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Production Host Setup Recommendations

Jailer Configuration

Using Jailer in a production Firecracker deployment is highly recommended, as it provides additional security boundaries for the microVM. The Jailer process applies cgroup, namespace, seccomp isolation and drops privileges of the Firecracker process.

To set up the jailer correctly, you'll need to:

  • Create a dedicated non-privileged POSIX user and group to run Firecracker under. Use the created POSIX user and group IDs in Jailer's --uid <uid> and --gid <gid> flags, respectively. This will run the Firecracker as the created non-privileged user and group. All file system resources used for Firecracker should be owned by this user and group. Apply least privilege to the resource files owned by this user and group to prevent other accounts from unauthorized file access. When running multiple Firecracker instances it is recommended that each runs with its unique uid and gid to provide an extra layer of security for their individually owned resources in the unlikely case where any one of the jails is broken out of.

Additional details of Jailer features can be found in the Jailer documentation.

Host Security Configuration

Mitigating Side-Channel Issues

When deploying Firecracker microVMs to handle multi-tenant workloads, the following host environment configurations are strongly recommended to guard against side-channel security issues.

Disable Simultaneous Multithreading (SMT)

Disabling SMT will help mitigate side-channels issues between sibling threads on the same physical core.

SMT can be disabled by adding the following Kernel boot parameter to the host:

nosmt=force

Verification can be done by running:

(grep -q "^forceoff$\|^notsupported$" /sys/devices/system/cpu/smt/control && echo "Hyperthreading: DISABLED (OK)") || echo "Hyperthreading: ENABLED (Recommendation: DISABLED)"

Check Kernel Page-Table Isolation (KPTI) support

KPTI is used to prevent certain side-channel issues that allow access to protected kernel memory pages that are normally inaccessible to guests. Some variants of Meltdown can be mitigated by enabling this feature.

Verification can be done by running:

(grep -q "^Mitigation: PTI$" /sys/devices/system/cpu/vulnerabilities/meltdown && echo "KPTI: SUPPORTED (OK)") || echo "KPTI: NOT SUPPORTED (Recommendation: SUPPORTED)"

Disable Kernel Same-page Merging (KSM)

Disabling KSM mitigates side-channel issues which rely on de-duplication to reveal what memory line was accessed by another process.

KSM can be disabled by executing the following as root:

echo "0" > /sys/kernel/mm/ksm/run

Verification can be done by running:

(grep -q "^0$" /sys/kernel/mm/ksm/run && echo "KSM: DISABLED (OK)") || echo "KSM: ENABLED (Recommendation: DISABLED)"

Check for speculative branch prediction issue mitigation

Use a kernel compiled with retpoline and run on hardware with microcode supporting Indirect Branch Prediction Barriers (IBPB) and Indirect Branch Restricted Speculation (IBRS).

These features provide side-channel mitigation for variants of Spectre such as the Branch Target Injection variant.

Verification can be done by running:

(grep -q "^Mitigation: Full generic retpoline, IBPB, IBRS_FW$" /sys/devices/system/cpu/vulnerabilities/spectre_v2 && echo "retpoline, IBPB, IBRS: ENABLED (OK)") || echo "retpoline, IBPB, IBRS: DISABLED (Recommendation: ENABLED)"

Apply L1 Terminal Fault (L1TF) mitigation

These features provide mitigation for Foreshadow/L1TF side-channel issue on affected hardware.

They can be enabled by adding the following Linux kernel boot parameter:

l1tf=full,force

which will also implicitly disable SMT. This will apply the mitigation when execution context switches into microVMs.

Verification can be done by running:

declare -a CONDITIONS=("Mitigation: PTE Inversion" "VMX: cache flushes")
for cond in "${CONDITIONS[@]}"; do (grep -q "$cond" /sys/devices/system/cpu/vulnerabilities/l1tf && echo "$cond: ENABLED (OK)") || echo "$cond: DISABLED (Recommendation: ENABLED)"; done

See more details here.

Apply Speculative Store Bypass (SSBD) mitigation

This will mitigate variants of Spectre side-channel issues such as Speculative Store Bypass and SpectreNG.

It can be enabled by adding the following Linux kernel boot parameter:

spec_store_bypass_disable=seccomp

which will apply SSB if seccomp is enabled by Firecracker's jailer.

Verification can be done by running:

cat /proc/$(pgrep firecracker | head -n1)/status | grep Speculation_Store_Bypass

Output shows one of the following:

  • vulnerable
  • not vulnerable
  • thread mitigated
  • thread force mitigated
  • globally mitigated

Use memory with Rowhammer mitigation support

Rowhammer is a memory side-channel issue that can lead to unauthorized cross- process memory changes.

Using DDR4 memory that supports Target Row Refresh (TRR) with error-correcting code (ECC) is recommended. Use of pseudo target row refresh (pTRR) for systems with pTRR-compliant DDR3 memory can help mitigate the issue, but it also incurs a performance penalty.

Disable swapping to disk or enable secure swap

Memory pressure on a host can cause memory to be written to drive storage when swapping is enabled. Disabling swap mitigates data remanence issues related to having guest memory contents on microVM storage devices.

Verify that swap is disabled by running:

grep -q "/dev" /proc/swaps && echo "swap partitions present (Recommendation: no swap)" || echo "no swap partitions (OK)"

Known kernel issues

General recommendation: Keep the host and the guest kernels up to date.

CVE-2019-3016

Description

In a Linux KVM guest that has PV TLB enabled, a process in the guest kernel may be able to read memory locations from another process in the same guest.

Impact

Under certain conditions the TLB will contain invalid entries. A malicious attacker running on the guest can get access to the memory of other running process on that guest.

Vulnerable systems

The vulnerability affects systems where all the following conditions are present:

  • the host kernel >= 4.10.
  • the guest kernel >= 4.16.
  • the KVM_FEATURE_PV_TLB_FLUSH is set in the CPUID of the guest. This is the EAX bit 9 in the KVM_CPUID_FEATURES (0x40000001) entry.

This can be checked by running

cpuid -r

and by searching for the entry corresponding to the leaf 0x40000001.

Example output:

0x40000001 0x00: eax=0x200 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
EAX 010004fb = 0010 0000 0000
EAX Bit 9: KVM_FEATURE_PV_TLB_FLUSH = 1
Mitigation

The vulnerability is fixed by the following host kernel patches.

The fix was integrated in the mainline kernel and in 4.19.103, 5.4.19, 5.5.3 stable kernel releases. Please follow kernel.org and once the fix is available in your stable release please update the host kernel. If you are not using a vanilla kernel, please check with Linux distro provider.