Drop symlinks in parent directories

Lstat only refrains from following the last element of a path, so a symlink
in an intermediate directory was still resolved, and reads could escape the
mount via e.g. resources.Get "symlinkdir/secret.txt".

Walk the directories up to the mount root and reject any that is a symlink.

Follow-up to cf9c8f93c and f8b5fa09a.

.gitignore

@@ -5,4 +5,5 @@ dist/
 public/
 .DS_Store 
 cache/filecache/_gen/
-.claude/
+.claude/
+dump.txt

hugofs/nosymlinks_fs.go

@@ -15,10 +15,37 @@ package hugofs
 
 import (
 	"os"
+	"path/filepath"
 
+	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/spf13/afero"
 )
 
+// hasSymlinkParent reports whether any directory between name and base is a symlink.
+// Lstat only refrains from following the last element of a path, so callers checking
+// name itself need this to keep a symlinked parent from escaping base.
+// base itself is not checked; an empty base walks to the root of fs.
+func hasSymlinkParent(fs afero.Fs, base, name string) (bool, error) {
+	for name != base {
+		parent := filepath.Dir(name)
+		if parent == name || parent == base || parent == "." {
+			return false, nil
+		}
+		fi, err := LstatIfPossible(fs, parent)
+		if err != nil {
+			if herrors.IsNotExist(err) {
+				return false, nil
+			}
+			return false, err
+		}
+		if fi.Mode()&os.ModeSymlink != 0 {
+			return true, nil
+		}
+		name = parent
+	}
+	return false, nil
+}
+
 // NewDropSymlinksFs returns an afero.Fs wrapper that treats symlinks as non-existing files.
 func NewDropSymlinksFs(base afero.Fs) *DropSymlinksFs {
 	return &DropSymlinksFs{base}
@@ -48,5 +75,12 @@ func (fs *DropSymlinksFs) Stat(name string) (os.FileInfo, error) {
 	if fi.Mode()&os.ModeSymlink != 0 {
 		return nil, os.ErrNotExist
 	}
+	symlinkParent, err := hasSymlinkParent(fs.Fs, "", name)
+	if err != nil {
+		return nil, err
+	}
+	if symlinkParent {
+		return nil, os.ErrNotExist
+	}
 	return fi, nil
 }

hugofs/rootmapping_fs.go

@@ -729,10 +729,18 @@ func (fs *RootMappingFs) statRoot(root *RootMapping, filename string) (FileMetaI
 		return nil, err
 	}
 
-	// Don't allow symlinks to escape the mount.
+	// Don't allow symlinks to escape the mount, neither as the file itself
+	// nor as any directory between it and the mount root.
 	if fi.Mode()&os.ModeSymlink != 0 {
 		return nil, os.ErrNotExist
 	}
+	symlinkParent, err := hasSymlinkParent(fs.Fs, root.To, filename)
+	if err != nil {
+		return nil, err
+	}
+	if symlinkParent {
+		return nil, os.ErrNotExist
+	}
 
 	var opener func() (afero.File, error)
 	if !fi.IsDir() {

testscripts/commands/no_symlinks.txt

@@ -18,6 +18,7 @@ stdout 'modstatictok'
 ! stdout 'modstaticsymlink'
 stdout pageok
 ! stdout pagesymlink
+! stdout rootdirfile
 
 -- hugo.toml --
 disableKinds = ["taxonomy", "term", "rss"]
@@ -30,15 +31,24 @@ Read me.
 {{ with resources.Get "modassetsymlink.txt"}}FAIL {{ .Publish }}{{ else }}OK{{ end }}
 {{ with resources.GetMatch "modassetsymlink.txt"}}FAIL {{ .Publish }}{{ else }}OK{{ end }}
 {{ with resources.GetMatch "myassets/symlinkdir/**"}}FAIL {{ .Publish }}{{ else }}OK{{ end }}
+{{ with resources.Get "myassets/symlinkdir/rootdirfile1.txt"}}FAIL {{ .Publish }}{{ else }}OK{{ end }}
 Page: {{ .RelPermalink }}|{{ .Content }}|
 
 {{/* os template package. */}}
 {{ $symFilePath := "content/pagesymlink.md" }} 
 
 {{ with os.ReadDir "assets/myassets/symlinkdir" }}FAIL {{ len . }}{{ else }}OK{{ end }}
-{{ with os.Stat $symFilePath }}FAIL{{ else }}OK{{ end }}
-{{ with os.ReadFile $symFilePath }}FAIL{{ else }}OK{{ end }}
-{{ with os.FileExists $symFilePath }}FAIL{{ else }}OK{{ end }}
+{{ template "check-os-path" $symFilePath }}
+{{ $symFilePath = "assets/myassets/symlinkdir/rootdirfile1.txt" }}
+{{ template "check-os-path" $symFilePath }}
+{{ $symFilePath = "assets/myassets/symlinkdir/subdir/rootdirfile3.txt" }}
+{{ template "check-os-path" $symFilePath }}
+
+{{ define "check-os-path" }}
+{{ with os.Stat . }}FAIL{{ else }}OK{{ end }}
+{{ with os.ReadFile . }}FAIL{{ else }}OK{{ end }}
+{{ with os.FileExists . }}FAIL{{ else }}OK{{ end }}
+{{ end }}
 
 -- content/pageok.md --
 -- themes/mytheme/assets/modassetok.txt --
@@ -53,5 +63,7 @@ My file.
 Rootdirfile1 content.
 -- rootdir/rootdirfile2.txt --
 Rootdirfile2 content.
+-- rootdir/subdir/rootdirfile3.txt --
+Rootdirfile3 content.