From a1d1f97955fd9fd3cee77dc04e2eb5e5fa29d243 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Fri, 8 Nov 2024 03:58:23 +0000
Subject: [PATCH 01/13] Provide option to drop gratuitous ARP packets

---
 README.md                               |  3 +++
 usr/lib/sysctl.d/990-security-misc.conf | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/README.md b/README.md
index c23d261..3404414 100644
--- a/README.md
+++ b/README.md
@@ -121,6 +121,9 @@ Networking:
 
 - Optional - Enable IPv6 Privacy Extensions.
 
+- Optional - Drop gratuitous ARP packets to prevent ARP cache poisoning
+  via man-in-the-middle and denial-of-service attacks.
+
 ### Boot parameters
 
 Mitigations for known CPU vulnerabilities are enabled in their strictest form
diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 6009fc4..2bec703 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -526,3 +526,14 @@ net.ipv4.tcp_timestamps=0
 ## The use of IPv6 Privacy Extensions is currently disabled due to these breakages.
 ##
 #net.ipv6.conf.*.use_tempaddr=2
+
+## Drop gratuitous ARP (Address Resolution Protocol) packets.
+## Stops ARP responses sent by a device without being explicitly requested.
+## Prevents ARP cache poisoning by rejecting fake ARP entries into a network.
+## Prevents man-in-the-middle and denial-of-service attacks.
+## May cause breakages when ARP proxies are used in the network.
+##
+## https://patchwork.ozlabs.org/project/netdev/patch/1428652454-1224-3-git-send-email-johannes@sipsolutions.net/
+## https://www.practicalnetworking.net/series/arp/gratuitous-arp/
+##
+#net.ipv4.conf.*.drop_gratuitous_arp=1

From c2aae73ce161811571e4c85609a0b043399c1b65 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Wed, 13 Nov 2024 05:38:03 +0000
Subject: [PATCH 02/13] Add reference and move text

---
 README.md                               |  6 +++---
 usr/lib/sysctl.d/990-security-misc.conf | 23 ++++++++++++-----------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/README.md b/README.md
index 3404414..e167a87 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,9 @@ Networking:
 - Disable ICMP redirect acceptance and redirect sending messages to prevent
   man-in-the-middle attacks and minimize information disclosure.
 
+- Optional - Drop gratuitous ARP packets to prevent ARP cache poisoning
+  via man-in-the-middle and denial-of-service attacks.
+
 - Ignore ICMP echo requests to prevent clock fingerprinting and Smurf attacks.
 
 - Ignore bogus ICMP error responses.
@@ -121,9 +124,6 @@ Networking:
 
 - Optional - Enable IPv6 Privacy Extensions.
 
-- Optional - Drop gratuitous ARP packets to prevent ARP cache poisoning
-  via man-in-the-middle and denial-of-service attacks.
-
 ### Boot parameters
 
 Mitigations for known CPU vulnerabilities are enabled in their strictest form
diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 2bec703..054dcbf 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -443,6 +443,18 @@ net.ipv4.conf.*.send_redirects=0
 net.ipv6.conf.*.accept_redirects=0
 #net.ipv4.conf.*.secure_redirects=1
 
+## Drop gratuitous ARP (Address Resolution Protocol) packets.
+## Stops ARP responses sent by a device without being explicitly requested.
+## Prevents ARP cache poisoning by rejecting fake ARP entries into a network.
+## Prevents man-in-the-middle and denial-of-service attacks.
+## May cause breakages when ARP proxies are used in the network.
+##
+## https://cyber.gouv.fr/sites/default/files/document/linux_configuration-en-v2.pdf
+## https://patchwork.ozlabs.org/project/netdev/patch/1428652454-1224-3-git-send-email-johannes@sipsolutions.net/
+## https://www.practicalnetworking.net/series/arp/gratuitous-arp/
+##
+#net.ipv4.conf.*.drop_gratuitous_arp=1
+
 ## Ignore ICMP echo requests.
 ## Prevents clock fingerprinting through ICMP timestamps and Smurf attacks.
 ##
@@ -526,14 +538,3 @@ net.ipv4.tcp_timestamps=0
 ## The use of IPv6 Privacy Extensions is currently disabled due to these breakages.
 ##
 #net.ipv6.conf.*.use_tempaddr=2
-
-## Drop gratuitous ARP (Address Resolution Protocol) packets.
-## Stops ARP responses sent by a device without being explicitly requested.
-## Prevents ARP cache poisoning by rejecting fake ARP entries into a network.
-## Prevents man-in-the-middle and denial-of-service attacks.
-## May cause breakages when ARP proxies are used in the network.
-##
-## https://patchwork.ozlabs.org/project/netdev/patch/1428652454-1224-3-git-send-email-johannes@sipsolutions.net/
-## https://www.practicalnetworking.net/series/arp/gratuitous-arp/
-##
-#net.ipv4.conf.*.drop_gratuitous_arp=1

From a25d4f8df88908e83e56049204aa625f1196a948 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Wed, 13 Nov 2024 05:40:21 +0000
Subject: [PATCH 03/13] Provide option to enable ARP filtering

---
 README.md                               | 3 +++
 usr/lib/sysctl.d/990-security-misc.conf | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/README.md b/README.md
index e167a87..baa450d 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,9 @@ Networking:
 - Disable ICMP redirect acceptance and redirect sending messages to prevent
   man-in-the-middle attacks and minimize information disclosure.
 
+- Optional - Enable ARP filtering to mitigate some ARP spoofing and ARP
+  cache poisoning attacks.
+
 - Optional - Drop gratuitous ARP packets to prevent ARP cache poisoning
   via man-in-the-middle and denial-of-service attacks.
 
diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 054dcbf..52d31de 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -443,6 +443,15 @@ net.ipv4.conf.*.send_redirects=0
 net.ipv6.conf.*.accept_redirects=0
 #net.ipv4.conf.*.secure_redirects=1
 
+## Enable ARP (Address Resolution Protocol) filtering.
+## Prevents the Linux kernel from handling the ARP table globally
+## Can mitigate some ARP spoofing and ARP cache poisoning attacks.
+## Improper filtering can lead to increased ARP traffic and inadvertently block legitimate ARP requests.
+##
+## https://cyber.gouv.fr/sites/default/files/document/linux_configuration-en-v2.pdf
+##
+#net.ipv4.conf.*.arp_filter=1
+
 ## Drop gratuitous ARP (Address Resolution Protocol) packets.
 ## Stops ARP responses sent by a device without being explicitly requested.
 ## Prevents ARP cache poisoning by rejecting fake ARP entries into a network.

From 18aec201bfb0477fee8800ad1388099e11920016 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Wed, 13 Nov 2024 05:41:25 +0000
Subject: [PATCH 04/13] Provide option to harden response to ARP requests

---
 README.md                               | 3 +++
 usr/lib/sysctl.d/990-security-misc.conf | 7 +++++++
 2 files changed, 10 insertions(+)

diff --git a/README.md b/README.md
index baa450d..ed9e4d7 100644
--- a/README.md
+++ b/README.md
@@ -105,6 +105,9 @@ Networking:
 - Optional - Enable ARP filtering to mitigate some ARP spoofing and ARP
   cache poisoning attacks.
 
+- Optional - Respond to ARP requests only if the target IP address is
+  on-link, preventing some IP spoofing attacks.
+  
 - Optional - Drop gratuitous ARP packets to prevent ARP cache poisoning
   via man-in-the-middle and denial-of-service attacks.
 
diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 52d31de..a2c3198 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -452,6 +452,13 @@ net.ipv6.conf.*.accept_redirects=0
 ##
 #net.ipv4.conf.*.arp_filter=1
 
+## Respond to ARP (Address Resolution Protocol) requests only if the target IP address is on-link.
+## Reduces IP spoofing attacks by limiting the scope of allowable ARP responses.
+##
+## https://cyber.gouv.fr/sites/default/files/document/linux_configuration-en-v2.pdf
+##
+#net.ipv4.conf.*.arp_ignore=2
+
 ## Drop gratuitous ARP (Address Resolution Protocol) packets.
 ## Stops ARP responses sent by a device without being explicitly requested.
 ## Prevents ARP cache poisoning by rejecting fake ARP entries into a network.

From 141b84c40de76988ec78bdccf1c1d67fc4367b3f Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Wed, 13 Nov 2024 05:42:56 +0000
Subject: [PATCH 05/13] Provide option to deny sending and receiving shared
 media redirects

---
 README.md                               |  5 ++++-
 usr/lib/sysctl.d/990-security-misc.conf | 11 +++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index ed9e4d7..50b3cf0 100644
--- a/README.md
+++ b/README.md
@@ -102,12 +102,15 @@ Networking:
 - Disable ICMP redirect acceptance and redirect sending messages to prevent
   man-in-the-middle attacks and minimize information disclosure.
 
+- Optional - Deny sending and receiving shared media redirects to reduce
+  the risk of IP spoofing attacks.
+
 - Optional - Enable ARP filtering to mitigate some ARP spoofing and ARP
   cache poisoning attacks.
 
 - Optional - Respond to ARP requests only if the target IP address is
   on-link, preventing some IP spoofing attacks.
-  
+
 - Optional - Drop gratuitous ARP packets to prevent ARP cache poisoning
   via man-in-the-middle and denial-of-service attacks.
 
diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index a2c3198..c43ca19 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -443,6 +443,17 @@ net.ipv4.conf.*.send_redirects=0
 net.ipv6.conf.*.accept_redirects=0
 #net.ipv4.conf.*.secure_redirects=1
 
+## Deny sending and receiving RFC1620 shared media redirects.
+## Relevant mainly for network interfaces that operate over shared media such as Ethernet hubs.
+## Stops the kernel from sending ICMP redirects to specific networks from the connected network.
+## This variable overrides the use secure_redirects.
+##
+## https://cyber.gouv.fr/sites/default/files/document/linux_configuration-en-v2.pdf
+## https://datatracker.ietf.org/doc/html/rfc1620
+## https://www.frozentux.net/ipsysctl-tutorial/chunkyhtml/theconfvariables.html
+##
+#net.ipv4.conf.*.shared_media=0
+
 ## Enable ARP (Address Resolution Protocol) filtering.
 ## Prevents the Linux kernel from handling the ARP table globally
 ## Can mitigate some ARP spoofing and ARP cache poisoning attacks.

From c1167968542a62d0677517e11505f6e9222ec378 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Thu, 12 Dec 2024 06:36:47 +0000
Subject: [PATCH 06/13] `arp_ignore`: Add reference to 2024-12-10 Mullvad VPN
 audit details

---
 usr/lib/sysctl.d/990-security-misc.conf | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index c43ca19..93efe26 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -467,6 +467,9 @@ net.ipv6.conf.*.accept_redirects=0
 ## Reduces IP spoofing attacks by limiting the scope of allowable ARP responses.
 ##
 ## https://cyber.gouv.fr/sites/default/files/document/linux_configuration-en-v2.pdf
+## https://github.com/mullvad/mullvadvpn-app/blob/main/audits/2024-12-10-X41-D-Sec.md#mllvd-cr-24-03-virtual-ip-address-of-tunnel-device-leaks-to-network-adjacent-participant-severity-medium
+## https://github.com/mullvad/mullvadvpn-app/pull/7141
+## https://www.x41-dsec.de/static/reports/X41-Mullvad-Audit-Public-Report-2024-12-10.pdf
 ##
 #net.ipv4.conf.*.arp_ignore=2
 

From ca3a73ac13d805515f71f1be7ecedc33d3a1b519 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Tue, 17 Dec 2024 11:37:10 +0000
Subject: [PATCH 07/13] Typo

---
 usr/lib/sysctl.d/990-security-misc.conf | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 6009fc4..81e40a2 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -281,7 +281,7 @@ fs.protected_fifos=2
 fs.protected_regular=2
 
 ## Enable ASLR for mmap base, stack, VDSO pages, and heap.
-## Forces shared libraries to be loaded to random addresses
+## Forces shared libraries to be loaded to random addresses.
 ## Start location of PIE-linked binaries is randomized.
 ## Heap randomization can lead to breakages with legacy applications.
 ##

From 943c421889ce5dfe3869380e4587ca22724f2ce7 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Tue, 17 Dec 2024 11:40:38 +0000
Subject: [PATCH 08/13] Minor refactoring

---
 etc/default/grub.d/41_recovery_restrict.cfg | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/etc/default/grub.d/41_recovery_restrict.cfg b/etc/default/grub.d/41_recovery_restrict.cfg
index cfe68b0..530a728 100644
--- a/etc/default/grub.d/41_recovery_restrict.cfg
+++ b/etc/default/grub.d/41_recovery_restrict.cfg
@@ -7,10 +7,15 @@
 ## KSPP=no: not (currently) compliant with recommendations by the KSPP
 ## If there is no explicit KSPP compliance notice, the setting is not mentioned by the KSPP.
 
-## Disable access to single-user mode (i.e. recovery mode).
+## Disable access to single-user (recovery) mode.
+##
 ## https://forums.kicksecure.com/t/remove-linux-recovery-mode-boot-option-from-default-grub-boot-menu/727
-GRUB_DISABLE_RECOVERY='true'
+##
+GRUB_DISABLE_RECOVERY="true"
 
 ## Disable access to Dracut's recovery console.
+##
 ## https://forums.kicksecure.com/t/harden-dracut-initramfs-generator-by-disabling-recovery-console/724
-GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT rd.shell=0 rd.emergency=halt"
+##
+GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT rd.emergency=halt"
+GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT rd.shell=0"
\ No newline at end of file

From defba1f2450b2c8bbc668bf5f6f6f0d101338cc7 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Tue, 17 Dec 2024 11:42:03 +0000
Subject: [PATCH 09/13] Refactor CPU mitigations

---
 etc/default/grub.d/40_cpu_mitigations.cfg | 80 ++++++++++++++++-------
 1 file changed, 55 insertions(+), 25 deletions(-)

diff --git a/etc/default/grub.d/40_cpu_mitigations.cfg b/etc/default/grub.d/40_cpu_mitigations.cfg
index 1b900c0..c1828dc 100644
--- a/etc/default/grub.d/40_cpu_mitigations.cfg
+++ b/etc/default/grub.d/40_cpu_mitigations.cfg
@@ -17,7 +17,7 @@
 ## https://www.intel.com/content/www/us/en/developer/topic-technology/software-security-guidance/advisory-guidance.html
 ## https://www.intel.com/content/www/us/en/developer/topic-technology/software-security-guidance/disclosure-documentation.html
 
-## Enable a subset of known mitigations for CPU vulnerabilities and disable SMT.
+## Enable a subset of known mitigations for some CPU vulnerabilities and disable SMT.
 ##
 ## KSPP=yes
 ## KSPP sets the kernel parameters.
@@ -27,7 +27,7 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mitigations=auto,nosmt"
 ## Disable SMT as it has been the cause of and amplified numerous CPU exploits.
 ## The only full mitigation of cross-HT attacks is to disable SMT.
 ## Disabling will significantly decrease system performance on multi-threaded tasks.
-## To enable SMT, remove this line and all other occurrences of "nosmt" in this file.
+## Note, this setting will prevent re-enabling SMT via the sysfs interface.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/core-scheduling.html
 ## https://forums.whonix.org/t/should-all-kernel-patches-for-cpu-bugs-be-unconditionally-enabled-vs-performance-vs-applicability/7647/17
@@ -36,95 +36,125 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mitigations=auto,nosmt"
 ## KSPP=yes
 ## KSPP sets the kernel parameter.
 ##
+## To re-enable SMT:
+## - Remove "nosmt=force".
+## - Remove all occurrences of ",nosmt" in this file (note the comma ",").
+## - Downgrade "l1tf=full,force" protection to "l1tf=flush".
+##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX nosmt=force"
 
-## Enable mitigations for both Spectre Variant 2 (indirect branch speculation)
-## and Intel branch history injection (BHI) vulnerabilities.
+## Spectre Side Channels (BTI and BHI):
+## Unconditionally enable mitigation for Spectre Variant 2 (branch target injection).
+## Enable mitigation for the Intel branch history injection vulnerability.
+## Currently affects both AMD and Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/spectre.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX spectre_v2=on"
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX spectre_bhi=on"
 
-## Disable Speculative Store Bypass (Spectre Variant 4).
-## Unconditionally enable mitigation for both kernel and userspace.
+## Speculative Store Bypass (SSB):
+## Mitigate Spectre Variant 4 by disabling speculative store bypass system-wide.
+## Unconditionally enable the mitigation for both kernel and userspace.
+## Currently affects both AMD and Intel CPUs.
 ##
+## https://en.wikipedia.org/wiki/Speculative_Store_Bypass
 ## https://www.suse.com/support/kb/doc/?id=000019189
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX spec_store_bypass_disable=on"
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX ssbd=force-on"
 
-## Enable mitigations for the L1TF vulnerability through disabling SMT
-## and L1D flush runtime control.
+## L1 Terminal Fault (L1TF):
+## Mitigate the vulnerability by disabling L1D flush runtime control and SMT.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX l1tf=full,force"
 
-## Enable mitigations for the MDS vulnerability through clearing buffer cache
-## and disabling SMT.
+## Microarchitectural Data Sampling (MDS):
+## Mitigate the vulnerability by clearing the buffer cache and disabling SMT.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mds=full,nosmt"
 
-## Patches the TAA vulnerability by disabling TSX and enables mitigations using
-## TSX Async Abort along with disabling SMT.
+## TSX Asynchronous Abort (TAA):
+## Mitigate the vulnerability by disabling TSX.
+## If TSX is enabled, clear CPU buffer rings on transitions and disable SMT.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/tsx_async_abort.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX tsx=off"
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX tsx_async_abort=full,nosmt"
 
-## Mark all huge pages in the EPT as non-executable to mitigate iTLB multihit.
+## iTLB Multihit:
+## Mitigate the vulnerability by marking all huge pages in the EPT as non-executable.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/multihit.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm.nx_huge_pages=force"
 
-## Mitigations for SRBDS to prevent MDS attacks on RDRAND and RDSEED instructions
-## are only possible through microcode updates from Intel.
+## Special Register Buffer Data Sampling (SRBDS):
+## Mitigation of the vulnerability is only possible via microcode updates from Intel.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/special-register-buffer-data-sampling.html
 ## https://access.redhat.com/solutions/5142691
 
-## Enable the prctl() interface to prevent leaks from L1D on context switches.
+## L1D Flushing:
+## Mitigate leaks from the L1D cache on context switches by enabling the prctl() interface.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1d_flush.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX l1d_flush=on"
 
-## Mitigate numerous MMIO Stale Data vulnerabilities and disable SMT.
+## MMIO Stale Data:
+## Mitigate the vulnerability by appropriately clearing the CPU buffer and disabling SMT.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/processor_mmio_stale_data.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mmio_stale_data=full,nosmt"
 
-## Enable mitigations for RETBleed (Arbitrary Speculative Code Execution with
-## Return Instructions) vulnerability and disable SMT.
+## Arbitrary Speculative Code Execution with Return Instructions (Retbleed):
+## Mitigate the vulnerability through CPU-dependent implementation and disable SMT.
+## Currently affects both AMD Zen 1-2 and Intel CPUs.
 ##
+## https://en.wikipedia.org/wiki/Retbleed
+## https://comsec.ethz.ch/research/microarch/retbleed/
 ## https://www.suse.com/support/kb/doc/?id=000020693
+## https://access.redhat.com/solutions/retbleed
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX retbleed=auto,nosmt"
 
-## Control RAS overflow mitigation on AMD Zen CPUs.
+## Speculative Return Stack Overflow (SRSO):
+## Mitigate the vulnerability by ensureing all RET instructions speculate to a controlled location.
+## Currently affects AMD Zen 1-4 CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/srso.html
 ##
 ## The default kernel setting will be utilized until provided sufficient evidence to modify.
+## Using "spec_rstack_overflow=ipbp" may provide stronger security at a greater performance impact.
 ##
 #GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX spec_rstack_overflow=safe-ret"
 
-## Enable Gather Data Sampling (GDS) mitigation.
-## Note for systems that have not received a suitable microcode update this will
-## entirely disable use of the AVX instructions set.
+## Gather Data Sampling (GDS):
+## Mitigate the vulnerability either via microcode update or by disabling AVX.
+## Note, without a suitable microcode update, this will entirely disable use of the AVX instructions set.
+## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/gather_data_sampling.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX gather_data_sampling=force"
 
-## Enable Register File Data Sampling (RFDS) mitigation on Intel Atom CPUs which
-## encompasses E-cores on hybrid architectures.
+## Register File Data Sampling (RFDS):
+## Mitigate the vulnerability by appropriately clearing the CPU buffer.
+## Currently affects Intel Atom CPUs (which encompasses E-cores on hybrid architectures).
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/reg-file-data-sampling.html
 ##

From 45355aabdc180a6a2fdd4a374c6f7d72f4d36240 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Tue, 17 Dec 2024 11:42:52 +0000
Subject: [PATCH 10/13] Enable `kvm-intel.vmentry_l1d_flush=always`

---
 etc/default/grub.d/40_cpu_mitigations.cfg | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/etc/default/grub.d/40_cpu_mitigations.cfg b/etc/default/grub.d/40_cpu_mitigations.cfg
index c1828dc..8236c99 100644
--- a/etc/default/grub.d/40_cpu_mitigations.cfg
+++ b/etc/default/grub.d/40_cpu_mitigations.cfg
@@ -66,11 +66,13 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX ssbd=force-on"
 
 ## L1 Terminal Fault (L1TF):
 ## Mitigate the vulnerability by disabling L1D flush runtime control and SMT.
+## If conditional L1D flushing, mitigate the vulnerability for certain KVM hypervisor configurations.
 ## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX l1tf=full,force"
+GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm-intel.vmentry_l1d_flush=always"
 
 ## Microarchitectural Data Sampling (MDS):
 ## Mitigate the vulnerability by clearing the buffer cache and disabling SMT.

From 3e96fdd9ccb6268403d6c4f9a061c4a33e6f6dd2 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Tue, 17 Dec 2024 11:44:11 +0000
Subject: [PATCH 11/13] Enable `kvm.mitigate_smt_rsb=1`

---
 etc/default/grub.d/40_cpu_mitigations.cfg | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/etc/default/grub.d/40_cpu_mitigations.cfg b/etc/default/grub.d/40_cpu_mitigations.cfg
index 8236c99..b7984c5 100644
--- a/etc/default/grub.d/40_cpu_mitigations.cfg
+++ b/etc/default/grub.d/40_cpu_mitigations.cfg
@@ -134,6 +134,14 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mmio_stale_data=full,nosmt"
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX retbleed=auto,nosmt"
 
+## Cross-Thread Return Address Predictions:
+## Mitigate the vulnerability for certain KVM hypervisor configurations.
+## Currently affects AMD Zen 1-2 CPUs.
+##
+## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/cross-thread-rsb.html
+##
+GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm.mitigate_smt_rsb=1"
+
 ## Speculative Return Stack Overflow (SRSO):
 ## Mitigate the vulnerability by ensureing all RET instructions speculate to a controlled location.
 ## Currently affects AMD Zen 1-4 CPUs.

From 0dff2cd28fd769955757cdef1b7f9d637a1180c5 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Wed, 18 Dec 2024 03:32:35 +0000
Subject: [PATCH 12/13] Minor additions

---
 etc/default/grub.d/40_cpu_mitigations.cfg | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/etc/default/grub.d/40_cpu_mitigations.cfg b/etc/default/grub.d/40_cpu_mitigations.cfg
index b7984c5..fa750b7 100644
--- a/etc/default/grub.d/40_cpu_mitigations.cfg
+++ b/etc/default/grub.d/40_cpu_mitigations.cfg
@@ -8,6 +8,7 @@
 ## If there is no explicit KSPP compliance notice, the setting is not mentioned by the KSPP.
 
 ## Enable known mitigations for CPU vulnerabilities.
+## Note, the mitigations for SSB and Retbleed are not currently mentioned in the first link.
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/index.html
 ## https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
 ## https://forums.whonix.org/t/should-all-kernel-patches-for-cpu-bugs-be-unconditionally-enabled-vs-performance-vs-applicability/7647
@@ -40,6 +41,7 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mitigations=auto,nosmt"
 ## - Remove "nosmt=force".
 ## - Remove all occurrences of ",nosmt" in this file (note the comma ",").
 ## - Downgrade "l1tf=full,force" protection to "l1tf=flush".
+## - Regenerate the dracut initramfs and then reboot system.
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX nosmt=force"
 
@@ -66,7 +68,7 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX ssbd=force-on"
 
 ## L1 Terminal Fault (L1TF):
 ## Mitigate the vulnerability by disabling L1D flush runtime control and SMT.
-## If conditional L1D flushing, mitigate the vulnerability for certain KVM hypervisor configurations.
+## If L1D flushing is conditional, mitigate the vulnerability for certain KVM hypervisor configurations.
 ## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html
@@ -75,7 +77,7 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX l1tf=full,force"
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm-intel.vmentry_l1d_flush=always"
 
 ## Microarchitectural Data Sampling (MDS):
-## Mitigate the vulnerability by clearing the buffer cache and disabling SMT.
+## Mitigate the vulnerability by clearing the CPU buffer cache and disabling SMT.
 ## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html
@@ -101,7 +103,7 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX tsx_async_abort=full,nosmt"
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm.nx_huge_pages=force"
 
 ## Special Register Buffer Data Sampling (SRBDS):
-## Mitigation of the vulnerability is only possible via microcode updates from Intel.
+## Mitigation of the vulnerability is only possible via microcode update from Intel.
 ## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/special-register-buffer-data-sampling.html
@@ -115,8 +117,8 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm.nx_huge_pages=force"
 ##
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX l1d_flush=on"
 
-## MMIO Stale Data:
-## Mitigate the vulnerability by appropriately clearing the CPU buffer and disabling SMT.
+## Processor MMIO Stale Data:
+## Mitigate the vulnerabilities by appropriately clearing the CPU buffer and disabling SMT.
 ## Currently affects Intel CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/processor_mmio_stale_data.html
@@ -143,7 +145,7 @@ GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX retbleed=auto,nosmt"
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kvm.mitigate_smt_rsb=1"
 
 ## Speculative Return Stack Overflow (SRSO):
-## Mitigate the vulnerability by ensureing all RET instructions speculate to a controlled location.
+## Mitigate the vulnerability by ensuring all RET instructions speculate to a controlled location.
 ## Currently affects AMD Zen 1-4 CPUs.
 ##
 ## https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/srso.html

From 3749f8ff097551a843e5ed80de52c6770a32e0c6 Mon Sep 17 00:00:00 2001
From: raja-grewal <rg_public@proton.me>
Date: Wed, 18 Dec 2024 03:36:09 +0000
Subject: [PATCH 13/13] Update presentation on user namespaces

---
 README.md                               |  4 +--
 usr/lib/sysctl.d/990-security-misc.conf | 43 ++++++++++++-------------
 2 files changed, 23 insertions(+), 24 deletions(-)

diff --git a/README.md b/README.md
index c23d261..d032762 100644
--- a/README.md
+++ b/README.md
@@ -37,11 +37,11 @@ Kernel space:
 - Entirely disable the SysRq key so that the Secure Attention Key (SAK)
   can no longer be utilized. See [documentation](https://www.kicksecure.com/wiki/SysRq).
 
+- Optional - Disable all use of user namespaces.
+
 - Optional - Restrict user namespaces to `CAP_SYS_ADMIN` as they can lead to substantial
   privilege escalation.
 
-- Optional - Disable all use of user namespaces.
-
 - Restrict kernel profiling and the performance events system to `CAP_PERFMON`.
 
 - Force the kernel to panic on both "oopses", which can potentially indicate and thwart
diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 81e40a2..10f7a12 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -117,42 +117,41 @@ vm.unprivileged_userfaultfd=0
 ##
 kernel.sysrq=0
 
-## Restrict user namespaces to users with CAP_SYS_ADMIN.
+## Disable user namespaces entirely.
 ## User namespaces aim to improve sandboxing and accessibility for unprivileged users.
-## Unprivileged user namespaces pose substantial privilege escalation risks.
-## Restricting may lead to breakages in numerous software packages.
-##
-## Flatpak requires unprivileged users to create new user namespaces for sandboxing.
-## https://github.com/flatpak/flatpak/wiki/User-namespace-requirements
-## https://salsa.debian.org/debian/bubblewrap/-/blob/debian/latest/debian/README.Debian
-## https://forums.kicksecure.com/t/can-not-run-flatpak-apps-after-kicksecure-update/592
-##
 ## Disabling entirely will reduce compatibility with some AppArmor profiles.
 ## Disabling entirely is known to break the UPower systemd service.
-##
-## Also breaks (some?) AppImages.
-## https://forums.kicksecure.com/t/cannot-run-some-appimage-apps-after-kicksecure-upate/594
-##
-## Might also break evolution (e-mail client):
-## https://forums.kicksecure.com/t/impossible-to-start-evolution-app-since-the-last-update/601
+## Not recommended due to well-known breakages across numerous software packages.
 ##
 ## https://lwn.net/Articles/673597/
 ## https://madaidans-insecurities.github.io/linux.html#kernel
-## https://gitlab.com/apparmor/apparmor/-/wikis/unprivileged_userns_restriction
 ## https://github.com/a13xp0p0v/kernel-hardening-checker#questions-and-answers
 ## https://github.com/NixOS/nixpkgs/pull/84522#issuecomment-614640601
-## https://github.com/flatpak/flatpak/wiki/User-namespace-requirements
 ## https://github.com/Kicksecure/security-misc/pull/263
-## https://github.com/Kicksecure/security-misc/issues/274
 ##
 ## KSPP=no
-## KSPP sets user.max_user_namespaces=0 sysctl, a Linux mainline, stricter setting.
+## KSPP sets the sysctl.
 ##
-## kernel.unprivileged_userns_clone is a Debian specific kernel feature. Not Linux mainline.
-#kernel.unprivileged_userns_clone=0
-## Uncomment the following sysctl to entirely disable user namespaces.
 #user.max_user_namespaces=0
 
+## Restrict user namespaces to users with CAP_SYS_ADMIN.
+## See the user.max_user_namespaces setting for more details.
+## This is a Debian-specific kernel feature, not a Linux mainline setting.
+## Unprivileged user namespaces pose substantial privilege escalation risks.
+## Flatpak requires unprivileged users to create new user namespaces for sandboxing.
+## Restricting is known to cause breakages in some AppImages and the Evolution Email Client.
+## Not recommended due to widespread breakages across many software packages.
+##
+## https://salsa.debian.org/debian/bubblewrap/-/blob/debian/latest/debian/README.Debian
+## https://gitlab.com/apparmor/apparmor/-/wikis/unprivileged_userns_restriction
+## https://github.com/flatpak/flatpak/wiki/User-namespace-requirements
+## https://forums.kicksecure.com/t/can-not-run-flatpak-apps-after-kicksecure-update/592
+## https://forums.kicksecure.com/t/cannot-run-some-appimage-apps-after-kicksecure-upate/594
+## https://forums.kicksecure.com/t/impossible-to-start-evolution-app-since-the-last-update/601
+## https://github.com/Kicksecure/security-misc/issues/274
+##
+#kernel.unprivileged_userns_clone=0
+
 ## Restricts kernel profiling to users with CAP_PERFMON.
 ## The performance events system should not be accessible by unprivileged users.
 ## Other distributions such as Ubuntu and Fedora may permit further restricting.