to the root user and is hardened.

  • The ptrace system call is restricted to the root user only.

restricts access to the root account:

  • 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 to make above work so login as a user in a virtual console is still possible. debian/security-misc.postinst

  • Abort login for users with locked passwords. /usr/lib/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. (Deletion of /etc/securetty has a different effect.) /etc/securetty.security-misc

  • Console Lockdown. Allow members of group 'console' to use console. Everyone else except members of group 'console-unrestricted' are restricted from using console using ancient, unpopular login methods such as using /bin/login over networks, which might be exploitable. (CVE-2001-0797) Using pam_access. Not enabled by default in this package since this package does not know which users shall be added to group 'console' and would break console. /usr/share/pam-configs/console-lockdown-security-misc /etc/security/access-security-misc.conf

Protect Linux user accounts against brute force attacks. Lock user accounts after 50 failed login attempts using pam_tally2. /usr/share/pam-configs/tally2-security-misc

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.
  • /usr/share/pam-configs/tally2-security-misc
  • /usr/lib/security-misc/pam_tally2-info
  • /usr/lib/security-misc/pam-abort-on-locked-password

access rights restrictions:

  • Strong Linux User Account Separation. Removes read, write and execute access for others for all users who have home folders under folder /home by running for example "chmod o-rwx /home/user" during package installation, upgrade or pam mkhomedir. This will be done only once per folder in folder /home so users who wish to relax file permissions are free to do so. This is to protect previously created files in user home folder which were previously created with lax file permissions prior installation of this package. debian/security-misc.postinst /usr/lib/security-misc/permission-lockdown /usr/share/pam-configs/mkhomedir-security-misc

  • SUID / GUID removal and permission hardening. A systemd service removed SUID / GUID 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. https://forums.whonix.org/t/disable-suid-binaries/7706 /usr/lib/security-misc/permission-hardening /lib/systemd/system/permission-hardening.service /etc/permission-hardening.d/30_default.conf

access rights relaxations:

Redirect calls for pkexec to lxqt-sudo because pkexec is incompatible with hidepid. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=860040 https://forums.whonix.org/t/cannot-use-pkexec/8129 /usr/bin/pkexec.security-misc

This package does (not yet) automatically lock the root account password. It is not clear that would be sane in such a package. It is recommended to lock and expire the root account. In new Whonix builds, root account will be locked by package dist-base-files. https://www.whonix.org/wiki/Root https://www.whonix.org/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 Adverse security effects can be prevented by setting up BIOS password protection, grub password protection and/or full disk encryption. /etc/systemd/system/emergency.service.d/override.conf /etc/systemd/system/rescue.service.d/override.conf

Let the kernel only swap if it is absolutely necessary. /etc/sysctl.d/30_security-misc.conf

Disables TCP Time Stamps:

TCP time stamps (RFC 1323) allow for tracking clock information with millisecond resolution. This may or may not allow an attacker to learn information about the system clock at such a resolution, depending on various issues such as network lag. This information is available to anyone who monitors the network somewhere between the attacked system and the destination server. It may allow an attacker to find out how long a given system has been running, and to distinguish several systems running behind NAT and using the same IP address. It might also allow one to look for clocks that match an expected value to find the public IP used by a user.

Hence, this package disables this feature by shipping the /etc/sysctl.d/30_security-misc.conf configuration file.

Note that TCP time stamps normally have some usefulness. They are needed for:

  • the TCP protection against wrapped sequence numbers; however, to trigger a wrap, one needs to send roughly 2^32 packets in one minute: as said in RFC 1700, "The current recommended default time to live (TTL) for the Internet Protocol (IP) [45,105] is 64". So, this probably won't be a practical problem in the context of Anonymity Distributions.
  • "Round-Trip Time Measurement", which is only useful when the user manages to saturate their connection. When using Anonymity Distributions, probably the limiting factor for transmission speed is rarely the capacity of the user connection.

Application specific hardening:

  • 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.
  • Enables punycode (network.IDN_show_punycode) by default in Thunderbird to make phishing attacks more difficult. Fixing URL not showing real Domain Name (Homograph attack).
  • 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

Want more? Look into these:

Discussion:

Happening primarily in Whonix forums. https://forums.whonix.org/t/kernel-hardening/7296

How to install security-misc using apt-get

1. Download Whonix's Signing Key.

wget https://www.whonix.org/patrick.asc

Users can check Whonix Signing Key for better security.

2. Add Whonix's signing key.

sudo apt-key --keyring /etc/apt/trusted.gpg.d/whonix.gpg add ~/patrick.asc

3. Add Whonix's APT repository.

echo "deb https://deb.whonix.org buster main contrib non-free" | sudo tee /etc/apt/sources.list.d/whonix.list

4. Update your package lists.

sudo apt-get update

5. Install security-misc.

sudo apt-get install security-misc

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.)

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