Harden the host against a misbehaving Firecracker balloon (best-effort memory model)
domain: firecracker-microvm.github.io · 4 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Recognize the balloon is best-effort: the host must assume the Firecracker process may use all boot-allocated memory.
If the guest reports unexpected memory growth, decide on a response: migrate the VM to a higher-memory machine via snapshotting, kill the Firecracker process exceeding your limits, or enable swap with sufficient capacity.
Do not rely on balloon statistics as a security boundary — they come from the (untrusted) guest driver.
Rely on host-side enforcement (cgroups/memory limits, quotas) as the real guarantee that memory cannot leak between processes or out of the VM.
Known gotchas
Firecracker guarantees no memory leak BETWEEN processes or out of the VM even with a corrupt driver — this isolation comes from MAP_PRIVATE+MAP_ANONYMOUS guest memory and madvise(MADV_DONTNEED), not from balloon stats.
Migrate (snapshot) and kill are disruptive; enable swap before an incident rather than reactively.
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