Amazon Linux 2 — qemu-kvm — multiple vulnerabilities (10 CVEs) — patch and remediation guide

📖 ~4 min read  •  Source: Amazon Linux advisory ALAS2-2018-1073

Related CVEs: CVE-2018-11806 CVE-2018-7550 CVE-2017-13672 CVE-2017-13711 CVE-2017-15124 CVE-2017-15268 CVE-2018-3639 CVE-2018-5683  +2 more

Upstream summary: A heap buffer overflow issue was found in the way SLiRP networking back-end in QEMU processes fragmented packets. It could occur while reassembling the fragmented datagrams of an incoming packet. A privileged user/process inside guest could use this flaw to crash the QEMU process resulting in DoS or potentially leverage it to execute arbitrary code on the host with privileges of the QEMU process.(CVE-2018-11806) Quick Emulator (QEMU), compiled with the PC System Emulator with

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

Nothing visibly changes on an Amazon Linux 2 host until qemu-kvm is next invoked: it runs, exits and leaves no daemon behind, so there is no restart loop or service log to watch. Exposure is per-invocation, and the invocations that matter are the unattended ones — cron jobs, backup and deployment scripts, CI runners, and anything passing attacker-influenced input such as a remote URL, a supplied archive or an untrusted filename. Where qemu-kvm is run under sudo or from a privileged job, the same flaw becomes local privilege escalation. qemu-kvm ships command-line programs that run when invoked and then exit, so there is no unit to restart — systemctl and journalctl -u have nothing to act on; only the next invocation matters.

Environment & Reproduction

Reproduction targets Amazon Linux 2. Confirm release and the installed package:

cat /etc/system-release
cat /etc/os-release
rpm -q qemu-kvm
yum info qemu-kvm | head -20

Exercise the workload that uses qemu-kvm while collecting:

sudo journalctl -xe --no-pager | tail -200
sudo tail -200 /var/log/yum.log
sudo tail -200 /var/log/audit/audit.log
# For an evidence bundle bundle with sosreport (Amazon Linux ships it):
sudo sosreport --batch

For fleet-wide visibility, query Amazon Inspector and SSM at the same time:

aws inspector2 list-findings 
  --filter-criteria 'awsAccountId={comparison=EQUALS,value=<account-id>}' 
  --max-results 50
aws ssm describe-instance-patches --instance-id <i-xxxx> | head -40

Root Cause Analysis

Root cause is documented in Amazon Linux advisory ALAS2-2018-1073. The Amazon Linux Security Team shipped fixes in the corresponding qemu-kvm update for Amazon Linux 2; running an outdated AMI or unpatched instance leaves the host exposed to the failure modes described in the advisory. Correlate yum history with system logs:

sudo yum history | head
sudo yum history list qemu-kvm
sudo yum 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 Amazon Linux 2 to capture the current state of qemu-kvm:

rpm -q qemu-kvm                              # installed NVR
rpm -V qemu-kvm                              # verify shipped files
sudo yum check-update --security
sudo yum updateinfo list cves
systemctl --failed --no-pager
sudo firewall-cmd --list-all 2>/dev/null || sudo iptables -L -n
getenforce && sestatus
dnf repoquery -l qemu-kvm 2>/dev/null | grep -E '/lib/systemd/system/.*\.(service|socket|timer)$'  # same check WITHOUT installing (EL7/Amazon Linux 2: yum install yum-utils, then: repoquery -l qemu-kvm)
systemctl show --no-pager -p Type,RemainAfterExit <UNIT>  # Type=oneshot (often paired with a .timer) means a one-shot job, NOT a daemon -- do not 'restart' it, let the timer fire

Step-by-Step Diagnosis

1. List failed systemd units.

   systemctl --failed --no-pager

2. Tail the journal for qemu-kvm and the system bus.

   sudo journalctl -xe -f --no-pager

3. Inspect firewall / security-group posture from inside the instance.

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

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 qemu-kvm integrity and reinstall if anything is altered.

   sudo rpm -V qemu-kvm
   sudo yum reinstall -y qemu-kvm

6. Correlate findings with /var/log/yum.log, yum history, Amazon Inspector findings, and Amazon Linux advisory ALAS2-2018-1073 to pin the change that introduced the regression.

Solution – Primary Fix

Apply the corrective yum transaction referenced by Amazon Linux advisory ALAS2-2018-1073, then reload affected systemd units:

sudo yum -y makecache
sudo yum -y update --security              # apply ALL security errata (recommended)
# Or target a single package:
sudo yum -y update qemu-kvm
# Unit name may differ from pkg name; check first:
rpm -q qemu-kvm                                # confirm new NVR

For kernel / glibc / systemd / openssl advisories a reboot is required (or Live Patching where available):

sudo needs-restarting -r                    # report whether reboot needed
sudo systemctl reboot                       # or: sudo shutdown -r now
# Amazon Linux Live Patching for the kernel (when enabled on the instance):
sudo yum install -y kernel-livepatch
sudo yum kernel-livepatch enable           # AL2 / AL2023
sudo yum kernel-livepatch status

Roll the same change across an Auto Scaling group / fleet with AWS Systems Manager Patch Manager:

aws ssm send-command 
  --document-name AWS-RunPatchBaseline 
  --targets Key=tag:Patch,Values=yes 
  --parameters 'Operation=Install,RebootOption=RebootIfNeeded' 
  --comment 'Apply Amazon Linux security errata'
aws ssm list-command-invocations --details --max-results 5
# Confirm the patch landed across the fleet:
aws ssm describe-instance-patch-states-for-patch-group 
  --patch-group <patch-group>

For immutable infrastructure, rebuild the golden AMI in EC2 Image Builder so newly launched instances start patched (do not rely on in-place patching alone):

aws imagebuilder start-image-pipeline-execution 
  --image-pipeline-arn arn:aws:imagebuilder:<region>:<acct>:image-pipeline/<name>
aws imagebuilder list-images --owner Self --max-results 5
# Then update the launch template / ASG to the new AMI:
aws ec2 create-launch-template-version --launch-template-id <lt-id> 
  --source-version '$Latest' --launch-template-data 'ImageId=<new-ami-id>'

Solution – Alternative Approaches

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

• Roll back the offending yum transaction:

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

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

  sudo yum install -y yum-plugin-versionlock
  sudo yum versionlock add qemu-kvm
  sudo yum versionlock list
  sudo yum versionlock delete qemu-kvm      # remove the lock

• Install an older NVR if a regression is suspected:

  yum --showduplicates list qemu-kvm | tac | head
  sudo yum install -y --allowerasing qemu-kvm-<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 EBS snapshot of the root volume before kernel / glibc upgrades for fast rollback:

  INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)
  VOL_ID=$(aws ec2 describe-instances --instance-ids $INSTANCE_ID 
    --query 'Reservations[].Instances[].BlockDeviceMappings[?DeviceName==`/dev/xvda`].Ebs.VolumeId' 
    --output text)
  aws ec2 create-snapshot --volume-id $VOL_ID 
    --description 'pre-patch-snapshot' --tag-specifications 
    'ResourceType=snapshot,Tags=[{Key=Name,Value=pre-patch}]'

• For immutable workloads, swap the ASG to a previous AMI version instead of patching in place:

  aws ec2 describe-launch-template-versions --launch-template-id <lt-id> --max-results 5
  aws autoscaling update-auto-scaling-group --auto-scaling-group-name <asg> 
    --launch-template LaunchTemplateId=<lt-id>,Version='<prev-version>'
  aws autoscaling start-instance-refresh --auto-scaling-group-name <asg>

• Where Kernel Live Patching is enabled, apply kernel fixes without reboot:

  sudo yum kernel-livepatch status
  sudo yum kernel-livepatch enable
  sudo yum update -y --advisory=ALAS2-2018-1073 kernel

Verification & Acceptance Criteria

All of these should pass after the fix:

rpm -q qemu-kvm                                            # expected fixed NVR
sudo yum updateinfo list cves --installed              # CVEs above no longer listed
sudo firewall-cmd --list-services 2>/dev/null || sudo iptables -L -n
getenforce
sudo needs-restarting -r
# Inspector should drop the finding within ~24h:
aws inspector2 list-findings 
  --filter-criteria 'vulnerabilityId={comparison=EQUALS,value=CVE-2018-11806}'

The conditions described in the advisory must no longer be reported for qemu-kvm across two consecutive runs.

Rollback Plan

Capture state before any change:

rpm -qa > /root/rpm-pre.txt
sudo yum history list > /root/yum-history-pre.txt
# Optional pre-patch EBS snapshot of the root volume (run from inside the instance):
INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)
aws ec2 create-snapshot --volume-id <vol-id> --description 'pre-patch'

To revert if the patch is bad:

sudo yum history undo <id>
# Or downgrade just the package:
sudo yum install -y --allowerasing qemu-kvm-<older-NVR>
# Or replace the instance from a snapshot/older AMI via ASG instance refresh:
aws autoscaling start-instance-refresh --auto-scaling-group-name <asg>
# Custom SELinux policy cleanup:
sudo semodule -r mylocal

Prevention & Hardening

Reduce the chance of this recurring on Amazon Linux 2:

• Enable automatic security patching on each instance:

  sudo yum install -y yum-cron
  sudo sed -i 's/^update_cmd.*/update_cmd = security/' /etc/yum-cron/yum-cron.conf 2>/dev/null || true
  sudo sed -i 's/^upgrade_type.*/upgrade_type = security/' /etc/dnf/automatic.conf 2>/dev/null || true
  sudo sed -i 's/^apply_updates.*/apply_updates = yes/' /etc/dnf/automatic.conf 2>/dev/null || true
  sudo systemctl enable --now yum-cron.service

• Drive fleet-wide patching through AWS Systems Manager Patch Manager (preferred for any fleet bigger than a handful of instances):

  aws ssm send-command --document-name AWS-RunPatchBaseline 
    --targets Key=tag:Patch,Values=yes 
    --parameters 'Operation=Install,RebootOption=RebootIfNeeded'
  aws ssm describe-patch-baselines --filters Key=OWNER,Values=AWS
  aws ssm get-default-patch-baseline --operating-system AMAZON_LINUX_2

• Enable Amazon Inspector for continuous CVE / package vulnerability scanning:

  aws inspector2 enable --resource-types EC2 ECR LAMBDA
  aws inspector2 list-findings 
    --filter-criteria 'severity={comparison=EQUALS,value=HIGH}' --max-results 20
  aws inspector2 batch-get-account-status

• Bake patched golden AMIs with EC2 Image Builder and roll them via ASG instance refresh instead of in-place patching for immutable infrastructure:

  aws imagebuilder list-image-pipelines
  aws imagebuilder start-image-pipeline-execution 
    --image-pipeline-arn arn:aws:imagebuilder:<region>:<acct>:image-pipeline/<name>
  aws autoscaling start-instance-refresh --auto-scaling-group-name <asg>

• Subscribe to alas.aws.amazon.com and watch AWS security bulletins for upstream changes.

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

  sudo yum install -y yum-plugin-versionlock
  sudo yum versionlock add qemu-kvm

• Monitor file integrity with AIDE:

  sudo yum 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 Kernel Live Patching so kernel CVEs can be remediated without reboot:

  sudo yum install -y kernel-livepatch
  sudo yum kernel-livepatch enable
  sudo yum kernel-livepatch status

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

• Apply CIS Amazon Linux 2 Benchmark hardening and remove unused packages.

Related Errors & Cross-Refs

Issues that commonly surface alongside a qemu-kvm update: yum lock contention, systemd unit ordering cycles, SELinux AVC bursts, security-group / NACL drift, and kernel taint flags after out-of-tree modules. Useful triage:

sudo yum check
systemd-analyze critical-chain
sudo ausearch -m AVC -ts today | tail
sudo firewall-cmd --get-active-zones 2>/dev/null || sudo iptables -L -n
cat /proc/sys/kernel/tainted
sudo needs-restarting -r
aws ssm describe-instance-patches --instance-id <i-xxxx> | tail -40

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References & Further Reading

Primary reference: Amazon Linux advisory ALAS2-2018-1073. Manual pages useful on Amazon Linux 2:

man yum
man yum.conf
man systemctl
man journalctl
man firewall-cmd
man semanage
man audit2allow
man sosreport

Other resources: alas.aws.amazon.com, SSM Patch Manager docs, Amazon Inspector docs, EC2 Image Builder docs, and per-package notes in /usr/share/doc/qemu-kvm/ for components implicated in this advisory.

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