libct: close rootFd ASAP in maskPaths

Close the root file descriptor immediately after use in maskPaths to
reduce the window during which an attacker could potentially exploit
an open fd to access or manipulate the root filesystem. This follows
the principle of least privilege and mitigates risks in compromised
or malicious container scenarios.

Co-authored-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: lifubang <lifubang@acmcoder.com>
(cherry picked from commit b88635e57e)
Signed-off-by: lifubang <lifubang@acmcoder.com>

libcontainer/rootfs_linux.go

@@ -1345,7 +1345,10 @@ func maskDir(path, mountLabel string) error {
 // mounts ( proc/kcore ).
 // For files, maskPath bind mounts /dev/null over the top of the specified path.
 // For directories, maskPath mounts read-only tmpfs over the top of the specified path.
-func maskPaths(rootFd *os.File, paths []string, mountLabel string) error {
+func maskPaths(rootFs string, paths []string, mountLabel string) error {
+	if len(paths) == 0 {
+		return nil
+	}
 	devNull, err := os.OpenFile("/dev/null", unix.O_PATH, 0)
 	if err != nil {
 		return fmt.Errorf("can't mask paths: %w", err)
@@ -1403,7 +1406,7 @@ func maskPaths(rootFd *os.File, paths []string, mountLabel string) error {
 					// resolves the underlying inode via procfs and re-opens it through
 					// rootFd, so the resulting fd is anchored to the real path inside the
 					// container rootfs even if path was a /proc/self/fd/N alias.
-					reopened, err := reopenAfterMount(rootFd, dstFh, unix.O_PATH|unix.O_CLOEXEC)
+					reopened, err := reopenAfterMount(rootFs, dstFh, unix.O_PATH|unix.O_CLOEXEC)
 					if err != nil {
 						return fmt.Errorf("can't reopen shared directory mask: %w", err)
 					}

libcontainer/standard_init_linux.go

@@ -141,17 +141,10 @@ func (l *linuxStandardInit) Init() error {
 		}
 	}
 
-	if len(l.config.Config.MaskPaths) > 0 {
-		rootFd, err := os.OpenFile("/", unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_PATH, 0)
-		if err != nil {
-			return fmt.Errorf("open rootfs handle for masked paths: %w", err)
-		}
-		err = maskPaths(rootFd, l.config.Config.MaskPaths, l.config.Config.MountLabel)
-		rootFd.Close()
-		if err != nil {
-			return err
-		}
+	if err := maskPaths("/", l.config.Config.MaskPaths, l.config.Config.MountLabel); err != nil {
+		return err
 	}
+
 	pdeath, err := system.GetParentDeathSignal()
 	if err != nil {
 		return fmt.Errorf("can't get pdeath signal: %w", err)