Support recursive mount attrs ("rro", "rnosuid", "rnodev", ...)

The new mount option "rro" makes the mount point recursively read-only,
by calling `mount_setattr(2)` with `MOUNT_ATTR_RDONLY` and `AT_RECURSIVE`.
https://man7.org/linux/man-pages/man2/mount_setattr.2.html

Requires kernel >= 5.12.

The "rro" option string conforms to the proposal in util-linux/util-linux Issue 1501.

Fix issue 2823

Similary, this commit also adds the following mount options:
- rrw
- r[no]{suid,dev,exec,relatime,atime,strictatime,diratime,symfollow}

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

libcontainer/configs/mount.go

@@ -30,6 +30,9 @@ type Mount struct {
 	// Relabel source if set, "z" indicates shared, "Z" indicates unshared.
 	Relabel string `json:"relabel"`
 
+	// RecAttr represents mount properties to be applied recursively (AT_RECURSIVE), see mount_setattr(2).
+	RecAttr *unix.MountAttr `json:"rec_attr"`
+
 	// Extensions are additional flags that are specific to runc.
 	Extensions int `json:"extensions"`
 

libcontainer/rootfs_linux.go

@@ -493,6 +493,9 @@ func mountToRootfs(m *configs.Mount, c *mountConfig) error {
 		}
 		return mountPropagate(m, rootfs, mountLabel, mountFd)
 	}
+	if err := setRecAttr(m, rootfs); err != nil {
+		return err
+	}
 	return nil
 }
 
@@ -1123,3 +1126,12 @@ func mountPropagate(m *configs.Mount, rootfs string, mountLabel string, mountFd
 	}
 	return nil
 }
+
+func setRecAttr(m *configs.Mount, rootfs string) error {
+	if m.RecAttr == nil {
+		return nil
+	}
+	return utils.WithProcfd(rootfs, m.Destination, func(procfd string) error {
+		return unix.MountSetattr(-1, procfd, unix.AT_RECURSIVE, m.RecAttr)
+	})
+}

libcontainer/specconv/spec_linux.go

@@ -26,9 +26,13 @@ import (
 )
 
 var (
-	initMapsOnce               sync.Once
-	namespaceMapping           map[specs.LinuxNamespaceType]configs.NamespaceType
-	mountPropagationMapping    map[string]int
+	initMapsOnce            sync.Once
+	namespaceMapping        map[specs.LinuxNamespaceType]configs.NamespaceType
+	mountPropagationMapping map[string]int
+	recAttrFlags            map[string]struct {
+		clear bool
+		flag  uint64
+	}
 	mountFlags, extensionFlags map[string]struct {
 		clear bool
 		flag  int
@@ -99,6 +103,32 @@ func initMaps() {
 			"sync":          {false, unix.MS_SYNCHRONOUS},
 			"symfollow":     {true, unix.MS_NOSYMFOLLOW}, // since kernel 5.10
 		}
+
+		recAttrFlags = map[string]struct {
+			clear bool
+			flag  uint64
+		}{
+			"rro":            {false, unix.MOUNT_ATTR_RDONLY},
+			"rrw":            {true, unix.MOUNT_ATTR_RDONLY},
+			"rnosuid":        {false, unix.MOUNT_ATTR_NOSUID},
+			"rsuid":          {true, unix.MOUNT_ATTR_NOSUID},
+			"rnodev":         {false, unix.MOUNT_ATTR_NODEV},
+			"rdev":           {true, unix.MOUNT_ATTR_NODEV},
+			"rnoexec":        {false, unix.MOUNT_ATTR_NOEXEC},
+			"rexec":          {true, unix.MOUNT_ATTR_NOEXEC},
+			"rnodiratime":    {false, unix.MOUNT_ATTR_NODIRATIME},
+			"rdiratime":      {true, unix.MOUNT_ATTR_NODIRATIME},
+			"rrelatime":      {false, unix.MOUNT_ATTR_RELATIME},
+			"rnorelatime":    {true, unix.MOUNT_ATTR_RELATIME},
+			"rnoatime":       {false, unix.MOUNT_ATTR_NOATIME},
+			"ratime":         {true, unix.MOUNT_ATTR_NOATIME},
+			"rstrictatime":   {false, unix.MOUNT_ATTR_STRICTATIME},
+			"rnostrictatime": {true, unix.MOUNT_ATTR_STRICTATIME},
+			"rnosymfollow":   {false, unix.MOUNT_ATTR_NOSYMFOLLOW}, // since kernel 5.14
+			"rsymfollow":     {true, unix.MOUNT_ATTR_NOSYMFOLLOW},  // since kernel 5.14
+			// No support for MOUNT_ATTR_IDMAP yet (needs UserNS FD)
+		}
+
 		extensionFlags = map[string]struct {
 			clear bool
 			flag  int
@@ -133,6 +163,9 @@ func KnownMountOptions() []string {
 			res = append(res, k)
 		}
 	}
+	for k := range recAttrFlags {
+		res = append(res, k)
+	}
 	for k := range extensionFlags {
 		res = append(res, k)
 	}
@@ -924,8 +957,9 @@ func setupUserNamespace(spec *specs.Spec, config *configs.Config) error {
 // structure with fields that depends on options set accordingly.
 func parseMountOptions(options []string) *configs.Mount {
 	var (
-		data []string
-		m    configs.Mount
+		data                   []string
+		m                      configs.Mount
+		recAttrSet, recAttrClr uint64
 	)
 	initMaps()
 	for _, o := range options {
@@ -940,6 +974,17 @@ func parseMountOptions(options []string) *configs.Mount {
 			}
 		} else if f, exists := mountPropagationMapping[o]; exists && f != 0 {
 			m.PropagationFlags = append(m.PropagationFlags, f)
+		} else if f, exists := recAttrFlags[o]; exists {
+			if f.clear {
+				recAttrClr |= f.flag
+			} else {
+				recAttrSet |= f.flag
+				if f.flag&unix.MOUNT_ATTR__ATIME == f.flag {
+					// https://man7.org/linux/man-pages/man2/mount_setattr.2.html
+					// "cannot simply specify the access-time setting in attr_set, but must also include MOUNT_ATTR__ATIME in the attr_clr field."
+					recAttrClr |= unix.MOUNT_ATTR__ATIME
+				}
+			}
 		} else if f, exists := extensionFlags[o]; exists && f.flag != 0 {
 			if f.clear {
 				m.Extensions &= ^f.flag
@@ -951,6 +996,12 @@ func parseMountOptions(options []string) *configs.Mount {
 		}
 	}
 	m.Data = strings.Join(data, ",")
+	if recAttrSet != 0 || recAttrClr != 0 {
+		m.RecAttr = &unix.MountAttr{
+			Attr_set: recAttrSet,
+			Attr_clr: recAttrClr,
+		}
+	}
 	return &m
 }
 

tests/integration/mounts_recursive.bats

@@ -0,0 +1,78 @@
+#!/usr/bin/env bats
+
+load helpers
+
+TESTVOLUME="${BATS_RUN_TMPDIR}/mounts_recursive"
+
+function setup_volume() {
+	# requires root (in the current user namespace) to mount tmpfs outside runc
+	requires root
+
+	mkdir -p "${TESTVOLUME}"
+	mount -t tmpfs none "${TESTVOLUME}"
+	echo "foo" >"${TESTVOLUME}/foo"
+
+	mkdir "${TESTVOLUME}/subvol"
+	mount -t tmpfs none "${TESTVOLUME}/subvol"
+	echo "bar" >"${TESTVOLUME}/subvol/bar"
+}
+
+function teardown_volume() {
+	umount -R "${TESTVOLUME}"
+}
+
+function setup() {
+	setup_volume
+	setup_busybox
+}
+
+function teardown() {
+	teardown_volume
+	teardown_bundle
+}
+
+@test "runc run [rbind,ro mount is read-only but not recursively]" {
+	update_config ".mounts += [{source: \"${TESTVOLUME}\" , destination: \"/mnt\", options: [\"rbind\",\"ro\"]}]"
+
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_rbind_ro
+	[ "$status" -eq 0 ]
+
+	runc exec test_rbind_ro touch /mnt/foo
+	[ "$status" -eq 1 ]
+	[[ "${output}" == *"Read-only file system"* ]]
+
+	runc exec test_rbind_ro touch /mnt/subvol/bar
+	[ "$status" -eq 0 ]
+}
+
+@test "runc run [rbind,rro mount is recursively read-only]" {
+	requires_kernel 5.12
+	update_config ".mounts += [{source: \"${TESTVOLUME}\" , destination: \"/mnt\", options: [\"rbind\",\"rro\"]}]"
+
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_rbind_rro
+	[ "$status" -eq 0 ]
+
+	runc exec test_rbind_rro touch /mnt/foo
+	[ "$status" -eq 1 ]
+	[[ "${output}" == *"Read-only file system"* ]]
+
+	runc exec test_rbind_rro touch /mnt/subvol/bar
+	[ "$status" -eq 1 ]
+	[[ "${output}" == *"Read-only file system"* ]]
+}
+
+@test "runc run [rbind,ro,rro mount is recursively read-only too]" {
+	requires_kernel 5.12
+	update_config ".mounts += [{source: \"${TESTVOLUME}\" , destination: \"/mnt\", options: [\"rbind\",\"ro\",\"rro\"]}]"
+
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_rbind_ro_rro
+	[ "$status" -eq 0 ]
+
+	runc exec test_rbind_ro_rro touch /mnt/foo
+	[ "$status" -eq 1 ]
+	[[ "${output}" == *"Read-only file system"* ]]
+
+	runc exec test_rbind_ro_rro touch /mnt/subvol/bar
+	[ "$status" -eq 1 ]
+	[[ "${output}" == *"Read-only file system"* ]]
+}