local: fix panic on Range request past the end of a symlink GHSA-p6m2-r3w9-mpxw CVE-PENDING

With --links/-l, a symlink is served as a .rclonelink object whose
content is the target path. A Range request with a start offset beyond
the target length (e.g. "Range: bytes=99999999999-") reached
openTranslatedLink and sliced the target string at that offset, panicking
with "slice bounds out of range".

Clamp the offset to the target length so an out-of-range start reads
empty, matching how a real file read past EOF behaves.

backend/local/local.go

@@ -1423,6 +1423,13 @@ func (o *Object) openTranslatedLink(offset, limit int64) (lrc io.ReadCloser, err
 	if err != nil {
 		return nil, err
 	}
+	// Clamp offset into range to avoid panic
+	if offset < 0 {
+		offset = 0
+	}
+	if offset > int64(len(linkdst)) {
+		offset = int64(len(linkdst))
+	}
 	return readers.NewLimitedReadCloser(io.NopCloser(strings.NewReader(linkdst[offset:])), limit), nil
 }
 

backend/local/local_internal_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"math"
 	"os"
 	"path"
 	"path/filepath"
@@ -210,6 +211,32 @@ func TestSymlink(t *testing.T) {
 	require.NoError(t, in.Close())
 }
 
+// TestSymlinkRangeBeyondEnd checks range requests on a translated
+// symlink's target string don't panic.
+func TestSymlinkRangeBeyondEnd(t *testing.T) {
+	ctx := context.Background()
+	r := fstest.NewRun(t)
+	f := r.Flocal.(*Fs)
+	linksMode(f)
+
+	const target = "file.txt"
+	require.NoError(t, putLink(ctx, f, "symlink.txt", target))
+
+	o, err := f.NewObject(ctx, "symlink.txt"+fs.LinkSuffix)
+	require.NoError(t, err)
+
+	// An offset just past the end and a wildly large offset must both read
+	// empty rather than panicking.
+	for _, start := range []int64{int64(len(target)), int64(len(target)) + 1, math.MaxInt64} {
+		in, err := o.Open(ctx, &fs.RangeOption{Start: start, End: -1})
+		require.NoError(t, err)
+		contents, err := io.ReadAll(in)
+		require.NoError(t, err)
+		require.Empty(t, string(contents))
+		require.NoError(t, in.Close())
+	}
+}
+
 func TestSymlinkError(t *testing.T) {
 	m := configmap.Simple{
 		"links":      "true",