AlmaLinux 8 — libsepol — multiple vulnerabilities (4 CVEs) — patch and remediation guide

📖 ~4 min read  •  Source: AlmaLinux ALSA ALSA-2021:4513

Related CVEs: CVE-2021-36084 CVE-2021-36085 CVE-2021-36086 CVE-2021-36087

Upstream summary: The libsepol library provides an API for the manipulation of SELinux binary policies. It is used by checkpolicy (the policy compiler) and similar tools, as well as by programs like load_policy that need to perform specific transformations on binary policies (for example, customizing policy boolean settings).

Security Fix(es):

* libsepol: use-after-free in __cil_verify_classperms() (CVE-2021-36084)

* libsepol: use-after-free in __cil_verify_classperms() (CVE-2021-36085)

*

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

On AlmaLinux 8 hosts that have libsepol installed, operators report behaviour consistent with AlmaLinux ALSA ALSA-2021:4513: dnf refuses to install or restart affected services, SELinux AVC denials appear in /var/log/audit/audit.log, and — for security-rated advisories — the host is exposed to the vulnerability set above. Impact ranges from a single service-restart loop to wider availability incidents whenever libsepol sits on the serving path.

Environment & Reproduction

Reproduction targets AlmaLinux 8. Confirm release and the installed package:

cat /etc/almalinux-release
cat /etc/os-release
rpm -q libsepol
dnf info libsepol | head -20

Trigger the workflow that exposes libsepol — multiple vulnerabilities (4 CVEs) — patch and remediation guide while collecting:

sudo journalctl -u libsepol -b --no-pager | tail -200
sudo journalctl -xe --no-pager | tail -200
sudo tail -200 /var/log/dnf.log
sudo tail -200 /var/log/audit/audit.log
# For an evidence bundle bundle with sosreport:
sudo sosreport --batch

Root Cause Analysis

Root cause is documented in AlmaLinux ALSA ALSA-2021:4513. AlmaLinux / Red Hat maintainers shipped fixes in the corresponding libsepol update for AlmaLinux 8; running an outdated build leaves the host exposed to the failure modes described in the advisory. Correlate dnf history with system logs:

sudo dnf history | head
sudo dnf history list libsepol
sudo dnf history info <id>
sudo ausearch -m AVC,USER_AVC -ts today | tail -100
cat /proc/sys/kernel/tainted   # non-zero = tainted kernel / out-of-tree modules

Quick Triage

Run these on AlmaLinux 8 to capture the current state of libsepol:

rpm -q libsepol                              # installed NVR
rpm -V libsepol                              # verify shipped files
sudo dnf check-update --security
sudo dnf updateinfo list cves
systemctl --failed --no-pager
sudo firewall-cmd --list-all
getenforce && sestatus
# If libsepol ships a systemd unit (unit name may differ from pkg name, e.g.
# bind→named, postgresql-server→postgresql, php-fpm→php-fpm):
systemctl list-unit-files | grep -i libsepol | head

Step-by-Step Diagnosis

1. List failed systemd units.

   systemctl --failed --no-pager

2. Tail the journal for libsepol and the system bus.

   sudo journalctl -u libsepol -f --no-pager
   sudo journalctl -xe -f --no-pager

3. Inspect firewall posture.

   sudo firewall-cmd --list-all-zones --permanent
   sudo nft list ruleset 2>/dev/null | head -50

4. Surface SELinux denials and author a local policy module if needed.

   sudo ausearch -m AVC,USER_AVC -ts today
   sudo ausearch -m AVC -ts today | audit2allow -a -M /tmp/local-fix
   sudo semodule -i /tmp/local-fix.pp

5. Verify libsepol integrity and reinstall if anything is altered.

   sudo rpm -V libsepol
   sudo dnf reinstall libsepol

6. Correlate findings with /var/log/dnf.log, dnf history, and AlmaLinux ALSA ALSA-2021:4513 to pin the change that introduced libsepol — multiple vulnerabilities (4 CVEs) — patch and remediation guide.

Solution – Primary Fix

Apply the corrective dnf transaction referenced by AlmaLinux ALSA ALSA-2021:4513, then reload affected systemd units:

sudo dnf -y makecache
sudo dnf -y upgrade --security              # apply ALL security errata (recommended)
# Or target a single package:
sudo dnf -y upgrade libsepol
sudo systemctl daemon-reload
# Unit name may differ from pkg name; check first:
systemctl list-unit-files | grep -i libsepol | head
sudo systemctl restart libsepol
rpm -q libsepol                                # confirm new NVR
systemctl is-active libsepol 2>/dev/null       # confirm running (if a unit exists)

For kernel / glibc / systemd / openssl advisories a reboot is required (or kpatch where licensed):

sudo needs-restarting -r                    # report whether reboot needed
sudo systemctl reboot                       # or: sudo shutdown -r now
# kpatch (Red Hat / Oracle) avoids reboot for many kernel CVEs:
sudo dnf install -y kpatch kpatch-dnf
sudo dnf kpatch auto                        # enable auto-patching
sudo kpatch list

Solution – Alternative Approaches

If the primary patch is not viable, choose from these:

• Roll back the offending dnf transaction:

  sudo dnf history list | head
  sudo dnf history info <id>
  sudo dnf history undo <id>

• Version-lock the package so dnf cannot upgrade it:

  sudo dnf install -y python3-dnf-plugin-versionlock
  sudo dnf versionlock add libsepol
  sudo dnf versionlock list
  sudo dnf versionlock delete libsepol      # remove the lock

• Install an older NVR if a regression is suspected:

  dnf --showduplicates list libsepol | tac | head
  sudo dnf install -y --allowerasing libsepol-<older-NVR>

• Switch SELinux to permissive briefly to confirm policy is the cause, then re-enforce:

  sudo setenforce 0
  # reproduce, capture denials, author a custom module:
  sudo ausearch -m AVC -ts recent | audit2allow -a -M mylocal
  sudo semodule -i mylocal.pp
  sudo setenforce 1

• Take an LVM snapshot before kernel / glibc upgrades for fast rollback:

  sudo lvs
  sudo lvcreate -s -n preupgrade -L 4G /dev/<vg>/<lv>
  # revert later via:
  sudo lvconvert --merge /dev/<vg>/preupgrade && sudo systemctl reboot

• Where kpatch is licensed, apply kernel fixes without reboot:

  sudo kpatch list
  sudo kpatch load /usr/lib/modules/$(uname -r)/extra/kpatch/*.ko

Verification & Acceptance Criteria

All of these should pass after the fix:

rpm -q libsepol                                            # expected fixed NVR
sudo dnf updateinfo list cves --installed               # CVEs above no longer listed
systemctl is-active libsepol 2>/dev/null
sudo journalctl -u libsepol --since "5 minutes ago" --no-pager | grep -iE "error|fail" || echo OK
sudo firewall-cmd --list-services
getenforce
sudo needs-restarting -r

The original reproduction for libsepol — multiple vulnerabilities (4 CVEs) — patch and remediation guide must not trigger across two consecutive runs.

Rollback Plan

Capture state before any change:

rpm -qa > /root/rpm-pre.txt
sudo dnf history list > /root/dnf-history-pre.txt
# Optional LVM snapshot of the root LV:
sudo lvcreate -s -n preupgrade -L 4G /dev/<vg>/<lv>

To revert if the patch is bad:

sudo dnf history undo <id>
# Or downgrade just the package:
sudo dnf install -y --allowerasing libsepol-<older-NVR>
sudo systemctl daemon-reload
sudo systemctl restart libsepol
# Or merge the LVM snapshot and reboot:
sudo lvconvert --merge /dev/<vg>/preupgrade && sudo systemctl reboot
# Custom SELinux policy cleanup:
sudo semodule -r mylocal

Prevention & Hardening

Reduce the chance of this recurring on AlmaLinux 8:

• Enable automatic security patching:

  sudo dnf install -y dnf-automatic
  sudo sed -i 's/^upgrade_type.*/upgrade_type = security/' /etc/dnf/automatic.conf
  sudo sed -i 's/^apply_updates.*/apply_updates = yes/' /etc/dnf/automatic.conf
  sudo systemctl enable --now dnf-automatic.timer

• Subscribe to almalinux-announce and watch Red Hat security updates for upstream changes.

• Mirror through a local Pulp / Foreman / Spacewalk-style repo for controlled rollouts:

  sudo dnf install -y dnf-utils createrepo_c
  sudo reposync --download-metadata --downloadcomps -p /srv/mirror -- repoid=baseos
  sudo createrepo_c /srv/mirror/baseos

• Version-lock sensitive packages so they cannot be auto-upgraded:

  sudo dnf install -y python3-dnf-plugin-versionlock
  sudo dnf versionlock add libsepol

• Monitor file integrity with AIDE:

  sudo dnf install -y aide
  sudo aide --init && sudo mv /var/lib/aide/aide.db.new.gz /var/lib/aide/aide.db.gz
  sudo aide --check

• Enable kpatch so kernel CVEs can be remediated without reboot:

  sudo dnf install -y kpatch kpatch-dnf
  sudo dnf kpatch auto
  sudo kpatch list

• Keep SELinux in enforcing mode and review custom modules in /etc/selinux/targeted/ after every package upgrade.

• Apply CIS AlmaLinux 8 Benchmark hardening and remove unused packages.

Related Errors & Cross-Refs

Issues that commonly surface alongside libsepol — multiple vulnerabilities (4 CVEs) — patch and remediation guide: dnf lock contention, systemd unit ordering cycles, SELinux AVC bursts, firewalld zone drift, and kernel taint flags. Useful triage:

sudo dnf check
systemd-analyze critical-chain
sudo ausearch -m AVC -ts today | tail
sudo firewall-cmd --get-active-zones
cat /proc/sys/kernel/tainted
sudo needs-restarting -r

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References & Further Reading

Primary reference: AlmaLinux ALSA ALSA-2021:4513. Manual pages useful on AlmaLinux 8:

man dnf
man dnf.conf
man systemctl
man journalctl
man firewall-cmd
man semanage
man audit2allow
man kpatch
man sosreport

Other resources: wiki.almalinux.org, Red Hat CVE database, AlmaLinux errata, and per-package notes in /usr/share/doc/libsepol/ for components implicated in libsepol — multiple vulnerabilities (4 CVEs) — patch and remediation guide.

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