selfupdate: fix hash check trusting unsigned data in the signed SHA256SUMS GHSA-w33h-3qgq-8mv8 CVE-PENDING

rclone selfupdate checked the PGP signature on the clearsigned block
of SHA256SUMS but then looked up the archive hash in the whole
downloaded file. Unsigned data can appear in that file without
invalidating the signature. A hash line placed before or after the
signed text was accepted, allowing whoever controlled the download to
install an arbitrary unsigned archive.

The hash is now only looked up in the signed text.

Affects stable (non --beta) selfupdate since v1.55.0. --beta updates
are not signature checked.

cmd/selfupdate/verify.go

@@ -86,7 +86,8 @@ func verifyHashsumDownloaded(ctx context.Context, sumsBuf []byte, archive string
 		return fmt.Errorf("invalid hashsum signature: %w", err)
 	}
 
-	wantHash, err := findFileHash(sumsBuf, archive)
+	// Only read hashes from the the signed text
+	wantHash, err := findFileHash(block.Bytes, archive)
 	if err != nil {
 		return err
 	}

cmd/selfupdate/verify_test.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"encoding/hex"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -39,4 +40,22 @@ func TestVerify(t *testing.T) {
 		err = verifyHashsumDownloaded(ctx, sumsBuf, "archive.zipX", hash)
 		assert.ErrorContains(t, err, "unable to find hash")
 	})
+	t.Run("UnsignedPrefix", func(t *testing.T) {
+		hash[0] ^= 1 // forge an unsigned hash line before the signed block
+		prefixed := append([]byte(hex.EncodeToString(hash)+"  archive.zip\n"), sumsBuf...)
+		err = verifyHashsumDownloaded(ctx, prefixed, "archive.zip", hash)
+		assert.ErrorContains(t, err, "archive hash mismatch")
+		hash[0] ^= 1 // undo the change
+	})
+	t.Run("UnsignedInArmor", func(t *testing.T) {
+		// Dash escaping the signed line doesn't change the signed text but hides
+		// it from a raw scan, and lines after the armor checksum are ignored.
+		badHash := hex.EncodeToString([]byte{0x01})
+		armored := strings.Replace(string(sumsBuf), "b20b", "- b20b", 1)
+		armored = strings.Replace(armored, "=1GTr\n", "=1GTr\n"+badHash+"  archive.zip\n", 1)
+		err = verifyHashsumDownloaded(ctx, []byte(armored), "archive.zip", []byte{0x01})
+		assert.ErrorContains(t, err, "archive hash mismatch")
+		err = verifyHashsumDownloaded(ctx, []byte(armored), "archive.zip", hash)
+		require.NoError(t, err)
+	})
 }