archive: resolve absolute symlinks and hard links within extraction root

archive.go

@@ -439,6 +439,82 @@ func (ta *tarAppender) addTarFile(srcPath, archivePath string) error {
 	return nil
 }
 
+// resolveArchivePath resolves intermediate symlinks in name using chroot-like
+// semantics when os.Root cannot traverse them. The final path component is
+// intentionally preserved because archive extraction may create or replace it.
+//
+// This is a compatibility workaround rather than the preferred long-term
+// implementation. It resolves the path separately before the actual operation,
+// so a concurrent filesystem change may cause the operation to affect a
+// different path within root. The subsequent os.Root operation still confines
+// the operation to root and prevents such a change from escaping it.
+//
+// Paths with missing components are supported. Existing symlinks are resolved,
+// and any remaining nonexistent components are retained for later creation.
+//
+// This helper should eventually be replaced by handle-relative resolution and
+// operations with resolve-in-root semantics, avoiding the resolution/use race
+// and repeated path traversal.
+func resolveArchivePath(root *os.Root, name string) (string, error) {
+	parent, base := filepath.Split(name)
+	if parent == "" {
+		return name, nil
+	}
+
+	parent = filepath.Clean(parent)
+
+	// Follow the final parent component: it is an intermediate component of name,
+	// and an absolute symlink there must trigger the resolve-in-root fallback.
+	_, statErr := root.Stat(parent)
+	switch {
+	case statErr == nil:
+		return name, nil
+	case !os.IsNotExist(statErr) && !isPathEscapes(statErr):
+		return "", statErr
+	}
+
+	// Resolve the parent both to handle ENOENT from missing components or dangling
+	// symlinks, and to determine whether an os.Root breakout was caused by an
+	// absolute symlink. Relative symlink escapes preserve the original Stat error.
+	resolved, err := resolveFSRootPath(root.Name(), parent)
+	if err != nil {
+		return "", err
+	}
+
+	if isPathEscapes(statErr) && (!resolved.followedAbsoluteLink || resolved.relativeEscapeBeforeAbsolute) {
+		return "", statErr
+	}
+
+	relParent, err := filepath.Rel(root.Name(), resolved.path)
+	if err != nil {
+		return "", breakoutError(fmt.Errorf(
+			"could not make resolved parent %q relative to root %q: %w",
+			resolved.path,
+			root.Name(),
+			err,
+		))
+	}
+	if relParent != "." && !filepath.IsLocal(relParent) {
+		return "", breakoutError(fmt.Errorf(
+			"resolved parent %q escapes root %q",
+			resolved.path,
+			root.Name(),
+		))
+	}
+
+	return filepath.Join(relParent, base), nil
+}
+
+// resolveHardlinkTarget validates a POSIX hardlink target and resolves it to
+// the native, root-relative filesystem path used for extraction.
+func resolveHardlinkTarget(root *os.Root, linkname string) (string, error) {
+	cleaned := path.Clean(linkname)
+	if cleaned == "." || !filepath.IsLocal(cleaned) {
+		return "", breakoutError(fmt.Errorf("invalid hardlink target %q", linkname))
+	}
+	return resolveArchivePath(root, filepath.FromSlash(cleaned))
+}
+
 // createTarFile extracts a single tar entry into the given root. dstPath is the
 // root-relative path of the entry being extracted, in native (host-separator)
 // form so it can be passed directly to os.Root methods and fsRootPath.
@@ -462,6 +538,15 @@ func createTarFile(root *os.Root, dstPath string, hdr *tar.Header, reader io.Rea
 	// so use hdrInfo.Mode() (they differ for e.g. setuid bits)
 	hdrInfo := hdr.FileInfo()
 
+	var hardlinkTarget string
+	if hdr.Typeflag == tar.TypeLink {
+		var err error
+		hardlinkTarget, err = resolveHardlinkTarget(root, hdr.Linkname)
+		if err != nil {
+			return err
+		}
+	}
+
 	switch hdr.Typeflag {
 	case tar.TypeDir:
 		// Create directory unless it already exists as one; merge in that case.
@@ -511,13 +596,7 @@ func createTarFile(root *os.Root, dstPath string, hdr *tar.Header, reader io.Rea
 		}
 
 	case tar.TypeLink:
-		// Defence in depth: root.Link's containment is limited when
-		// dest is a volume root.
-		linkname := path.Clean(hdr.Linkname)
-		if linkname == "." || !filepath.IsLocal(linkname) {
-			return breakoutError(fmt.Errorf("invalid hardlink target %q", hdr.Linkname))
-		}
-		if err := root.Link(filepath.FromSlash(linkname), dstPath); err != nil {
+		if err := root.Link(hardlinkTarget, dstPath); err != nil {
 			return err
 		}
 
@@ -593,7 +672,7 @@ func createTarFile(root *os.Root, dstPath string, hdr *tar.Header, reader io.Rea
 
 	// There is no LChmod, so ignore mode for symlink. Also, this
 	// must happen after chown, as that can modify the file mode
-	if err := handleLChmod(root, dstPath, hdr, hdrInfo); err != nil {
+	if err := handleLChmod(root, dstPath, hardlinkTarget, hdr, hdrInfo); err != nil {
 		return err
 	}
 
@@ -608,7 +687,7 @@ func createTarFile(root *os.Root, dstPath string, hdr *tar.Header, reader io.Rea
 		}
 	case tar.TypeLink:
 		// Follow the hardlink only when its target is not itself a symlink.
-		fi, err := root.Lstat(filepath.FromSlash(path.Clean(hdr.Linkname)))
+		fi, err := root.Lstat(hardlinkTarget)
 		if err == nil && fi.Mode()&os.ModeSymlink == 0 {
 			if err := chtimes(root, dstPath, aTime, mTime); err != nil {
 				return err
@@ -931,7 +1010,10 @@ loop:
 		// dstPath is the native (host-separator) form of the entry name,
 		// used at all filesystem boundaries (os.Root methods, fsRootPath).
 		// hdr.Name stays POSIX (forward-slash) for logical string checks.
-		dstPath := filepath.FromSlash(hdr.Name)
+		dstPath, err := resolveArchivePath(root, filepath.FromSlash(hdr.Name))
+		if err != nil {
+			return err
+		}
 
 		// If dstPath exists we almost always just want to remove and replace it.
 		// The only exception is when it is a directory *and* the file from
@@ -969,7 +1051,7 @@ loop:
 		//
 		// This must be done before whiteoutConverter.ConvertRead, which
 		// may set xattrs on the directory or create whiteout files.
-		if err := createImpliedDirectories(root, hdr, options); err != nil {
+		if err := createImpliedDirectories(root, dstPath, options); err != nil {
 			return err
 		}
 
@@ -1024,17 +1106,19 @@ func unrepresentableOnWindows(hdr *tar.Header) error {
 	return nil
 }
 
-// createImpliedDirectories will create all parent directories of the current path with default permissions, if they do
-// not already exist. This is possible as the tar format supports 'implicit' directories, where their existence is
-// defined by the paths of files in the tar, but there are no header entries for the directories themselves, and thus
-// we most both create them and choose metadata like permissions.
+// createImpliedDirectories creates all parent directories of dstPath with
+// default permissions if they do not already exist. This is necessary because
+// the tar format permits implicit directories whose existence is defined only
+// by file paths, without corresponding directory headers from which metadata
+// could be restored.
 //
-// The caller must have normalized hdr.Name (no leading ".." components).
-// All directory creation is performed via root so it is bounded within the
-// destination at the OS level (openat(2) semantics), preventing escape via
-// symlinks in the destination tree.
-func createImpliedDirectories(root *os.Root, hdr *tar.Header, options *TarOptions) error {
-	parent := filepath.FromSlash(path.Dir(strings.TrimSuffix(hdr.Name, "/")))
+// The caller must pass a normalized, root-relative local path. Any archive-path
+// conversion and resolve-in-root handling must already have been applied.
+// Directory creation is performed through root, so it remains confined to the
+// extraction destination even if the destination tree changes concurrently.
+func createImpliedDirectories(root *os.Root, dstPath string, options *TarOptions) error {
+	parent := filepath.Dir(dstPath)
+
 	// Skip when the parent is the root itself; nothing to create.
 	if parent == "." || parent == "" {
 		return nil

archive_unix.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"math"
 	"os"
-	"path"
 	"path/filepath"
 	"strings"
 	"syscall"
@@ -88,15 +87,15 @@ func handleTarTypeBlockCharFifo(root *os.Root, hdr *tar.Header, dstPath string)
 // handleLChmod applies the mode from hdrInfo to dstPath within root, skipping
 // symlinks (there is no lchmod). For hardlinks, the mode is applied only when
 // the link target is itself not a symlink.
-func handleLChmod(root *os.Root, dstPath string, hdr *tar.Header, hdrInfo os.FileInfo) error {
+func handleLChmod(root *os.Root, dstPath string, hardlinkTarget string, hdr *tar.Header, hdrInfo os.FileInfo) error {
 	switch hdr.Typeflag {
 	case tar.TypeSymlink:
 		return nil
 
 	case tar.TypeLink:
 		// If the target is a symlink, there is no way to chmod the hardlink
 		// without following it.
-		fi, err := root.Lstat(filepath.FromSlash(path.Clean(hdr.Linkname)))
+		fi, err := root.Lstat(hardlinkTarget)
 		if err != nil || fi.Mode()&os.ModeSymlink != 0 {
 			return nil
 		}