Only allow proc mount if it is procfs

Fixes #2128

This allows proc to be bind mounted for host and rootless namespace usecases but
it removes the ability to mount over the top of proc with a directory.

```bash
> sudo docker run --rm  apparmor
docker: Error response from daemon: OCI runtime create failed:
container_linux.go:346: starting container process caused "process_linux.go:449:
container init caused \"rootfs_linux.go:58: mounting
\\\"/var/lib/docker/volumes/aae28ea068c33d60e64d1a75916cf3ec2dc3634f97571854c9ed30c8401460c1/_data\\\"
to rootfs
\\\"/var/lib/docker/overlay2/a6be5ae911bf19f8eecb23a295dec85be9a8ee8da66e9fb55b47c841d1e381b7/merged\\\"
at \\\"/proc\\\" caused
\\\"\\\\\\\"/var/lib/docker/overlay2/a6be5ae911bf19f8eecb23a295dec85be9a8ee8da66e9fb55b47c841d1e381b7/merged/proc\\\\\\\"
cannot be mounted because it is not of type proc\\\"\"": unknown.

> sudo docker run --rm -v /proc:/proc apparmor

docker-default (enforce)        root     18989  0.9  0.0   1288     4 ?
Ss   16:47   0:00 sleep 20
```

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

libcontainer/container_linux.go

@@ -19,7 +19,7 @@ import (
 	"syscall" // only for SysProcAttr and Signal
 	"time"
 
-	"github.com/cyphar/filepath-securejoin"
+	securejoin "github.com/cyphar/filepath-securejoin"
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/intelrdt"
@@ -1176,7 +1176,7 @@ func (c *linuxContainer) makeCriuRestoreMountpoints(m *configs.Mount) error {
 		if err != nil {
 			return err
 		}
-		if err := checkMountDestination(c.config.Rootfs, dest); err != nil {
+		if err := checkProcMount(c.config.Rootfs, dest, ""); err != nil {
 			return err
 		}
 		m.Destination = dest

libcontainer/rootfs_linux.go

@@ -13,7 +13,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/cyphar/filepath-securejoin"
+	securejoin "github.com/cyphar/filepath-securejoin"
 	"github.com/mrunalp/fileutils"
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/configs"
@@ -197,7 +197,7 @@ func prepareBindMount(m *configs.Mount, rootfs string) error {
 	if dest, err = securejoin.SecureJoin(rootfs, m.Destination); err != nil {
 		return err
 	}
-	if err := checkMountDestination(rootfs, dest); err != nil {
+	if err := checkProcMount(rootfs, dest, m.Source); err != nil {
 		return err
 	}
 	// update the mount with the correct dest after symlinks are resolved.
@@ -414,7 +414,7 @@ func mountToRootfs(m *configs.Mount, rootfs, mountLabel string, enableCgroupns b
 		if dest, err = securejoin.SecureJoin(rootfs, m.Destination); err != nil {
 			return err
 		}
-		if err := checkMountDestination(rootfs, dest); err != nil {
+		if err := checkProcMount(rootfs, dest, m.Source); err != nil {
 			return err
 		}
 		// update the mount with the correct dest after symlinks are resolved.
@@ -461,12 +461,12 @@ func getCgroupMounts(m *configs.Mount) ([]*configs.Mount, error) {
 	return binds, nil
 }
 
-// checkMountDestination checks to ensure that the mount destination is not over the top of /proc.
+// checkProcMount checks to ensure that the mount destination is not over the top of /proc.
 // dest is required to be an abs path and have any symlinks resolved before calling this function.
-func checkMountDestination(rootfs, dest string) error {
-	invalidDestinations := []string{
-		"/proc",
-	}
+//
+// if source is nil, don't stat the filesystem.  This is used for restore of a checkpoint.
+func checkProcMount(rootfs, dest, source string) error {
+	const procPath = "/proc"
 	// White list, it should be sub directories of invalid destinations
 	validDestinations := []string{
 		// These entries can be bind mounted by files emulated by fuse,
@@ -489,16 +489,40 @@ func checkMountDestination(rootfs, dest string) error {
 			return nil
 		}
 	}
-	for _, invalid := range invalidDestinations {
-		path, err := filepath.Rel(filepath.Join(rootfs, invalid), dest)
+	path, err := filepath.Rel(filepath.Join(rootfs, procPath), dest)
+	if err != nil {
+		return err
+	}
+	// pass if the mount path is located outside of /proc
+	if strings.HasPrefix(path, "..") {
+		return nil
+	}
+	if path == "." {
+		// an empty source is pasted on restore
+		if source == "" {
+			return nil
+		}
+		// only allow a mount on-top of proc if it's source is "proc"
+		isproc, err := isProc(source)
 		if err != nil {
 			return err
 		}
-		if path != "." && !strings.HasPrefix(path, "..") {
-			return fmt.Errorf("%q cannot be mounted because it is located inside %q", dest, invalid)
+		// pass if the mount is happening on top of /proc and the source of
+		// the mount is a proc filesystem
+		if isproc {
+			return nil
 		}
+		return fmt.Errorf("%q cannot be mounted because it is not of type proc", dest)
 	}
-	return nil
+	return fmt.Errorf("%q cannot be mounted because it is inside /proc", dest)
+}
+
+func isProc(path string) (bool, error) {
+	var s unix.Statfs_t
+	if err := unix.Statfs(path, &s); err != nil {
+		return false, err
+	}
+	return s.Type == unix.PROC_SUPER_MAGIC, nil
 }
 
 func setupDevSymlinks(rootfs string) error {

libcontainer/rootfs_linux_test.go

@@ -10,7 +10,7 @@ import (
 
 func TestCheckMountDestOnProc(t *testing.T) {
 	dest := "/rootfs/proc/sys"
-	err := checkMountDestination("/rootfs", dest)
+	err := checkProcMount("/rootfs", dest, "")
 	if err == nil {
 		t.Fatal("destination inside proc should return an error")
 	}
@@ -18,7 +18,7 @@ func TestCheckMountDestOnProc(t *testing.T) {
 
 func TestCheckMountDestOnProcChroot(t *testing.T) {
 	dest := "/rootfs/proc/"
-	err := checkMountDestination("/rootfs", dest)
+	err := checkProcMount("/rootfs", dest, "/proc")
 	if err != nil {
 		t.Fatal("destination inside proc when using chroot should not return an error")
 	}
@@ -26,7 +26,7 @@ func TestCheckMountDestOnProcChroot(t *testing.T) {
 
 func TestCheckMountDestInSys(t *testing.T) {
 	dest := "/rootfs//sys/fs/cgroup"
-	err := checkMountDestination("/rootfs", dest)
+	err := checkProcMount("/rootfs", dest, "")
 	if err != nil {
 		t.Fatal("destination inside /sys should not return an error")
 	}
@@ -34,7 +34,7 @@ func TestCheckMountDestInSys(t *testing.T) {
 
 func TestCheckMountDestFalsePositive(t *testing.T) {
 	dest := "/rootfs/sysfiles/fs/cgroup"
-	err := checkMountDestination("/rootfs", dest)
+	err := checkProcMount("/rootfs", dest, "")
 	if err != nil {
 		t.Fatal(err)
 	}