specconv: emit an error when using MS_PRIVATE with --no-pivot

Due to the semantics of chroot(2) when it comes to mount namespaces, it
is not generally safe to use MS_PRIVATE as a mount propgation when using
chroot(2). The reason for this is that this effectively results in a set
of mount references being held by the chroot'd namespace which the
namespace cannot free. pivot_root(2) does not have this issue because
the @old_root can be unmounted by the process.

Ultimately, --no-pivot is not really necessary anymore as a commonly
used option since f8e6b5a ("rootfs: make pivot_root not use a
temporary directory") resolved the read-only issue. But if someone
really needs to use it, MS_PRIVATE is never a good idea.

Signed-off-by: Aleksa Sarai <[email protected]>

Author: cyphar

libcontainer/specconv/spec_linux.go

@@ -203,6 +203,9 @@ func CreateLibcontainerConfig(opts *CreateOpts) (*configs.Config, error) {
 		if config.RootPropagation, exists = mountPropagationMapping[spec.Linux.RootfsPropagation]; !exists {
 			return nil, fmt.Errorf("rootfsPropagation=%v is not supported", spec.Linux.RootfsPropagation)
 		}
+		if config.NoPivotRoot && (config.RootPropagation&unix.MS_PRIVATE != 0) {
+			return nil, fmt.Errorf("rootfsPropagation of [r]private is not safe without pivot_root")
+		}
 
 		for _, ns := range spec.Linux.Namespaces {
 			t, exists := namespaceMapping[ns.Type]