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chroot.setupChrootBindMounts: pay more attention to flags
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Pay better attention to dev/nodev/exec/noexec/suid/nosuid/ro/rw flags on
bind, overlay, and tmpfs mounts when any of them are specified.  Stop
quietly adding "nodev" when it isn't asked for.

Signed-off-by: Nalin Dahyabhai <[email protected]>
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@nalind
nalind committed Oct 11, 2023
1 parent 21badab commit 2ef4837
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Showing 5 changed files with 198 additions and 74 deletions.
126 changes: 72 additions & 54 deletions chroot/run_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -236,7 +236,9 @@ func makeReadOnly(mntpoint string, flags uintptr) error {
return fmt.Errorf("checking if directory %q was bound read-only: %w", mntpoint, err)
}
if fs.Flags&unix.ST_RDONLY == 0 {
if err := unix.Mount(mntpoint, mntpoint, "bind", flags|unix.MS_REMOUNT, ""); err != nil {
// All callers currently pass MS_RDONLY in "flags", but in case they stop doing
// that at some point in the future...
if err := unix.Mount(mntpoint, mntpoint, "bind", flags|unix.MS_RDONLY|unix.MS_REMOUNT, ""); err != nil {
return fmt.Errorf("remounting %s in mount namespace read-only: %w", mntpoint, err)
}
}
Expand Down Expand Up @@ -268,7 +270,7 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
// Now bind mount all of those things to be under the rootfs's location in this
// mount namespace.
commonFlags := uintptr(unix.MS_BIND | unix.MS_REC | unix.MS_PRIVATE)
bindFlags := commonFlags | unix.MS_NODEV
bindFlags := commonFlags
devFlags := commonFlags | unix.MS_NOEXEC | unix.MS_NOSUID | unix.MS_RDONLY
procFlags := devFlags | unix.MS_NODEV
sysFlags := devFlags | unix.MS_NODEV
Expand Down Expand Up @@ -351,7 +353,7 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
}
logrus.Debugf("bind mounted %q to %q", "/sys", filepath.Join(spec.Root.Path, "/sys"))

// Bind mount in everything we've been asked to mount.
// Bind, overlay, or tmpfs mount everything we've been asked to mount.
for _, m := range spec.Mounts {
// Skip anything that we just mounted.
switch m.Destination {
Expand All @@ -369,45 +371,44 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
continue
}
}
// Skip anything that isn't a bind or tmpfs mount.
// Skip anything that isn't a bind or overlay or tmpfs mount.
if m.Type != "bind" && m.Type != "tmpfs" && m.Type != "overlay" {
logrus.Debugf("skipping mount of type %q on %q", m.Type, m.Destination)
continue
}
// If the target is there, we can just mount it.
// If the target is already there, we can just mount over it.
var srcinfo os.FileInfo
switch m.Type {
case "bind":
srcinfo, err = os.Stat(m.Source)
if err != nil {
return undoBinds, fmt.Errorf("examining %q for mounting in mount namespace: %w", m.Source, err)
}
case "overlay":
fallthrough
case "tmpfs":
case "overlay", "tmpfs":
srcinfo, err = os.Stat("/")
if err != nil {
return undoBinds, fmt.Errorf("examining / to use as a template for a %s: %w", m.Type, err)
return undoBinds, fmt.Errorf("examining / to use as a template for a %s mount: %w", m.Type, err)
}
}
target := filepath.Join(spec.Root.Path, m.Destination)
// Check if target is a symlink
// Check if target is a symlink.
stat, err := os.Lstat(target)
// If target is a symlink, follow the link and ensure the destination exists
// If target is a symlink, follow the link and ensure the destination exists.
if err == nil && stat != nil && (stat.Mode()&os.ModeSymlink != 0) {
target, err = copier.Eval(spec.Root.Path, m.Destination, copier.EvalOptions{})
if err != nil {
return nil, fmt.Errorf("evaluating symlink %q: %w", target, err)
}
// Stat the destination of the evaluated symlink
// Stat the destination of the evaluated symlink.
_, err = os.Stat(target)
}
if err != nil {
// If the target can't be stat()ted, check the error.
if !errors.Is(err, os.ErrNotExist) {
return undoBinds, fmt.Errorf("examining %q for mounting in mount namespace: %w", target, err)
}
// The target isn't there yet, so create it.
// The target isn't there yet, so create it. If the source is a directory,
// we need a directory, otherwise we need a non-directory (i.e., a file).
if srcinfo.IsDir() {
if err = os.MkdirAll(target, 0755); err != nil {
return undoBinds, fmt.Errorf("creating mountpoint %q in mount namespace: %w", target, err)
Expand All @@ -423,79 +424,94 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
file.Close()
}
}
// Sort out which flags we're asking for, and what statfs() should be telling us
// if we successfully mounted with them.
requestFlags := uintptr(0)
expectedFlags := uintptr(0)
expectedImportantFlags := uintptr(0)
importantFlags := uintptr(0)
possibleImportantFlags := uintptr(unix.MS_NODEV | unix.ST_NOEXEC | unix.ST_NOSUID | unix.MS_RDONLY)
for _, option := range m.Options {
switch option {
case "nodev":
requestFlags |= unix.MS_NODEV
expectedFlags |= unix.ST_NODEV
importantFlags |= unix.MS_NODEV
expectedImportantFlags |= unix.ST_NODEV
case "dev":
requestFlags &= ^uintptr(unix.MS_NODEV)
expectedFlags &= ^uintptr(unix.ST_NODEV)
importantFlags |= unix.MS_NODEV
expectedImportantFlags &= ^uintptr(unix.ST_NODEV)
case "noexec":
requestFlags |= unix.MS_NOEXEC
expectedFlags |= unix.ST_NOEXEC
importantFlags |= unix.MS_NOEXEC
expectedImportantFlags |= unix.ST_NOEXEC
case "exec":
requestFlags &= ^uintptr(unix.MS_NOEXEC)
expectedFlags &= ^uintptr(unix.ST_NOEXEC)
importantFlags |= unix.MS_NOEXEC
expectedImportantFlags &= ^uintptr(unix.ST_NOEXEC)
case "nosuid":
requestFlags |= unix.MS_NOSUID
expectedFlags |= unix.ST_NOSUID
importantFlags |= unix.MS_NOSUID
expectedImportantFlags |= unix.ST_NOSUID
case "suid":
requestFlags &= ^uintptr(unix.MS_NOSUID)
expectedFlags &= ^uintptr(unix.ST_NOSUID)
importantFlags |= unix.MS_NOSUID
expectedImportantFlags &= ^uintptr(unix.ST_NOSUID)
case "ro":
requestFlags |= unix.MS_RDONLY
expectedFlags |= unix.ST_RDONLY
importantFlags |= unix.MS_RDONLY
expectedImportantFlags |= unix.ST_RDONLY
case "rw":
requestFlags &= ^uintptr(unix.MS_RDONLY)
expectedFlags &= ^uintptr(unix.ST_RDONLY)
importantFlags |= unix.MS_RDONLY
expectedImportantFlags &= ^uintptr(unix.ST_RDONLY)
}
}
switch m.Type {
case "bind":
// Do the bind mount.
logrus.Debugf("bind mounting %q on %q", m.Destination, filepath.Join(spec.Root.Path, m.Destination))
if err := unix.Mount(m.Source, target, "", bindFlags|requestFlags, ""); err != nil {
// Do the initial bind mount. We'll worry about the flags in a bit.
logrus.Debugf("bind mounting %q on %q %v", m.Destination, filepath.Join(spec.Root.Path, m.Destination), m.Options)
if err = unix.Mount(m.Source, target, "", bindFlags|requestFlags, ""); err != nil {
return undoBinds, fmt.Errorf("bind mounting %q from host to %q in mount namespace (%q): %w", m.Source, m.Destination, target, err)
}
if (requestFlags & unix.MS_RDONLY) != 0 {
if err = unix.Statfs(target, &fs); err != nil {
return undoBinds, fmt.Errorf("checking if directory %q was bound read-only: %w", target, err)
}
// we need to make sure these flags are maintained in the REMOUNT operation
additionalFlags := uintptr(fs.Flags) & (unix.MS_NOEXEC | unix.MS_NOSUID | unix.MS_NODEV)
if err := unix.Mount("", target, "", unix.MS_REMOUNT|unix.MS_BIND|unix.MS_RDONLY|additionalFlags, ""); err != nil {
return undoBinds, fmt.Errorf("setting flags on the bind mount %q from host to %q in mount namespace (%q): %w", m.Source, m.Destination, target, err)
}
}
logrus.Debugf("bind mounted %q to %q", m.Source, target)
case "tmpfs":
// Mount a tmpfs.
if err := mount.Mount(m.Source, target, m.Type, strings.Join(append(m.Options, "private"), ",")); err != nil {
return undoBinds, fmt.Errorf("mounting tmpfs to %q in mount namespace (%q, %q): %w", m.Destination, target, strings.Join(m.Options, ","), err)
// Mount a tmpfs. We'll worry about the flags in a bit.
if err = mount.Mount(m.Source, target, m.Type, strings.Join(append(m.Options, "private"), ",")); err != nil {
return undoBinds, fmt.Errorf("mounting tmpfs to %q in mount namespace (%q, %q): %w", m.Destination, target, strings.Join(append(m.Options, "private"), ","), err)
}
logrus.Debugf("mounted a tmpfs to %q", target)
case "overlay":
// Mount a overlay.
if err := mount.Mount(m.Source, target, m.Type, strings.Join(append(m.Options, "private"), ",")); err != nil {
return undoBinds, fmt.Errorf("mounting overlay to %q in mount namespace (%q, %q): %w", m.Destination, target, strings.Join(m.Options, ","), err)
// Mount an overlay. We'll worry about the flags in a bit.
if err = mount.Mount(m.Source, target, m.Type, strings.Join(append(m.Options, "private"), ",")); err != nil {
return undoBinds, fmt.Errorf("mounting overlay to %q in mount namespace (%q, %q): %w", m.Destination, target, strings.Join(append(m.Options, "private"), ","), err)
}
logrus.Debugf("mounted a overlay to %q", target)
}
// Time to worry about the flags.
if err = unix.Statfs(target, &fs); err != nil {
return undoBinds, fmt.Errorf("checking if directory %q was bound read-only: %w", target, err)
return undoBinds, fmt.Errorf("checking if volume %q was mounted with requested flags: %w", target, err)
}
if uintptr(fs.Flags)&expectedFlags != expectedFlags {
if err := unix.Mount(target, target, "bind", requestFlags|unix.MS_REMOUNT, ""); err != nil {
return undoBinds, fmt.Errorf("remounting %q in mount namespace with expected flags: %w", target, err)
effectiveImportantFlags := uintptr(fs.Flags) & importantFlags
if effectiveImportantFlags != expectedImportantFlags {
// Do a remount to try to get the desired flags to stick.
effectiveUnimportantFlags := uintptr(fs.Flags) & ^possibleImportantFlags
if err = unix.Mount(target, target, m.Type, unix.MS_REMOUNT|requestFlags|effectiveUnimportantFlags, ""); err != nil {
return undoBinds, fmt.Errorf("remounting %q in mount namespace with flags %#x instead of %#x: %w", target, requestFlags, effectiveImportantFlags, err)
}
// Check if the desired flags stuck.
if err = unix.Statfs(target, &fs); err != nil {
return undoBinds, fmt.Errorf("checking if directory %q was remounted with requested flags %#x instead of %#x: %w", target, requestFlags, effectiveImportantFlags, err)
}
effectiveImportantFlags = uintptr(fs.Flags) & importantFlags
if effectiveImportantFlags != expectedImportantFlags {
return undoBinds, fmt.Errorf("unable to remount %q with requested flags %#x instead of %#x: %w", target, requestFlags, effectiveImportantFlags, err)
}
}
}

// Set up any read-only paths that we need to. If we're running inside
// of a container, some of these locations will already be read-only.
// of a container, some of these locations will already be read-only, in
// which case can declare victory and move on.
for _, roPath := range spec.Linux.ReadonlyPaths {
r := filepath.Join(spec.Root.Path, roPath)
target, err := filepath.EvalSymlinks(r)
Expand All @@ -515,12 +531,13 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
}
return undoBinds, fmt.Errorf("checking if directory %q is already read-only: %w", target, err)
}
if fs.Flags&unix.ST_RDONLY != 0 {
if fs.Flags&unix.ST_RDONLY == unix.ST_RDONLY {
continue
}
// Mount the location over itself, so that we can remount it as read-only.
roFlags := uintptr(unix.MS_NODEV | unix.MS_NOEXEC | unix.MS_NOSUID | unix.MS_RDONLY)
if err := unix.Mount(target, target, "", roFlags|unix.MS_BIND|unix.MS_REC, ""); err != nil {
// Mount the location over itself, so that we can remount it as read-only, making
// sure to preserve any combination of nodev/noexec/nosuid that's already in play.
roFlags := uintptr(fs.Flags) | unix.MS_RDONLY
if err := unix.Mount(target, target, "", bindFlags|roFlags, ""); err != nil {
if errors.Is(err, os.ErrNotExist) {
// No target, no problem.
continue
Expand All @@ -532,7 +549,7 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
return undoBinds, fmt.Errorf("checking if directory %q was bound read-only: %w", target, err)
}
if fs.Flags&unix.ST_RDONLY == 0 {
if err := unix.Mount(target, target, "", roFlags|unix.MS_BIND|unix.MS_REMOUNT, ""); err != nil {
if err := unix.Mount(target, target, "", unix.MS_REMOUNT|unix.MS_RDONLY|uintptr(fs.Flags), ""); err != nil {
return undoBinds, fmt.Errorf("remounting %q in mount namespace read-only: %w", target, err)
}
}
Expand All @@ -541,6 +558,7 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
return undoBinds, fmt.Errorf("checking if directory %q was remounted read-only: %w", target, err)
}
if fs.Flags&unix.ST_RDONLY == 0 {
// Still not read only.
return undoBinds, fmt.Errorf("verifying that %q in mount namespace was remounted read-only: %w", target, err)
}
}
Expand Down Expand Up @@ -641,11 +659,11 @@ func setupChrootBindMounts(spec *specs.Spec, bundlePath string) (undoBinds func(
return undoBinds, fmt.Errorf("masking directory %q in mount namespace: %w", target, err)
}
if err = unix.Statfs(target, &fs); err != nil {
return undoBinds, fmt.Errorf("checking if directory %q was mounted read-only in mount namespace: %w", target, err)
return undoBinds, fmt.Errorf("checking if masked directory %q was mounted read-only in mount namespace: %w", target, err)
}
if fs.Flags&unix.ST_RDONLY == 0 {
if err = unix.Mount(target, target, "", roFlags|syscall.MS_REMOUNT, ""); err != nil {
return undoBinds, fmt.Errorf("making sure directory %q in mount namespace is read only: %w", target, err)
if err = unix.Mount(target, target, "", syscall.MS_REMOUNT|roFlags|uintptr(fs.Flags), ""); err != nil {
return undoBinds, fmt.Errorf("making sure masked directory %q in mount namespace is read only: %w", target, err)
}
}
}
Expand Down
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