security: hardening по результатам аудита безопасности

pkg/adapters/crypto/composite/composite.go

@@ -74,6 +74,11 @@ var (
 	// ErrUnexpectedEOF indicates the chunk stream ended before any chunk
 	// with flags&0x01==1 (logical end-of-stream marker) was observed.
 	ErrUnexpectedEOF = errors.New("composite: unexpected end of stream")
+
+	// ErrPlaintextTooLarge indicates the decrypted payload would exceed the
+	// configured maximum plaintext size; decryption is aborted before the
+	// limit is crossed to prevent unbounded disk consumption.
+	ErrPlaintextTooLarge = errors.New("composite: plaintext size limit exceeded")
 )
 
 // Format constants.
@@ -89,6 +94,7 @@ const (
 	wrappedCekLen             int    = 48 // 32-byte CEK + 16-byte GCM tag
 	firstPayloadNonceLen      int    = 12
 	kekLen                    int    = 32
+	maxPlaintextSize          int64  = 1 << 40 // 1 TiB cap on decrypted output
 	infoPq                    string = "git.tswf.io/infra/go-synapse-backupper/v2/kek/pq"
 	infoComposite             string = "git.tswf.io/infra/go-synapse-backupper/v2/kek/composite"
 )
@@ -505,7 +511,24 @@ func (d *decryptor) Decrypt(
 		return err
 	}
 
-	return decryptChunks(src, plaintext, payloadGcm, firstPayloadNonce)
+	plaintextLimiter := &limitedWriter{writer: plaintext, remaining: maxPlaintextSize}
+	return decryptChunks(src, plaintextLimiter, payloadGcm, firstPayloadNonce)
+}
+
+// limitedWriter wraps an io.Writer and rejects writes that would exceed a
+// maximum byte budget.
+type limitedWriter struct {
+	writer    io.Writer
+	remaining int64
+}
+
+func (lw *limitedWriter) Write(p []byte) (int, error) {
+	if int64(len(p)) > lw.remaining {
+		return 0, ErrPlaintextTooLarge
+	}
+	n, err := lw.writer.Write(p)
+	lw.remaining -= int64(n)
+	return n, err
 }
 
 // decryptChunks reads and decrypts chunk records until a final chunk