libcontainer: init: only pass stateDirFd when creating a container

If we pass a file descriptor to the host filesystem while joining a
container, there is a race condition where a process inside the
container can ptrace(2) the joining process and stop it from closing its
file descriptor to the stateDirFd. Then the process can access the
*host* filesystem from that file descriptor. This was fixed in part by
5d93fed3d2 ("Set init processes as non-dumpable"), but that fix is
more of a hail-mary than an actual fix for the underlying issue.

To fix this, don't open or pass the stateDirFd to the init process
unless we're creating a new container. A proper fix for this would be to
remove the need for even passing around directory file descriptors
(which are quite dangerous in the context of mount namespaces).

There is still an issue with containers that have CAP_SYS_PTRACE and are
using the setns(2)-style of joining a container namespace. Currently I'm
not really sure how to fix it without rampant layer violation.

Fixes: CVE-2016-9962
Fixes: 5d93fed3d2 ("Set init processes as non-dumpable")
Signed-off-by: Aleksa Sarai <asarai@suse.de>

libcontainer/process_linux.go

@@ -51,7 +51,6 @@ type setnsProcess struct {
 	fds           []string
 	process       *Process
 	bootstrapData io.Reader
-	rootDir       *os.File
 }
 
 func (p *setnsProcess) startTime() (string, error) {
@@ -70,7 +69,6 @@ func (p *setnsProcess) start() (err error) {
 	defer p.parentPipe.Close()
 	err = p.cmd.Start()
 	p.childPipe.Close()
-	p.rootDir.Close()
 	if err != nil {
 		return newSystemErrorWithCause(err, "starting setns process")
 	}