tiktak: принимать legacy-токены для старых видео + fail-open в probe

MUMBLE "failed to get access using private key": чекер держит share-токен,
выданный ещё старым keygen, а после перехода на HMAC ValidateKey его
отвергал.

- keygen/legacy.go: восстановленный из keygen.a алгоритм
  seed=(seed*17+42)%62 принимается ТОЛЬКО для video.id <= cutoff, то есть
  для видео, существовавших на момент перехода. Выше cutoff -- лишь HMAC.
- cutoff берётся из max(video.id) при первом старте и пишется в
  public/.legacy_cutoff на volume: рестарт не должен расширять окно.
  Не смогли прочитать -- fail closed, legacy выключен.
- Когда старые флаги протухнут: echo 0 > public/.legacy_cutoff + рестарт,
  и legacy отключается полностью.

Ещё MUMBLE "cannot create video" -- он же DoS, у NOP-команды то же самое:
- checkGeometry сделан FAIL-OPEN. Неразобранный ffprobe больше не отклоняет
  загрузку, режем только успешно прочитанную и абсурдную геометрию.
- ffmpeg -max_alloc 128M: 1080p кадру нужно ~8 МБ, бомбе ~1 ГБ. Работает
  даже когда probe промолчал.

check_tiktak.sh: smoke-тест для vulnbox, гоняет сценарий чекера целиком
и отдельно проверяет, что патчи реально в задеплоенном билде.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/tiktak/db/db.go

@@ -138,3 +138,11 @@ func (s *Storage) AddAccess(ctx context.Context, id int, userId int64) error {
 		ExecContext(ctx)
 	return err
 }
+
+// MaxVideoID reports the highest video id currently stored, 0 when empty.
+func (s *Storage) MaxVideoID(ctx context.Context) (int64, error) {
+	sess := s.conn.NewSession(nil)
+	var id int64
+	err := sess.Select("COALESCE(MAX(id), 0)").From("video").LoadOneContext(ctx, &id)
+	return id, err
+}

services/tiktak/keygen/keygen.go

@@ -38,5 +38,10 @@ func GenerateKey(vid int) string {
 
 func ValidateKey(token string, vid int) bool {
 	// Constant-time: never leak how much of the token was correct.
-	return hmac.Equal([]byte(token), []byte(GenerateKey(vid)))
+	if hmac.Equal([]byte(token), []byte(GenerateKey(vid))) {
+		return true
+	}
+	// Grandfather in tokens handed out by the legacy algorithm, but only for
+	// videos that already existed when we switched. See legacy.go.
+	return legacyAccepted(token, vid)
 }

services/tiktak/keygen/legacy.go

@@ -0,0 +1,37 @@
+package keygen
+
+import "crypto/subtle"
+
+// Reconstructed from legacy/keygen.a (GenerateToken), kept ONLY for backwards
+// compatibility: the checker may still hold tokens it was handed before the
+// HMAC switch, and rejecting those costs SLA ("failed to get access using
+// private key").
+//
+//	int seed = n;
+//	for (i = 0; i < 30; i++) { seed = (seed*17 + 42) % 62; out[i] = alpha[seed]; }
+//
+// The state collapses into n%62 after the first round, so this yields only 62
+// distinct tokens overall -- which is exactly why it had to go.
+func legacyKey(vid int) string {
+	seed := vid
+	out := make([]byte, tokenLen)
+	for i := 0; i < tokenLen; i++ {
+		seed = (seed*17 + 42) % 62
+		out[i] = alphabet[seed]
+	}
+	return string(out)
+}
+
+// legacyCutoff is the highest video id that existed when the HMAC patch was
+// deployed. Videos above it were never shared with a legacy token, so they must
+// never accept one. 0 disables legacy acceptance entirely.
+var legacyCutoff int
+
+func SetLegacyCutoff(id int) { legacyCutoff = id }
+
+func legacyAccepted(token string, vid int) bool {
+	if legacyCutoff <= 0 || vid <= 0 || vid > legacyCutoff {
+		return false
+	}
+	return subtle.ConstantTimeCompare([]byte(token), []byte(legacyKey(vid))) == 1
+}

services/tiktak/keygen/legacy_test.go

@@ -0,0 +1,36 @@
+package keygen
+
+import "testing"
+
+func TestLegacyDualValidation(t *testing.T) {
+	// Reference values produced independently from the disassembly.
+	if got, want := legacyKey(1), "71npXhLZP5t3VJrvBfdFlRDNx9zHTb"; got != want {
+		t.Fatalf("legacyKey(1) = %q, want %q", got, want)
+	}
+	if legacyKey(1) != legacyKey(63) {
+		t.Fatal("legacy period should be 62")
+	}
+
+	SetLegacyCutoff(0)
+	if ValidateKey(legacyKey(5), 5) {
+		t.Fatal("cutoff 0 must reject every legacy token")
+	}
+
+	SetLegacyCutoff(100)
+	if !ValidateKey(legacyKey(5), 5) {
+		t.Fatal("old video must still accept its legacy token")
+	}
+	if ValidateKey(legacyKey(101), 101) {
+		t.Fatal("video created after the switch must NOT accept a legacy token")
+	}
+	if !ValidateKey(GenerateKey(101), 101) {
+		t.Fatal("new HMAC token must work above the cutoff")
+	}
+	if !ValidateKey(GenerateKey(5), 5) {
+		t.Fatal("new HMAC token must work below the cutoff too")
+	}
+	if ValidateKey(legacyKey(6), 5) || ValidateKey("garbage", 5) {
+		t.Fatal("wrong token accepted")
+	}
+	SetLegacyCutoff(0)
+}

services/tiktak/main.go

@@ -6,10 +6,14 @@ import (
 	_ "github.com/go-sql-driver/mysql"
 	"github.com/gocraft/dbr/v2"
 	"github.com/thanhpk/randstr"
+	"io/ioutil"
 	"net/http"
 	"os"
 	"os/signal"
+	"strconv"
+	"strings"
 	"tiktak/db"
+	"tiktak/keygen"
 	"tiktak/server"
 	"tiktak/server/auth"
 	"tiktak/video"
@@ -29,13 +33,21 @@ func main() {
 	}
 	defer conn.Close()
 
+	storage := db.New(conn)
+
 	auth.SetSalt(randstr.Hex(20))
+
+	// Videos that already existed when the HMAC keygen was deployed may still
+	// be shared with a legacy token the checker is holding. Freeze that cutoff
+	// on first boot and persist it, so a container restart cannot widen the
+	// window (a wider window means more videos accepting forgeable tokens).
+	keygen.SetLegacyCutoff(legacyCutoff(storage))
 	config := server.Config{
 		StaticFolder:    "public/static",
 		VttFolder:       "public/vtt",
 		TemplatesFolder: "templates",
 	}
-	srv := server.NewServer(db.New(conn), &video.Storage{Directory: "public/video"}, config)
+	srv := server.NewServer(storage, &video.Storage{Directory: "public/video"}, config)
 
 	// Start server
 	go func() {
@@ -54,3 +66,27 @@ func main() {
 		panic(err)
 	}
 }
+
+// legacyCutoffFile lives on the ./public volume so it survives restarts.
+const legacyCutoffFile = "public/.legacy_cutoff"
+
+func legacyCutoff(storage *db.Storage) int {
+	if b, err := ioutil.ReadFile(legacyCutoffFile); err == nil {
+		if n, err := strconv.Atoi(strings.TrimSpace(string(b))); err == nil {
+			fmt.Println("legacy token cutoff (pinned):", n)
+			return n
+		}
+	}
+
+	id, err := storage.MaxVideoID(context.Background())
+	if err != nil {
+		// Fail closed: no legacy tokens rather than an unbounded window.
+		fmt.Println("cannot read max video id, legacy tokens disabled:", err)
+		return 0
+	}
+	if err := ioutil.WriteFile(legacyCutoffFile, []byte(strconv.FormatInt(id, 10)), 0644); err != nil {
+		fmt.Println("cannot pin legacy cutoff:", err)
+	}
+	fmt.Println("legacy token cutoff (fresh):", id)
+	return int(id)
+}

services/tiktak/video/preview.go

@@ -25,7 +25,11 @@ func generatePreview(ctx context.Context, inp string, out string) error {
 	if err := checkGeometry(ctx, inp); err != nil {
 		return err
 	}
-	cmd := exec.CommandContext(ctx, "ffmpeg", "-y", "-threads", "1", "-i", inp, "-vframes", "1", out)
+	// -max_alloc caps a single ffmpeg allocation: a 1080p frame needs ~8 MB, a
+	// 16383x16383 bomb needs ~1 GB. Second line of defence behind checkGeometry,
+	// and it works even when the probe told us nothing.
+	cmd := exec.CommandContext(ctx, "ffmpeg", "-y", "-threads", "1",
+		"-max_alloc", "134217728", "-i", inp, "-vframes", "1", out)
 	s := strings.Builder{}
 	cmd.Stdout = &s
 	cmd.Stderr = &s

services/tiktak/video/probe.go

@@ -20,16 +20,24 @@ func checkGeometry(ctx context.Context, inp string) error {
 	args := []string{"-v", "error", "-select_streams", "v:0",
 		"-show_entries", "stream=width,height", "-of", "csv=s=x:p=0", inp}
 
+	// FAIL-OPEN on purpose. A probe that errors out or prints something we do
+	// not understand must NOT reject the upload: the checker's video would
+	// start failing with "cannot create video" and that costs more than the
+	// bomb does. Only a geometry we successfully read and that is absurd is
+	// rejected. Bombs need a real, huge, parseable resolution to be bombs.
 	out, err := exec.CommandContext(ctx, "ffprobe", args...).Output()
 	if err != nil {
-		return fmt.Errorf("cannot probe video: %v", err)
+		return nil
 	}
 
 	var w, h int
 	if _, err := fmt.Sscanf(strings.TrimSpace(string(out)), "%dx%d", &w, &h); err != nil {
-		return fmt.Errorf("cannot read video geometry")
+		return nil
 	}
-	if w <= 0 || h <= 0 || w > MaxDimension || h > MaxDimension || w*h > MaxPixels {
+	if w <= 0 || h <= 0 {
+		return nil
+	}
+	if w > MaxDimension || h > MaxDimension || w*h > MaxPixels {
 		return fmt.Errorf("video resolution %dx%d is not supported", w, h)
 	}
 	return nil