16 KiB
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.
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 toCAP_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 theTIOCSETD
ioctl which has been abused in a number of exploits before. -
Restricts the
userfaultfd()
syscall toCAP_SYS_PTRACE
asuserfaultfd()
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 ofCONFIG_ARCH_MMAP_RND_BITS_MAX
andCONFIG_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.
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) 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 forinitramfs-tools
only because this is not needed fordracut
becausedracut
does that by default, at least onsystemd
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.
/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 groupsudo
which prevents users from usingsu
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 groupsudo
. 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
withhidepid=2
at boot to prevent users from seeing another user's processes. This is disabled by default because it is incompatible withpkexec
. It can be enabled by executingsystemctl 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 executingsystemctl 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
Contact
Donate
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