geodata: validate entry and code tags before matching

common/geodata/geodat_loader.go

@@ -148,7 +148,8 @@ func seek(br *bufio.Reader, code []byte, size int64) (int, error) {
 	need := 2 + codeL // TODO: if code too long
 
 	for {
-		if _, err := br.ReadByte(); err != nil {
+		tag, err := br.ReadByte()
+		if err != nil {
 			return 0, err
 		}
 
@@ -170,7 +171,8 @@ func seek(br *bufio.Reader, code []byte, size int64) (int, error) {
 			}
 			return 0, err
 		}
-		if bodyL >= need && len(prefix) >= need && int(prefix[1]) == codeL && bytes.Equal(prefix[2:], code) {
+		if tag == 0x0a && bodyL >= need && len(prefix) >= need &&
+			prefix[0] == 0x0a && int(prefix[1]) == codeL && bytes.Equal(prefix[2:], code) {
 			return bodyL, nil
 		}
 		if _, err := br.Discard(bodyL); err != nil {

common/geodata/geodat_loader_test.go

@@ -1,6 +1,7 @@
 package geodata
 
 import (
+	"bytes"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -12,6 +13,19 @@ import (
 	"google.golang.org/protobuf/proto"
 )
 
+func TestFindChecksEntryAndCodeTags(t *testing.T) {
+	for _, payload := range [][]byte{
+		{0x12, 6, 0x0a, 4, 'T', 'E', 'S', 'T'}, // Unknown field posing as entry.
+		{0x08, 6, 0x0a, 4, 'T', 'E', 'S', 'T'}, // Wrong entry wire type.
+		{0x0a, 6, 0x1a, 4, 'T', 'E', 'S', 'T'}, // Unknown field posing as code.
+		{0x0a, 6, 0x08, 4, 'T', 'E', 'S', 'T'}, // Wrong code wire type.
+	} {
+		if _, err := find(bytes.NewReader(payload), []byte("TEST"), true, int64(len(payload))); err == nil {
+			t.Fatalf("matched incorrect entry/code tags in %x", payload)
+		}
+	}
+}
+
 func TestLoadFileRejectsVarintOverflow(t *testing.T) {
 	payload := []byte{
 		0x0a,