rootless: exec handle processes that create an user namespace

Manage the case where the main process of the container creates and
joins a new user namespace.

In this case we want to join only the first child in the new
hierarchy, which is the user namespace that was used to create the
container.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #1331
Approved by: rhatdan

2 files changed, 92 insertions, 0 deletions

diff --git a/pkg/rootless/rootless_linux.go b/pkg/rootless/rootless_linux.go
index 26f4b0b18..904d22ee2 100644
--- a/pkg/rootless/rootless_linux.go
+++ b/pkg/rootless/rootless_linux.go
@@ -9,9 +9,11 @@ import (
 	"os/exec"
 	gosignal "os/signal"
 	"os/user"
+	"path/filepath"
 	"runtime"
 	"strconv"
 	"syscall"
+	"unsafe"
 
 	"github.com/containers/storage/pkg/idtools"
 	"github.com/docker/docker/pkg/signal"
@@ -186,3 +188,86 @@ func BecomeRootInUserNS() (bool, int, error) {
 
 	return true, int(ret), nil
 }
+
+func readUserNs(path string) (string, error) {
+	b := make([]byte, 256)
+	_, err := syscall.Readlink(path, b)
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}
+
+func readUserNsFd(fd uintptr) (string, error) {
+	return readUserNs(filepath.Join("/proc/self/fd", fmt.Sprintf("%d", fd)))
+}
+
+func getParentUserNs(fd uintptr) (uintptr, error) {
+	const nsGetParent = 0xb702
+	ret, _, errno := syscall.Syscall(syscall.SYS_IOCTL, fd, uintptr(nsGetParent), 0)
+	if errno != 0 {
+		return 0, errno
+	}
+	return (uintptr)(unsafe.Pointer(ret)), nil
+}
+
+// GetUserNSForPid returns an open FD for the first direct child user namespace that created the process
+// Each container creates a new user namespace where the runtime runs.  The current process in the container
+// might have created new user namespaces that are child of the initial namespace we created.
+// This function finds the initial namespace created for the container that is a child of the current namespace.
+//
+//                                     current ns
+//                                       /     \
+//                           TARGET ->  a   [other containers]
+//                                     /
+//                                    b
+//                                   /
+//        NS READ USING THE PID ->  c
+func GetUserNSForPid(pid uint) (*os.File, error) {
+	currentNS, err := readUserNs("/proc/self/ns/user")
+	if err != nil {
+		return nil, err
+	}
+
+	path := filepath.Join("/proc", fmt.Sprintf("%d", pid), "ns/user")
+	u, err := os.Open(path)
+	if err != nil {
+		return nil, errors.Wrapf(err, "cannot open %s", path)
+	}
+
+	fd := u.Fd()
+	ns, err := readUserNsFd(fd)
+	if err != nil {
+		return nil, errors.Wrapf(err, "cannot read user namespace")
+	}
+	if ns == currentNS {
+		return nil, errors.New("process running in the same user namespace")
+	}
+
+	for {
+		nextFd, err := getParentUserNs(fd)
+		if err != nil {
+			return nil, errors.Wrapf(err, "cannot get parent user namespace")
+		}
+
+		ns, err = readUserNsFd(nextFd)
+		if err != nil {
+			return nil, errors.Wrapf(err, "cannot read user namespace")
+		}
+
+		if ns == currentNS {
+			syscall.Close(int(nextFd))
+
+			// Drop O_CLOEXEC for the fd.
+			_, _, errno := syscall.Syscall(syscall.SYS_FCNTL, fd, syscall.F_SETFD, 0)
+			if errno != 0 {
+				syscall.Close(int(fd))
+				return nil, errno
+			}
+
+			return os.NewFile(fd, "userns child"), nil
+		}
+		syscall.Close(int(fd))
+		fd = nextFd
+	}
+}
diff --git a/pkg/rootless/rootless_unsupported.go b/pkg/rootless/rootless_unsupported.go
index 11dfd5aa4..93b04adfd 100644
--- a/pkg/rootless/rootless_unsupported.go
+++ b/pkg/rootless/rootless_unsupported.go
@@ -3,6 +3,8 @@
 package rootless
 
 import (
+	"os"
+
 	"github.com/pkg/errors"
 )
 
@@ -30,3 +32,8 @@ func SetSkipStorageSetup(bool) {
 func SkipStorageSetup() bool {
 	return false
 }
+
+// GetUserNSForPid returns an open FD for the first direct child user namespace that created the process
+func GetUserNSForPid(pid uint) (*os.File, error) {
+	return nil, errors.New("this function is not supported on this os")
+}