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Production Host Setup Recommendations

Firecracker relies on KVM and on the processor virtualization features for workload isolation. Security guarantees and defense in depth can only be upheld, if the following list of recommendations are implemented in production.

Firecracker Configuration

Seccomp

Firecracker uses seccomp filters to limit the system calls allowed by the host OS to the required minimum.

By default, Firecracker uses the most restrictive filters, which is the recommended option for production usage.

Production usage of the --seccomp-filter or --no-seccomp parameters is not recommended.

8250 Serial Device

Firecracker implements the 8250 serial device, which is visible from the guest side and is tied to the Firecracker/non-daemonized jailer process stdout. Without proper handling, because the guest has access to the serial device, this can lead to unbound memory or storage usage on the host side. Firecracker does not offer users the option to limit serial data transfer, nor does it impose any restrictions on stdout handling. Users are responsible for handling the memory and storage usage of the Firecracker process stdout. We suggest using any upper-bounded forms of storage, such as fixed-size or ring buffers, using programs like journald or logrotate, or redirecting to /dev/null or a named pipe. Furthermore, we do not recommend that users enable the serial device in production. To disable it in the guest kernel, use the 8250.nr_uarts=0 boot argument when configuring the boot source. Please be aware that the device can be reactivated from within the guest even if it was disabled at boot.

If Firecracker's stdout buffer is non-blocking and full (assuming it has a bounded size), any subsequent writes will fail, resulting in data loss, until the buffer is freed.

Log files

Firecracker outputs logging data into a named pipe, socket, or file using the path specified in the log_path field of logger configuration. Firecracker can generate log data as a result of guest operations and therefore the guest can influence the volume of data written in the logs. Users are responsible for consuming and storing this data safely. We suggest using any upper-bounded forms of storage, such as fixed-size or ring buffers, programs like journald or logrotate, or redirecting to a named pipe.

Logging and performance

We recommend adding quiet loglevel=1 to the host kernel command line to limit the number of messages written to the serial console. This is because some host configurations can have an effect on Firecracker's performance as the process will generate host kernel logs during normal operations.

The most recent example of this was the addition of console=ttyAMA0 host kernel command line argument on one of our testing setups. This enabled console logging, which degraded the snapshot restore time from 3ms to 8.5ms on aarch64. In this case, creating the tap device for snapshot restore generated host kernel logs, which were very slow to write.

Logging and signal handlers

Firecracker installs custom signal handlers for some of the POSIX signals, such as SIGSEGV, SIGSYS, etc.

The custom signal handlers used by Firecracker are not async-signal-safe, since they write logs and flush the metrics, which use locks for synchronization. While very unlikely, it is possible that the handler will intercept a signal on a thread which is already holding a lock to the log or metrics buffer. This can result in a deadlock, where the specific Firecracker thread becomes unresponsive.

While there is no security impact caused by the deadlock, we recommend that customers have an overwatcher process on the host, that periodically looks for Firecracker processes that are unresponsive, and kills them, by SIGKILL.

Jailer Configuration

For assuring secure isolation in production deployments, Firecracker should be started using the jailer binary that's part of each Firecracker release, or executed under process constraints equal or more restrictive than those in the jailer. For more about Firecracker sandboxing please see Firecracker design

The Jailer process applies cgroup, namespace 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.

Firecracker's customers are strongly advised to use the provided resource-limits and cgroup functionalities encapsulated within jailer, in order to control Firecracker's resource consumption in a way that makes the most sense to their specific workload. While aiming to provide as much control as possible, we cannot enforce aggressive default constraints resources such as memory or CPU because these are highly dependent on the workload type and usecase.

Here are some recommendations on how to limit the process's resources:

Disk

  • cgroup provides a Block IO Controller which allows users to control I/O operations through the following files:

    • blkio.throttle.io_serviced - bounds the number of I/Os issued to disk
    • blkio.throttle.io_service_bytes - sets a limit on the number of bytes transferred to/from the disk

  • Jailer's resource-limit provides control on the disk usage through:

    • fsize - limits the size in bytes for files created by the process
    • no-file - specifies a value greater than the maximum file descriptor number that can be opened by the process. If not specified, it defaults to 4096.

Memory

  • cgroup provides a Memory Resource Controller to allow setting upper limits to memory usage:
    • memory.limit_in_bytes - bounds the memory usage
    • memory.memsw.limit_in_bytes - limits the memory+swap usage
    • memory.soft_limit_in_bytes - enables flexible sharing of memory. Under normal circumstances, control groups are allowed to use as much of the memory as needed, constrained only by their hard limits set with the memory.limit_in_bytes parameter. However, when the system detects memory contention or low memory, control groups are forced to restrict their consumption to their soft limits.

vCPU

  • cgroup’s CPU Controller can guarantee a minimum number of CPU shares when a system is busy and provides CPU bandwidth control through:
    • cpu.shares - limits the amount of CPU that each group it is expected to get. The percentage of CPU assigned is the value of shares divided by the sum of all shares in all cgroups in the same level
    • cpu.cfs_period_us - bounds the duration in us of each scheduler period, for bandwidth decisions. This defaults to 100ms
    • cpu.cfs_quota_us - sets the maximum time in microseconds during each cfs_period_us for which the current group will be allowed to run
    • cpuacct.usage_percpu - limits the CPU time, in ns, consumed by the process in the group, separated by CPU

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

Host Security Configuration

Constrain CPU overhead caused by kvm-pit kernel threads

The current implementation results in host CPU usage increase on x86 CPUs when a guest injects timer interrupts with the help of kvm-pit kernel thread. kvm-pit kthread is by default part of the root cgroup.

To mitigate the CPU overhead we recommend two system level configurations.

  1. Use an external agent to move the kvm-pit/<pid of firecracker> kernel thread in the microVM’s cgroup (e.g., created by the Jailer). This cannot be done by Firecracker since the thread is created by the Linux kernel after guest start, at which point Firecracker is de-privileged.
  2. Configure the kvm limit to a lower value. This is a system-wide configuration available to users without Firecracker or Jailer changes. However, the same limit applies to APIC timer events, and users will need to test their workloads in order to apply this mitigation.

To modify the kvm limit for interrupts that can be injected in a second.

  1. sudo modprobe -r (kvm_intel|kvm_amd) kvm
  2. sudo modprobe kvm min_timer_period_us={new_value}
  3. sudo modprobe (kvm_intel|kvm_amd)

To have this change persistent across boots we can append the option to /etc/modprobe.d/kvm.conf:

echo "options kvm min_timer_period_us=" >> /etc/modprobe.d/kvm.conf

Mitigating Network flooding issues

Network can be flooded by creating connections and sending/receiving a significant amount of requests. This issue can be mitigated either by configuring rate limiters for the network interface as explained within Network Interface documentation, or by using one of the tools presented below:

  • tc qdisc - manipulate traffic control settings by configuring filters.

When traffic enters a classful qdisc, the filters are consulted and the packet is enqueued into one of the classes within. Besides containing other qdiscs, most classful qdiscs perform rate control.

  • netnamespace and iptables
    • --pid-owner - can be used to match packets based on the PID that was responsible for them
    • connlimit - restricts the number of connections for a destination IP address/from a source IP address, as well as limit the bandwidth

Mitigating Noisy-Neighbour Storage Device Contention

Data written to storage devices is managed in Linux with a page cache. Updates to these pages are written through to their mapped storage devices asynchronously at the host operating system's discretion. As a result, high storage output can result in this cache being filled quickly resulting in a backlog which can slow down I/O of other guests on the host.

To protect the resource access of the guests, make sure to tune each Firecracker process via the following tools:

  • Jailer: A wrapper environment designed to contain Firecracker and strictly control what the process and its guest has access to. Take note of the jailer operations guide, paying particular note to the --resource-limit parameter.
  • Rate limiting: Rate limiting functionality is supported for both networking and storage devices and is configured by the operator of the environment that launches the Firecracker process and its associated guest. See the block device documentation for examples of calling the API to configure rate limiting.

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.

Some of the mitigations are platform specific. When applicable, this information will be specified between brackets.

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$" /sys/devices/system/cpu/smt/control && \
echo "Hyperthreading: DISABLED (OK)") || \
(grep -q "^notsupported$\|^notimplemented$" \
/sys/devices/system/cpu/smt/control && \
echo "Hyperthreading: Not Supported (OK)") || \
echo "Hyperthreading: ENABLED (Recommendation: DISABLED)"

Note There are some newer aarch64 CPUs that also implement SMT, however AWS Graviton processors do not implement it.

Disable Unprivileged BPF

Disabling BPF for unprivileged users protects against various transient execution attacks. See "Spectre revisits BPF" for one synopsis.

Ensure unprivileged BPF remains disabled by setting unprivileged_bpf_disable to 1 as documented in the kernel admin guide:

echo "kernel.unprivileged_bpf_disabled = 1" >> /etc/sysctl.conf

The default setting is determined by the Linux kernel config BPF_UNPRIV_DEFAULT_OFF which defaults to Y starting with this commit first picked up in version 5.16. Amazon Linux kernel configs all use BPF_UNPRIV_DEFAULT_OFF=Y.

Verification can be done by running:

cat /proc/sys/kernel/unprivileged_bpf_disabled

The output should be 1

[Intel and ARM only] 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$" /sys/devices/system/cpu/vulnerabilities/meltdown \
&& echo "KPTI: SUPPORTED (OK)") || \
(grep -q "^Not affected$" /sys/devices/system/cpu/vulnerabilities/meltdown \
&& echo "KPTI: Not Affected (OK)") || \
echo "KPTI: NOT SUPPORTED (Recommendation: SUPPORTED)"

A full list of the ARM processors that are vulnerable to side-channel attacks and the mechanisms of these attacks can be found here. KPTI is implemented for ARM in version 4.16 and later of the Linux kernel.

Note Graviton-enabled hardware is not affected by this.

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 mitigations against Spectre Side Channels

In development we use an integration test to check for spectre vulnerability on the host.

The script we run in this test can be downloaded and executed like:

# Read https://meltdown.ovh before running it.
wget -O - https://meltdown.ovh | bash
Branch Target Injection mitigation (Spectre V2, including Spectre-BHB)
Intel and AMD

We recommend using a kernel compiled with eIBRS or IBRS, together with microcode supporting conditional Indirect Branch Prediction Barriers (IBPB).

Verification can be done by running:

cat /sys/devices/system/cpu/vulnerabilities/spectre_v2

The output should mention the following mitigations being in use:

  • One of Retpolines (pre-Skylake CPU), IBRS (Skylake), or Enhanced IBRS (Cascade Lake and later)
  • IBPB at least conditional
ARM64

We recommend using a kernel compiled with MITIGATE_SPECTRE_BRANCH_HISTORY.

More information on the processors vulnerable to this type of attack and detailed information on the mitigations can be found in the ARM security documentation.

Verification can be done by running:

grep -q "^(Mitigation: CSV2, BHB|Not affected)$" \
/sys/devices/system/cpu/vulnerabilities/spectre_v2 && \
echo "SPECTRE V2 -> OK" || echo "SPECTRE V2 -> NOT OK"
Bounds Check Bypass Store (Spectre V1)

Verification for mitigation against Spectre V1 can be done:

grep -q "^(Mitigation:|Not affected)$" \
/sys/devices/system/cpu/vulnerabilities/spectre_v1 && \
echo "SPECTRE V1 -> OK" || echo "SPECTRE V1 -> NOT OK"
[Intel only] 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 (Spectre v4) and SpectreNG.

We recommend applying SSBD to Firecracker and the host kernel.

X86_64

On x86_64 systems, this can be done using the following kernel cmdline parameter:

spec_store_bypass_disable=on

Unfortunately, this applies SSBD to all the other processes running on the host as well.

ARM64

On aarch64 systems, SSBD can be applied to the kernel by using the following kernel cmdline parameter:

ssbd=kernel

SSBD is applied to Firecracker by using the prctl interface. However, this is only available on host kernels Linux >=4.17 and also Amazon Linux 4.14. Alternatively, a global mitigation can be enabled by adding the following Linux kernel cmdline parameter:

ssbd=force-on

The following command can be used to check if SSBD is applied to Firecracker:

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
Hardening other processes

For any process running on the host that communicates with Firecracker and handles sensitive data, we recommend hardening it against spectre-like attacks by:

  • compiling it with speculative load hardening
  • compiling it with retpolines
  • applying SSBD to it

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.

CVE-2022-1789

Description

With shadow paging enabled, the INVPCID instruction results in a call to kvm_mmu_invpcid_gva. If INVPCID is executed with CR0.PG=0, the invlpg callback is not set and the result is a NULL pointer dereference.

Impact

A malicious attacker running on the guest can cause a DoS (Denial of Service).

Vulnerable systems

The vulnerability affects systems that have shadow paging enabled and use the following host kernel versions:

  • 5.10.x prior to 5.10.119
  • 5.15.x prior to 5.15.44
  • 5.17.x prior to 5.17.12

Systems that use extended page table are not susceptible to this attack. To verify that extended page table is enabled, run the following command:

cat /sys/module/kvm_intel/parameters/ept

If the output is Y then KVM uses extended page table, otherwise if N then KVM uses shadow pages.

Mitigation

The vulnerability is fixed by this commit. The fix was integrated in 5.10.119, 5.15.44 and 5.17.12 kernel releases.

CVE-2022-26373

Description

Isolation boundaries between processes are vulnerable to a return stack buffer underflow. This may result in some processors allowing neighbouring guests to access data in other processes via local access.

This issue is not impacted by environments that make use of RETPOLINE as this results in RSB stuffing implemented by KVM which Firecracker uses exclusively.

Impact

A malicious attacker running on a guest can access information in other guests running on the same host.

Vulnerable systems

The vulnerability affects systems that do not have RETPOLINE enabled and use the following host kernel versions:

  • 5.10.x prior to 5.10.135
  • 5.15.x prior to 5.15.57

See earlier in this document for checking RETPOLINE configuration. You can check the version of the kernel being used with:

uname -r
Mitigation

The vulnerability is fixed in these releases by the commits merged upstream.

[ARM only] Physical counter directly passed through to the guest

On ARM, the physical counter (i.e CNTPCT) it is returning the actual EL1 physical counter value of the host. From the discussions before merging this change upstream, this seems like a conscious design decision of the ARM code contributors, giving precedence to performance over the ability to trap and control this in the hypervisor.