Ask for a token before spending the card (V-673)

mavgpud reverse-proxied every path to llama-server with no authentication on
a LAN port. Any client on the network could submit model work, hold the model
resident by touching the idle clock, and read /slots, which returns the
prompts of whoever else was using the card.

It now reads a bearer token from token_file and requires it on every request,
/health included: /health reports whether the card is loaded and free, which
is what someone deciding to take it would ask. A listen address reachable
from the network with no token is a startup failure rather than a downgrade
to loopback. homesrv is the client and it is on the LAN, so a loopback
default would look safe and take the model arm down.

Beyond the token: an allowlist of the five paths Maven calls, so a leaked
token buys the model API and not llama-server's admin surface; a body cap and
an in-flight cap on the proxy; and header and idle timeouts on the server.
No read or write timeout — a completion on this card legitimately takes
minutes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ESv8hqNPseYt1CnotZpqDz

cmd/mavgpud/auth.go

@@ -0,0 +1,145 @@
+package main
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"strings"
+)
+
+// The boundary in front of the card.
+//
+// mavgpud has to listen on the LAN, because homesrv is the client and a
+// loopback default takes the model arm down. That makes this the one hop on the
+// workstation anything on the network could reach, and until 2026-08-11 it
+// reverse-proxied every path to llama-server unauthenticated: any client could
+// spend the card, hold the model resident by touching the idle clock, and read
+// /slots, which carries the prompts of whoever else was using it.
+//
+// So: a bearer token every request must carry, read from a file, and a path
+// allowlist so a token that leaks buys the model API and not the admin one. The
+// CW2 transcriber beside this daemon has worked this way since it shipped; this
+// is the same arrangement, not a new one.
+
+// readToken loads the bearer token. The file holds the token and nothing else,
+// trailing newline allowed. A path that is set and unreadable is fatal to the
+// caller: a supervisor that silently ran without its boundary is the failure
+// this exists to prevent.
+func readToken(path string) (string, error) {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("token_file: %w", err)
+	}
+	tok := strings.TrimSpace(string(b))
+	if tok == "" {
+		return "", fmt.Errorf("token_file %s is empty", path)
+	}
+	return tok, nil
+}
+
+// loopbackListen reports whether addr can only be reached from this machine.
+// An empty or wildcard host is not loopback, which is the case that matters:
+// ":8080" is the shipped default and it answers the whole LAN.
+func loopbackListen(addr string) bool {
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		host = addr
+	}
+	host = strings.Trim(host, "[]")
+	if host == "" {
+		return false
+	}
+	if host == "localhost" {
+		return true
+	}
+	ip := net.ParseIP(host)
+	return ip != nil && ip.IsLoopback()
+}
+
+// allowed is what a token buys. Everything llama-server exposes beyond this is
+// refused, because the endpoints Maven does not call are the expensive ones to
+// hand out: /slots returns other callers' prompts, and its save/restore actions
+// write files chosen by the request.
+//
+// Adding a caller means adding its path here. That is deliberate — the list is
+// short because Maven's use of the workstation is.
+var allowed = map[string]string{
+	"/v1/chat/completions": http.MethodPost,
+	"/v1/completions":      http.MethodPost,
+	"/v1/embeddings":       http.MethodPost,
+	"/v1/models":           http.MethodGet,
+	"/props":               http.MethodGet,
+}
+
+// requireToken authenticates, then bounds. Order matters: an unauthenticated
+// client must not be able to make this daemon allocate a body buffer.
+//
+// /health is not exempt. It reports whether the card is loaded and free, which
+// is exactly what someone deciding whether to take it from him would ask.
+func requireToken(token string, maxBody int64, next http.Handler) http.Handler {
+	want := sha256.Sum256([]byte(token))
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		got := sha256.Sum256([]byte(bearer(r)))
+		if subtle.ConstantTimeCompare(got[:], want[:]) != 1 {
+			w.Header().Set("WWW-Authenticate", "Bearer")
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+		r.Body = http.MaxBytesReader(w, r.Body, maxBody)
+		next.ServeHTTP(w, r)
+	})
+}
+
+// bearer pulls the credential out of the header. A malformed header yields the
+// empty string, which fails the comparison like any other wrong token — there
+// is no separate error for it, because telling a caller *how* it was wrong is
+// the only thing a probe learns from a 401.
+func bearer(r *http.Request) string {
+	h := r.Header.Get("Authorization")
+	const prefix = "Bearer "
+	if len(h) <= len(prefix) || !strings.EqualFold(h[:len(prefix)], prefix) {
+		return ""
+	}
+	return strings.TrimSpace(h[len(prefix):])
+}
+
+// allowlist refuses a path the model arm does not use. It answers 404 rather
+// than 403 so a scan cannot map llama-server's surface through this hop.
+func allowlist(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		method, ok := allowed[r.URL.Path]
+		if !ok {
+			http.NotFound(w, r)
+			return
+		}
+		if r.Method != method {
+			w.Header().Set("Allow", method)
+			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+// limitInflight caps concurrent proxied requests. A waiter leaves when its own
+// context ends, so a client that gave up does not keep a slot: llama-server
+// runs with -np 1 and queueing here is cheaper than queueing inside the child
+// with a body held in memory on both sides.
+func limitInflight(n int, next http.Handler) http.Handler {
+	if n <= 0 {
+		return next
+	}
+	slots := make(chan struct{}, n)
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		select {
+		case slots <- struct{}{}:
+			defer func() { <-slots }()
+			next.ServeHTTP(w, r)
+		case <-r.Context().Done():
+			http.Error(w, "client went away", http.StatusServiceUnavailable)
+		}
+	})
+}

cmd/mavgpud/main.go

@@ -36,6 +36,21 @@ type config struct {
 	Listen    string `json:"listen"`     // what Maven talks to
 	LlamaAddr string `json:"llama_addr"` // where llama-server binds
 	LlamaBin  string `json:"llama_bin"`
+
+	// TokenFile holds the bearer token every request must carry. It is a path
+	// and never the token itself, the rule mavpoll, mavmaild and the CW2
+	// transcriber already follow: a secret in a committed config is a secret
+	// in the history. Empty is allowed only on a loopback Listen, and
+	// requireToken is where that is decided.
+	TokenFile string `json:"token_file,omitempty"`
+
+	// MaxBody bounds a proxied request body. A completion is a prompt, and a
+	// prompt that does not fit here would not fit the context window either.
+	MaxBody int64 `json:"max_body_bytes,omitempty"`
+	// MaxInflight bounds how many proxied requests reach llama-server at once.
+	// It runs with -np 1, so anything above a handful only queues inside the
+	// child while holding a connection and a body in memory here.
+	MaxInflight int `json:"max_inflight,omitempty"`
 	// LlamaArgs must include the flags that bind LlamaAddr. They are passed
 	// through untouched so the model, context size and layer count stay the
 	// owner's business and not this daemon's schema.
@@ -88,6 +103,8 @@ func defaults() config {
 		MinFreeVRAM: 15 << 30,
 		EvictAfter:  2,
 		StartAfter:  5,
+		MaxBody:     8 << 20,
+		MaxInflight: 4,
 	}
 }
 
@@ -123,6 +140,20 @@ func main() {
 		log.Fatal("mavgpud: llama_bin is required")
 	}
 
+	// A LAN listener with no token is refused rather than downgraded to
+	// loopback. Downgrading would look like a safe default and would take the
+	// model arm down instead: homesrv is the client and it is on the LAN.
+	var token string
+	if cfg.TokenFile != "" {
+		var err error
+		if token, err = readToken(cfg.TokenFile); err != nil {
+			log.Fatalf("mavgpud: %v", err)
+		}
+	} else if !loopbackListen(cfg.Listen) {
+		log.Fatalf("mavgpud: listen %s is reachable from the network and token_file is unset — "+
+			"set token_file, or listen on 127.0.0.1 and accept that Maven cannot reach it", cfg.Listen)
+	}
+
 	base := "http://" + cfg.LlamaAddr
 	run := newRunner("llama-server", cfg.LlamaBin, cfg.LlamaArgs, base+"/health")
 	sup := &supervisor{
@@ -145,7 +176,20 @@ func main() {
 	if err != nil {
 		log.Fatalf("mavgpud: llama_addr: %v", err)
 	}
-	srv := &http.Server{Addr: cfg.Listen, Handler: sup.handler(target)}
+	var h http.Handler = sup.handler(target)
+	if token != "" {
+		h = requireToken(token, cfg.MaxBody, h)
+	}
+	srv := &http.Server{
+		Addr:    cfg.Listen,
+		Handler: h,
+		// A slow-loris client holds a connection and a header buffer for free
+		// otherwise. No ReadTimeout or WriteTimeout: a completion legitimately
+		// takes minutes on this card, and either one would cut it off.
+		ReadHeaderTimeout: 10 * time.Second,
+		IdleTimeout:       60 * time.Second,
+		MaxHeaderBytes:    1 << 16,
+	}
 	go func() {
 		log.Printf("mavgpud: listening on %s, model %s", cfg.Listen, cfg.LlamaBin)
 		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
@@ -203,14 +247,14 @@ func (s *supervisor) handler(target *url.URL) http.Handler {
 		w.Header().Set("Content-Type", "application/json")
 		_, _ = w.Write([]byte(`{"status":"ok"}`))
 	})
-	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+	mux.Handle("/", allowlist(limitInflight(s.cfg.MaxInflight, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if !s.run.isReady() {
 			http.Error(w, "model not loaded", http.StatusServiceUnavailable)
 			return
 		}
 		s.touch()
 		proxy.ServeHTTP(w, r)
-	})
+	}))))
 	return mux
 }
 

deploy/mavgpud.json

@@ -1,5 +1,14 @@
 {
   "listen": ":8080",
+  "//token_file": [
+    "The bearer token every request must carry. homesrv is the client and it",
+    "is on the LAN, so this port cannot be loopback and the token is what",
+    "stops anything else on the network spending the card or reading /slots.",
+    "A path, never the token: mavgpud refuses to start when listen is",
+    "reachable from the network and this is unset.",
+    "Same value as MAVEN_GPU_TOKEN in homesrv's deploy/telegram.env."
+  ],
+  "token_file": "/home/kami/.config/mavgpud.token",
   "llama_addr": "127.0.0.1:10000",
   "llama_bin": "llama-server",
   "//llama_args": [