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Mount /proc and /sys read-only, except in privileged containers.

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It has been pointed out that some files in /proc and /sys can be used
to break out of containers. However, if those filesystems are mounted
read-only, most of the known exploits are mitigated, since they rely
on writing some file in those filesystems.

This does not replace security modules (like SELinux or AppArmor), it
is just another layer of security. Likewise, it doesn't mean that the
other mitigations (shadowing parts of /proc or /sys with bind mounts)
are useless. Those measures are still useful. As such, the shadowing
of /proc/kcore is still enabled with both LXC and native drivers.

Special care has to be taken with /proc/1/attr, which still needs to
be mounted read-write in order to enable the AppArmor profile. It is
bind-mounted from a private read-write mount of procfs.

All that enforcement is done in dockerinit. The code doing the real
work is in libcontainer. The init function for the LXC driver calls
the function from libcontainer to avoid code duplication.

Docker-DCO-1.1-Signed-off-by: Jérôme Petazzoni <jerome@docker.com> (github: jpetazzo)
This commit is contained in:
Jérôme Petazzoni 2014-04-30 18:00:42 -07:00 committed by Michael Crosby
parent 559dc9a66a
commit 1c4202a614
7 changed files with 113 additions and 67 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
daemon/execdriver/lxc/driver.go
View file

@ -5,6 +5,7 @@ import (
"github.com/dotcloud/docker/daemon/execdriver"
"github.com/dotcloud/docker/pkg/cgroups"
"github.com/dotcloud/docker/pkg/label"
"github.com/dotcloud/docker/pkg/libcontainer/security/restrict"
"github.com/dotcloud/docker/pkg/system"
"github.com/dotcloud/docker/utils"
"io/ioutil"
@ -35,6 +36,10 @@ func init() {
return err
}
if err := restrict.Restrict("/", "/empty"); err != nil {
return err
}
if err := setupCapabilities(args); err != nil {
return err
}

23
daemon/execdriver/lxc/lxc_template.go
View file

@ -82,15 +82,12 @@ lxc.pivotdir = lxc_putold
# NOTICE: These mounts must be applied within the namespace
# WARNING: procfs is a known attack vector and should probably be disabled
# if your userspace allows it. eg. see http://blog.zx2c4.com/749
# WARNING: mounting procfs and/or sysfs read-write is a known attack vector.
# See e.g. http://blog.zx2c4.com/749 and http://bit.ly/T9CkqJ
# We mount them read-write here, but later, dockerinit will call the Restrict() function to remount them read-only.
# We cannot mount them directly read-only, because that would prevent loading AppArmor profiles.
lxc.mount.entry = proc {{escapeFstabSpaces $ROOTFS}}/proc proc nosuid,nodev,noexec 0 0
# WARNING: sysfs is a known attack vector and should probably be disabled
# if your userspace allows it. eg. see http://bit.ly/T9CkqJ
{{if .Privileged}}
lxc.mount.entry = sysfs {{escapeFstabSpaces $ROOTFS}}/sys sysfs nosuid,nodev,noexec 0 0
{{end}}
{{if .Tty}}
lxc.mount.entry = {{.Console}} {{escapeFstabSpaces $ROOTFS}}/dev/console none bind,rw 0 0
@ -111,14 +108,14 @@ lxc.mount.entry = {{$value.Source}} {{escapeFstabSpaces $ROOTFS}}/{{escapeFstabS
{{if .AppArmor}}
lxc.aa_profile = unconfined
{{else}}
# not unconfined
# Let AppArmor normal confinement take place (i.e., not unconfined)
{{end}}
{{else}}
# restrict access to proc
lxc.mount.entry = {{.RestrictionSource}} {{escapeFstabSpaces $ROOTFS}}/proc/sys none bind,ro 0 0
lxc.mount.entry = {{.RestrictionSource}} {{escapeFstabSpaces $ROOTFS}}/proc/irq none bind,ro 0 0
lxc.mount.entry = {{.RestrictionSource}} {{escapeFstabSpaces $ROOTFS}}/proc/acpi none bind,ro 0 0
lxc.mount.entry = {{escapeFstabSpaces $ROOTFS}}/dev/null {{escapeFstabSpaces $ROOTFS}}/proc/sysrq-trigger none bind,ro 0 0
# Restrict access to some stuff in /proc. Note that /proc is already mounted
# read-only, so we don't need to bother about things that are just dangerous
# to write to (like sysrq-trigger). Also, recent kernels won't let a container
# peek into /proc/kcore, but let's cater for people who might run Docker on
# older kernels. Just in case.
lxc.mount.entry = {{escapeFstabSpaces $ROOTFS}}/dev/null {{escapeFstabSpaces $ROOTFS}}/proc/kcore none bind,ro 0 0
{{end}}

2
daemon/execdriver/native/create.go
View file

@ -84,8 +84,6 @@ func (d *driver) setPrivileged(container *libcontainer.Container) error {
}
container.Cgroups.DeviceAccess = true
// add sysfs as a mount for privileged containers
container.Mounts = append(container.Mounts, libcontainer.Mount{Type: "sysfs"})
delete(container.Context, "restriction_path")
if apparmor.IsEnabled() {

38
integration-cli/docker_cli_run_test.go
View file

@ -725,24 +725,46 @@ func TestUnPrivilegedCannotMount(t *testing.T) {
logDone("run - test un-privileged cannot mount")
}
func TestSysNotAvaliableInNonPrivilegedContainers(t *testing.T) {
cmd := exec.Command(dockerBinary, "run", "busybox", "ls", "/sys/kernel")
func TestSysNotWritableInNonPrivilegedContainers(t *testing.T) {
cmd := exec.Command(dockerBinary, "run", "busybox", "touch", "/sys/kernel/profiling")
if code, err := runCommand(cmd); err == nil || code == 0 {
t.Fatal("sys should not be available in a non privileged container")
t.Fatal("sys should not be writable in a non privileged container")
}
deleteAllContainers()
logDone("run - sys not avaliable in non privileged container")
logDone("run - sys not writable in non privileged container")
}
func TestSysAvaliableInPrivilegedContainers(t *testing.T) {
cmd := exec.Command(dockerBinary, "run", "--privileged", "busybox", "ls", "/sys/kernel")
func TestSysWritableInPrivilegedContainers(t *testing.T) {
cmd := exec.Command(dockerBinary, "run", "--privileged", "busybox", "touch", "/sys/kernel/profiling")
if code, err := runCommand(cmd); err != nil || code != 0 {
t.Fatalf("sys should be available in privileged container")
t.Fatalf("sys should be writable in privileged container")
}
deleteAllContainers()
logDone("run - sys avaliable in privileged container")
logDone("run - sys writable in privileged container")
}
func TestProcNotWritableInNonPrivilegedContainers(t *testing.T) {
cmd := exec.Command(dockerBinary, "run", "busybox", "touch", "/proc/sysrq-trigger")
if code, err := runCommand(cmd); err == nil || code == 0 {
t.Fatal("proc should not be writable in a non privileged container")
}
deleteAllContainers()
logDone("run - proc not writable in non privileged container")
}
func TestProcWritableInPrivilegedContainers(t *testing.T) {
cmd := exec.Command(dockerBinary, "run", "--privileged", "busybox", "touch", "/proc/sysrq-trigger")
if code, err := runCommand(cmd); err != nil || code != 0 {
t.Fatalf("proc should be writable in privileged container")
}
deleteAllContainers()
logDone("run - proc writable in privileged container")
}

12
pkg/libcontainer/mount/init.go
View file

@ -11,7 +11,6 @@ import (
"github.com/dotcloud/docker/pkg/label"
"github.com/dotcloud/docker/pkg/libcontainer"
"github.com/dotcloud/docker/pkg/libcontainer/mount/nodes"
"github.com/dotcloud/docker/pkg/libcontainer/security/restrict"
"github.com/dotcloud/docker/pkg/system"
)
@ -51,11 +50,6 @@ func InitializeMountNamespace(rootfs, console string, container *libcontainer.Co
if err := nodes.CopyN(rootfs, nodes.DefaultNodes); err != nil {
return fmt.Errorf("copy dev nodes %s", err)
}
if restrictionPath := container.Context["restriction_path"]; restrictionPath != "" {
if err := restrict.Restrict(rootfs, restrictionPath); err != nil {
return fmt.Errorf("restrict %s", err)
}
}
if err := SetupPtmx(rootfs, console, container.Context["mount_label"]); err != nil {
return err
}
@ -124,10 +118,11 @@ func setupBindmounts(rootfs string, bindMounts libcontainer.Mounts) error {
}
// TODO: this is crappy right now and should be cleaned up with a better way of handling system and
// standard bind mounts allowing them to be more dymanic
// standard bind mounts allowing them to be more dynamic
func newSystemMounts(rootfs, mountLabel string, mounts libcontainer.Mounts) []mount {
systemMounts := []mount{
{source: "proc", path: filepath.Join(rootfs, "proc"), device: "proc", flags: defaultMountFlags},
{source: "sysfs", path: filepath.Join(rootfs, "sys"), device: "sysfs", flags: defaultMountFlags},
}
if len(mounts.OfType("devtmpfs")) == 1 {
@ -138,8 +133,5 @@ func newSystemMounts(rootfs, mountLabel string, mounts libcontainer.Mounts) []mo
mount{source: "devpts", path: filepath.Join(rootfs, "dev", "pts"), device: "devpts", flags: syscall.MS_NOSUID | syscall.MS_NOEXEC, data: label.FormatMountLabel("newinstance,ptmxmode=0666,mode=620,gid=5", mountLabel)},
)
if len(mounts.OfType("sysfs")) == 1 {
systemMounts = append(systemMounts, mount{source: "sysfs", path: filepath.Join(rootfs, "sys"), device: "sysfs", flags: defaultMountFlags})
}
return systemMounts
}

14
pkg/libcontainer/nsinit/init.go
View file

@ -16,6 +16,7 @@ import (
"github.com/dotcloud/docker/pkg/libcontainer/mount"
"github.com/dotcloud/docker/pkg/libcontainer/network"
"github.com/dotcloud/docker/pkg/libcontainer/security/capabilities"
"github.com/dotcloud/docker/pkg/libcontainer/security/restrict"
"github.com/dotcloud/docker/pkg/libcontainer/utils"
"github.com/dotcloud/docker/pkg/system"
"github.com/dotcloud/docker/pkg/user"
@ -68,18 +69,25 @@ func Init(container *libcontainer.Container, uncleanRootfs, consolePath string,
if err := system.Sethostname(container.Hostname); err != nil {
return fmt.Errorf("sethostname %s", err)
}
if err := FinalizeNamespace(container); err != nil {
return fmt.Errorf("finalize namespace %s", err)
}
runtime.LockOSThread()
if restrictionPath := container.Context["restriction_path"]; restrictionPath != "" {
if err := restrict.Restrict("/", restrictionPath); err != nil {
return err
}
}
if err := apparmor.ApplyProfile(os.Getpid(), container.Context["apparmor_profile"]); err != nil {
return err
}
if err := label.SetProcessLabel(container.Context["process_label"]); err != nil {
return fmt.Errorf("set process label %s", err)
}
if err := FinalizeNamespace(container); err != nil {
return fmt.Errorf("finalize namespace %s", err)
}
return system.Execv(args[0], args[0:], container.Env)
}

82
pkg/libcontainer/security/restrict/restrict.go
View file

@ -9,43 +9,67 @@ import (
"github.com/dotcloud/docker/pkg/system"
)
const flags = syscall.MS_BIND | syscall.MS_REC | syscall.MS_RDONLY
var restrictions = map[string]string{
// dirs
"/proc/sys": "",
"/proc/irq": "",
"/proc/acpi": "",
// files
"/proc/sysrq-trigger": "/dev/null",
"/proc/kcore": "/dev/null",
// "restrictions" are container paths (files, directories, whatever) that have to be masked.
// maskPath is a "safe" path to be mounted over maskedPath. It can take two special values:
// - if it is "", then nothing is mounted;
// - if it is "EMPTY", then an empty directory is mounted instead.
// If remountRO is true then the maskedPath is remounted read-only (regardless of whether a maskPath was used).
type restriction struct {
maskedPath string
maskPath string
remountRO bool
}
// Restrict locks down access to many areas of proc
// by using the asumption that the user does not have mount caps to
// revert the changes made here
func Restrict(rootfs, empty string) error {
for dest, source := range restrictions {
dest = filepath.Join(rootfs, dest)
var restrictions = []restriction{
{"/proc", "", true},
{"/sys", "", true},
{"/proc/kcore", "/dev/null", false},
}
// we don't have a "/dev/null" for dirs so have the requester pass a dir
// for us to bind mount
switch source {
case "":
// This has to be called while the container still has CAP_SYS_ADMIN (to be able to perform mounts).
// However, afterwards, CAP_SYS_ADMIN should be dropped (otherwise the user will be able to revert those changes).
// "empty" should be the path to an empty directory.
func Restrict(rootfs, empty string) error {
for _, restriction := range restrictions {
dest := filepath.Join(rootfs, restriction.maskedPath)
if restriction.maskPath != "" {
var source string
if restriction.maskPath == "EMPTY" {
source = empty
default:
source = filepath.Join(rootfs, source)
} else {
source = filepath.Join(rootfs, restriction.maskPath)
}
if err := system.Mount(source, dest, "bind", flags, ""); err != nil {
if os.IsNotExist(err) {
continue
if err := system.Mount(source, dest, "", syscall.MS_BIND, ""); err != nil {
return fmt.Errorf("unable to bind-mount %s over %s: %s", source, dest, err)
}
return fmt.Errorf("unable to mount %s over %s %s", source, dest, err)
}
if err := system.Mount("", dest, "bind", flags|syscall.MS_REMOUNT, ""); err != nil {
return fmt.Errorf("unable to mount %s over %s %s", source, dest, err)
if restriction.remountRO {
if err := system.Mount("", dest, "", syscall.MS_REMOUNT|syscall.MS_RDONLY, ""); err != nil {
return fmt.Errorf("unable to remount %s readonly: %s", dest, err)
}
}
}
// This weird trick will allow us to mount /proc read-only, while being able to use AppArmor.
// This is because apparently, loading an AppArmor profile requires write access to /proc/1/attr.
// So we do another mount of procfs, ensure it's write-able, and bind-mount a subset of it.
tmpProcPath := filepath.Join(rootfs, ".proc")
if err := os.Mkdir(tmpProcPath, 0700); err != nil {
return fmt.Errorf("unable to create temporary proc mountpoint %s: %s", tmpProcPath, err)
}
if err := system.Mount("proc", tmpProcPath, "proc", 0, ""); err != nil {
return fmt.Errorf("unable to mount proc on temporary proc mountpoint: %s", err)
}
if err := system.Mount("proc", tmpProcPath, "", syscall.MS_REMOUNT, ""); err != nil {
return fmt.Errorf("unable to remount proc read-write: %s", err)
}
rwAttrPath := filepath.Join(rootfs, ".proc", "1", "attr")
roAttrPath := filepath.Join(rootfs, "proc", "1", "attr")
if err := system.Mount(rwAttrPath, roAttrPath, "", syscall.MS_BIND, ""); err != nil {
return fmt.Errorf("unable to bind-mount %s on %s: %s", rwAttrPath, roAttrPath, err)
}
if err := system.Unmount(tmpProcPath, 0); err != nil {
return fmt.Errorf("unable to unmount temporary proc filesystem: %s", err)
}
return nil
}