bug(envd): PostInit reads request body with io.ReadAll and no size limit, enabling OOM via oversized payload

Summary

PostInit in packages/envd/internal/api/init.go reads the request body with an unbounded io.ReadAll:

// packages/envd/internal/api/init.go ~L141
body, err := io.ReadAll(r.Body)  // no MaxBytesReader
defer memguard.WipeBytes(body)

Any process inside the VM that can reach the envd HTTP port can send an arbitrarily large body, allocating heap memory until envd is OOM-killed.

Why it matters

envd is the sandbox control plane — it manages file I/O, process execution, cgroup freezing, NFS mounts, and live-upgrade handover. An OOM kill of envd:

• leaves all user processes running but orphaned (no envd to receive commands)
• prevents graceful sandbox teardown (cleanup callbacks, slot release)
• breaks any in-progress pause/resume sequence, potentially corrupting snapshot state

The /init endpoint is excluded from authExcludedPaths but does accept unauthenticated bodies — the auth check happens after the body is fully read. A guest process (e.g. user code running inside the sandbox) that discovers the envd port can therefore trigger OOM without any credentials.

Secondary issue: memguard.WipeBytes security value is reduced

memguard.WipeBytes(body) is deferred to scrub the access token from heap memory. But if body contains the token and is first replicated into a large allocation (e.g. a 500 MiB body), Go's allocator may have already copied the slice header or the GC may have paged parts to disk before the wipe runs. Capping the body to a size where the entire buffer fits comfortably in memory preserves the intent of the wipe.

Fix

Wrap r.Body with http.MaxBytesReader before reading. The largest legitimate /init payload contains EnvVars (many vars) and CaBundle (multiple PEM certs); 1 MiB is a generous upper bound that no real orchestrator payload will approach:

if r.Body != nil {
    // Cap body to 1 MiB. /init carries credentials and CA bundles but no
    // bulk data. An unbounded io.ReadAll lets a guest process OOM envd,
    // the sandbox control plane, by sending an oversized body.
    r.Body = http.MaxBytesReader(w, r.Body, 1<<20)
    body, err := io.ReadAll(r.Body)
    defer memguard.WipeBytes(body)
    if err != nil {
        var maxErr *http.MaxBytesError
        if errors.As(err, &maxErr) {
            logger.Error().Msg("request body exceeds 1 MiB limit")
            w.WriteHeader(http.StatusRequestEntityTooLarge)
        } else {
            logger.Error().Msgf("Failed to read request body: %v", err)
            w.WriteHeader(http.StatusBadRequest)
        }
        return
    }
    ...

No changes needed to imports (errors, io, net/http are all already imported).

Severity

Medium. The envd HTTP port is not externally exposed; exploitation requires code execution inside the VM. However sandboxes are explicitly designed to run untrusted LLM-generated code, so "arbitrary code inside the VM" is the normal threat model, not an unusual escalation.

Start in packages/envd/internal/api/init.go at PostInit and inspect how the /init request body is read before authentication. Confirm done when oversized bodies are rejected with the stated error response while legitimate EnvVars and CaBundle payloads still work, and the existing token-wiping behavior remains covered.