Merge pull request #4985 from cyphar/hallucinated-paths

pathrs: add "hallucination" helpers for SecureJoin magic

internal/pathrs/mkdirall.go

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: Apache-2.0
+/*
+ * Copyright (C) 2024-2025 Aleksa Sarai <cyphar@cyphar.com>
+ * Copyright (C) 2024-2025 SUSE LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package pathrs
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// MkdirAllParentInRoot is like [MkdirAllInRoot] except that it only creates
+// the parent directory of the target path, returning the trailing component so
+// the caller has more flexibility around constructing the final inode.
+//
+// Callers need to be very careful operating on the trailing path, as trivial
+// mistakes like following symlinks can cause security bugs. Most people
+// should probably just use [MkdirAllInRoot] or [CreateInRoot].
+func MkdirAllParentInRoot(root, unsafePath string, mode os.FileMode) (*os.File, string, error) {
+	// MkdirAllInRoot also does hallucinateUnsafePath, but we need to do it
+	// here first because when we split unsafePath into (dir, file) components
+	// we want to be doing so with the hallucinated path (so that trailing
+	// dangling symlinks are treated correctly).
+	unsafePath, err := hallucinateUnsafePath(root, unsafePath)
+	if err != nil {
+		return nil, "", fmt.Errorf("failed to construct hallucinated target path: %w", err)
+	}
+
+	dirPath, filename := filepath.Split(unsafePath)
+	if filepath.Join("/", filename) == "/" {
+		return nil, "", fmt.Errorf("create parent dir in root subpath %q has bad trailing component %q", unsafePath, filename)
+	}
+
+	dirFd, err := MkdirAllInRoot(root, dirPath, mode)
+	return dirFd, filename, err
+}

internal/pathrs/mkdirall_pathrslite.go

@@ -21,14 +21,13 @@ package pathrs
 import (
 	"fmt"
 	"os"
-	"path/filepath"
 
 	"github.com/cyphar/filepath-securejoin/pathrs-lite"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
 
-// MkdirAllInRootOpen attempts to make
+// MkdirAllInRoot attempts to make
 //
 //	path, _ := securejoin.SecureJoin(root, unsafePath)
 //	os.MkdirAll(path, mode)
@@ -49,19 +48,10 @@ import (
 // handling if unsafePath has already been scoped within the rootfs (this is
 // needed for a lot of runc callers and fixing this would require reworking a
 // lot of path logic).
-func MkdirAllInRootOpen(root, unsafePath string, mode os.FileMode) (*os.File, error) {
-	// If the path is already "within" the root, get the path relative to the
-	// root and use that as the unsafe path. This is necessary because a lot of
-	// MkdirAllInRootOpen callers have already done SecureJoin, and refactoring
-	// all of them to stop using these SecureJoin'd paths would require a fair
-	// amount of work.
-	// TODO(cyphar): Do the refactor to libpathrs once it's ready.
-	if IsLexicallyInRoot(root, unsafePath) {
-		subPath, err := filepath.Rel(root, unsafePath)
-		if err != nil {
-			return nil, err
-		}
-		unsafePath = subPath
+func MkdirAllInRoot(root, unsafePath string, mode os.FileMode) (*os.File, error) {
+	unsafePath, err := hallucinateUnsafePath(root, unsafePath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to construct hallucinated target path: %w", err)
 	}
 
 	// Check for any silly mode bits.
@@ -87,13 +77,3 @@ func MkdirAllInRootOpen(root, unsafePath string, mode os.FileMode) (*os.File, er
 		return pathrs.MkdirAllHandle(rootDir, unsafePath, mode)
 	})
 }
-
-// MkdirAllInRoot is a wrapper around MkdirAllInRootOpen which closes the
-// returned handle, for callers that don't need to use it.
-func MkdirAllInRoot(root, unsafePath string, mode os.FileMode) error {
-	f, err := MkdirAllInRootOpen(root, unsafePath, mode)
-	if err == nil {
-		_ = f.Close()
-	}
-	return err
-}

internal/pathrs/path.go

@@ -19,7 +19,11 @@
 package pathrs
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
+
+	securejoin "github.com/cyphar/filepath-securejoin"
 )
 
 // IsLexicallyInRoot is shorthand for strings.HasPrefix(path+"/", root+"/"),
@@ -32,3 +36,81 @@ func IsLexicallyInRoot(root, path string) bool {
 	path = strings.TrimRight(path, "/")
 	return strings.HasPrefix(path+"/", root+"/")
 }
+
+// LexicallyCleanPath makes a path safe for use with filepath.Join. This is
+// done by not only cleaning the path, but also (if the path is relative)
+// adding a leading '/' and cleaning it (then removing the leading '/'). This
+// ensures that a path resulting from prepending another path will always
+// resolve to lexically be a subdirectory of the prefixed path. This is all
+// done lexically, so paths that include symlinks won't be safe as a result of
+// using CleanPath.
+func LexicallyCleanPath(path string) string {
+	// Deal with empty strings nicely.
+	if path == "" {
+		return ""
+	}
+
+	// Ensure that all paths are cleaned (especially problematic ones like
+	// "/../../../../../" which can cause lots of issues).
+
+	if filepath.IsAbs(path) {
+		return filepath.Clean(path)
+	}
+
+	// If the path isn't absolute, we need to do more processing to fix paths
+	// such as "../../../../<etc>/some/path". We also shouldn't convert absolute
+	// paths to relative ones.
+	path = filepath.Clean(string(os.PathSeparator) + path)
+	// This can't fail, as (by definition) all paths are relative to root.
+	path, _ = filepath.Rel(string(os.PathSeparator), path)
+
+	return path
+}
+
+// LexicallyStripRoot returns the passed path, stripping the root path if it
+// was (lexicially) inside it. Note that both passed paths will always be
+// treated as absolute, and the returned path will also always be absolute. In
+// addition, the paths are cleaned before stripping the root.
+func LexicallyStripRoot(root, path string) string {
+	// Make the paths clean and absolute.
+	root, path = LexicallyCleanPath("/"+root), LexicallyCleanPath("/"+path)
+	switch {
+	case path == root:
+		path = "/"
+	case root == "/":
+		// do nothing
+	default:
+		path = strings.TrimPrefix(path, root+"/")
+	}
+	return LexicallyCleanPath("/" + path)
+}
+
+// hallucinateUnsafePath creates a new unsafePath which has all symlinks
+// (including dangling symlinks) fully resolved and any non-existent components
+// treated as though they are real. This is effectively just a wrapper around
+// [securejoin.SecureJoin] that strips the root. This path *IS NOT* safe to use
+// as-is, you *MUST* operate on the returned path with pathrs-lite.
+//
+// The reason for this methods is that in previous runc versions, we would
+// tolerate nonsense paths with dangling symlinks as path components.
+// pathrs-lite does not support this, so instead we have to emulate this
+// behaviour by doing SecureJoin *purely to get a semi-reasonable path to use*
+// and then we use pathrs-lite to operate on the path safely.
+//
+// It would be quite difficult to emulate this in a race-free way in
+// pathrs-lite, so instead we use [securejoin.SecureJoin] to simply produce a
+// new candidate path for operations like [MkdirAllInRoot] so they can then
+// operate on the new unsafePath as if it was what the user requested.
+//
+// If unsafePath is already lexically inside root, it is stripped before
+// re-resolving it (this is done to ensure compatibility with legacy callers
+// within runc that call SecureJoin before calling into pathrs).
+func hallucinateUnsafePath(root, unsafePath string) (string, error) {
+	unsafePath = LexicallyStripRoot(root, unsafePath)
+	weirdPath, err := securejoin.SecureJoin(root, unsafePath)
+	if err != nil {
+		return "", err
+	}
+	unsafePath = LexicallyStripRoot(root, weirdPath)
+	return unsafePath, nil
+}

internal/pathrs/path_test.go

@@ -51,3 +51,70 @@ func TestIsLexicallyInRoot(t *testing.T) {
 		})
 	}
 }
+
+func TestLexicallyCleanPath(t *testing.T) {
+	path := LexicallyCleanPath("")
+	if path != "" {
+		t.Errorf("expected to receive empty string and received %s", path)
+	}
+
+	path = LexicallyCleanPath("rootfs")
+	if path != "rootfs" {
+		t.Errorf("expected to receive 'rootfs' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("../../../var")
+	if path != "var" {
+		t.Errorf("expected to receive 'var' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("/../../../var")
+	if path != "/var" {
+		t.Errorf("expected to receive '/var' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("/foo/bar/")
+	if path != "/foo/bar" {
+		t.Errorf("expected to receive '/foo/bar' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("/foo/bar/../")
+	if path != "/foo" {
+		t.Errorf("expected to receive '/foo' and received %s", path)
+	}
+}
+
+func TestLexicallyStripRoot(t *testing.T) {
+	for _, test := range []struct {
+		root, path, out string
+	}{
+		// Works with multiple components.
+		{"/a/b", "/a/b/c", "/c"},
+		{"/hello/world", "/hello/world/the/quick-brown/fox", "/the/quick-brown/fox"},
+		// '/' must be a no-op.
+		{"/", "/a/b/c", "/a/b/c"},
+		// Must be the correct order.
+		{"/a/b", "/a/c/b", "/a/c/b"},
+		// Must be at start.
+		{"/abc/def", "/foo/abc/def/bar", "/foo/abc/def/bar"},
+		// Must be a lexical parent.
+		{"/foo/bar", "/foo/barSAMECOMPONENT", "/foo/barSAMECOMPONENT"},
+		// Must only strip the root once.
+		{"/foo/bar", "/foo/bar/foo/bar/baz", "/foo/bar/baz"},
+		// Deal with .. in a fairly sane way.
+		{"/foo/bar", "/foo/bar/../baz", "/foo/baz"},
+		{"/foo/bar", "../../../../../../foo/bar/baz", "/baz"},
+		{"/foo/bar", "/../../../../../../foo/bar/baz", "/baz"},
+		{"/foo/bar/../baz", "/foo/baz/bar", "/bar"},
+		{"/foo/bar/../baz", "/foo/baz/../bar/../baz/./foo", "/foo"},
+		// All paths are made absolute before stripping.
+		{"foo/bar", "/foo/bar/baz/bee", "/baz/bee"},
+		{"/foo/bar", "foo/bar/baz/beef", "/baz/beef"},
+		{"foo/bar", "foo/bar/baz/beets", "/baz/beets"},
+	} {
+		got := LexicallyStripRoot(test.root, test.path)
+		if got != test.out {
+			t.Errorf("LexicallyStripRoot(%q, %q) -- got %q, expected %q", test.root, test.path, got, test.out)
+		}
+	}
+}

internal/pathrs/root_pathrslite.go

@@ -19,9 +19,7 @@
 package pathrs
 
 import (
-	"fmt"
 	"os"
-	"path/filepath"
 
 	"github.com/cyphar/filepath-securejoin/pathrs-lite"
 	"golang.org/x/sys/unix"
@@ -50,12 +48,7 @@ func OpenInRoot(root, subpath string, flags int) (*os.File, error) {
 // include it in the passed flags. The fileMode argument uses unix.* mode bits,
 // *not* os.FileMode.
 func CreateInRoot(root, subpath string, flags int, fileMode uint32) (*os.File, error) {
-	dir, filename := filepath.Split(subpath)
-	if filepath.Join("/", filename) == "/" {
-		return nil, fmt.Errorf("create in root subpath %q has bad trailing component %q", subpath, filename)
-	}
-
-	dirFd, err := MkdirAllInRootOpen(root, dir, 0o755)
+	dirFd, filename, err := MkdirAllParentInRoot(root, subpath, 0o755)
 	if err != nil {
 		return nil, err
 	}

libcontainer/apparmor/apparmor_linux.go

@@ -9,7 +9,6 @@ import (
 	"golang.org/x/sys/unix"
 
 	"github.com/opencontainers/runc/internal/pathrs"
-	"github.com/opencontainers/runc/libcontainer/utils"
 )
 
 var (
@@ -29,7 +28,7 @@ func isEnabled() bool {
 }
 
 func setProcAttr(attr, value string) error {
-	attr = utils.CleanPath(attr)
+	attr = pathrs.LexicallyCleanPath(attr)
 	attrSubPath := "attr/apparmor/" + attr
 	if _, err := os.Stat("/proc/self/" + attrSubPath); errors.Is(err, os.ErrNotExist) {
 		// fall back to the old convention

libcontainer/criu_linux.go

@@ -25,6 +25,7 @@ import (
 	"google.golang.org/protobuf/proto"
 
 	"github.com/opencontainers/cgroups"
+	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/utils"
 )
@@ -555,16 +556,22 @@ func (c *Container) prepareCriuRestoreMounts(mounts []*configs.Mount) error {
 	umounts := []string{}
 	defer func() {
 		for _, u := range umounts {
-			_ = utils.WithProcfd(c.config.Rootfs, u, func(procfd string) error {
-				if e := unix.Unmount(procfd, unix.MNT_DETACH); e != nil {
-					if e != unix.EINVAL {
+			mntFile, err := pathrs.OpenInRoot(c.config.Rootfs, u, unix.O_PATH)
+			if err != nil {
+				logrus.Warnf("Error during cleanup unmounting %s: open handle: %v", u, err)
+				continue
+			}
+			_ = utils.WithProcfdFile(mntFile, func(procfd string) error {
+				if err := unix.Unmount(procfd, unix.MNT_DETACH); err != nil {
+					if err != unix.EINVAL {
 						// Ignore EINVAL as it means 'target is not a mount point.'
 						// It probably has already been unmounted.
-						logrus.Warnf("Error during cleanup unmounting of %s (%s): %v", procfd, u, e)
+						logrus.Warnf("Error during cleanup unmounting of %s (%s): %v", procfd, u, err)
 					}
 				}
 				return nil
 			})
+			_ = mntFile.Close()
 		}
 	}()
 	// Now go through all mounts and create the required mountpoints.

libcontainer/factory_linux.go

@@ -11,10 +11,10 @@ import (
 
 	"github.com/opencontainers/cgroups"
 	"github.com/opencontainers/cgroups/manager"
+	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/configs/validate"
 	"github.com/opencontainers/runc/libcontainer/intelrdt"
-	"github.com/opencontainers/runc/libcontainer/utils"
 )
 
 const (
@@ -211,7 +211,7 @@ func validateID(id string) error {
 
 	}
 
-	if string(os.PathSeparator)+id != utils.CleanPath(string(os.PathSeparator)+id) {
+	if string(os.PathSeparator)+id != pathrs.LexicallyCleanPath(string(os.PathSeparator)+id) {
 		return ErrInvalidID
 	}
 

libcontainer/rootfs_linux.go

@@ -12,7 +12,6 @@ import (
 	"syscall"
 	"time"
 
-	securejoin "github.com/cyphar/filepath-securejoin"
 	"github.com/cyphar/filepath-securejoin/pathrs-lite/procfs"
 	"github.com/moby/sys/mountinfo"
 	"github.com/moby/sys/userns"
@@ -91,7 +90,7 @@ func (m mountEntry) srcStatfs() (*unix.Statfs_t, error) {
 // needsSetupDev returns true if /dev needs to be set up.
 func needsSetupDev(config *configs.Config) bool {
 	for _, m := range config.Mounts {
-		if m.Device == "bind" && utils.CleanPath(m.Destination) == "/dev" {
+		if m.Device == "bind" && pathrs.LexicallyCleanPath(m.Destination) == "/dev" {
 			return false
 		}
 	}
@@ -257,7 +256,7 @@ func finalizeRootfs(config *configs.Config) (err error) {
 		if m.Flags&unix.MS_RDONLY != unix.MS_RDONLY {
 			continue
 		}
-		if m.Device == "tmpfs" || utils.CleanPath(m.Destination) == "/dev" {
+		if m.Device == "tmpfs" || pathrs.LexicallyCleanPath(m.Destination) == "/dev" {
 			if err := remountReadonly(m); err != nil {
 				return err
 			}
@@ -329,12 +328,15 @@ func mountCgroupV1(m mountEntry, c *mountConfig) error {
 			// We just created the tmpfs, and so we can just use filepath.Join
 			// here (not to mention we want to make sure we create the path
 			// inside the tmpfs, so we don't want to resolve symlinks).
+			// TODO: Why not just use b.Destination (c.root is the root here)?
 			subsystemPath := filepath.Join(c.root, b.Destination)
 			subsystemName := filepath.Base(b.Destination)
-			if err := pathrs.MkdirAllInRoot(c.root, subsystemPath, 0o755); err != nil {
+			subsystemDir, err := pathrs.MkdirAllInRoot(c.root, subsystemPath, 0o755)
+			if err != nil {
 				return err
 			}
-			if err := utils.WithProcfd(c.root, b.Destination, func(dstFd string) error {
+			defer subsystemDir.Close()
+			if err := utils.WithProcfdFile(subsystemDir, func(dstFd string) error {
 				flags := defaultMountFlags
 				if m.Flags&unix.MS_RDONLY != 0 {
 					flags = flags | unix.MS_RDONLY
@@ -503,10 +505,8 @@ func statfsToMountFlags(st unix.Statfs_t) int {
 	return flags
 }
 
-var errRootfsToFile = errors.New("config tries to change rootfs to file")
-
 func (m *mountEntry) createOpenMountpoint(rootfs string) (Err error) {
-	unsafePath := utils.StripRoot(rootfs, m.Destination)
+	unsafePath := pathrs.LexicallyStripRoot(rootfs, m.Destination)
 	dstFile, err := pathrs.OpenInRoot(rootfs, unsafePath, unix.O_PATH)
 	defer func() {
 		if dstFile != nil && Err != nil {
@@ -543,22 +543,10 @@ func (m *mountEntry) createOpenMountpoint(rootfs string) (Err error) {
 			}
 			dstIsFile = !fi.IsDir()
 		}
-
-		// In previous runc versions, we would tolerate nonsense paths with
-		// dangling symlinks as path components. pathrs-lite does not support
-		// this, so instead we have to emulate this behaviour by doing
-		// SecureJoin *purely to get a semi-reasonable path to use* and then we
-		// use pathrs-lite to operate on the path safely.
-		newUnsafePath, err := securejoin.SecureJoin(rootfs, unsafePath)
-		if err != nil {
-			return err
-		}
-		unsafePath = utils.StripRoot(rootfs, newUnsafePath)
-
 		if dstIsFile {
 			dstFile, err = pathrs.CreateInRoot(rootfs, unsafePath, unix.O_CREAT|unix.O_EXCL|unix.O_NOFOLLOW, 0o644)
 		} else {
-			dstFile, err = pathrs.MkdirAllInRootOpen(rootfs, unsafePath, 0o755)
+			dstFile, err = pathrs.MkdirAllInRoot(rootfs, unsafePath, 0o755)
 		}
 		if err != nil {
 			return fmt.Errorf("make mountpoint %q: %w", m.Destination, err)
@@ -620,7 +608,7 @@ func mountToRootfs(c *mountConfig, m mountEntry) error {
 		} else if !fi.IsDir() {
 			return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device)
 		}
-		dstFile, err := pathrs.MkdirAllInRootOpen(rootfs, dest, 0o755)
+		dstFile, err := pathrs.MkdirAllInRoot(rootfs, dest, 0o755)
 		if err != nil {
 			return err
 		}
@@ -952,7 +940,7 @@ func createDevices(config *configs.Config) error {
 	for _, node := range config.Devices {
 
 		// The /dev/ptmx device is setup by setupPtmx()
-		if utils.CleanPath(node.Path) == "/dev/ptmx" {
+		if pathrs.LexicallyCleanPath(node.Path) == "/dev/ptmx" {
 			continue
 		}
 
@@ -983,15 +971,7 @@ func createDeviceNode(rootfs string, node *devices.Device, bind bool) error {
 		// The node only exists for cgroup reasons, ignore it here.
 		return nil
 	}
-	destPath, err := securejoin.SecureJoin(rootfs, node.Path)
-	if err != nil {
-		return err
-	}
-	if destPath == rootfs {
-		return fmt.Errorf("%w: mknod over rootfs", errRootfsToFile)
-	}
-	destDirPath, destName := filepath.Split(destPath)
-	destDir, err := pathrs.MkdirAllInRootOpen(rootfs, destDirPath, 0o755)
+	destDir, destName, err := pathrs.MkdirAllParentInRoot(rootfs, node.Path, 0o755)
 	if err != nil {
 		return fmt.Errorf("mkdir parent of device inode %q: %w", node.Path, err)
 	}
@@ -1392,7 +1372,7 @@ func reopenAfterMount(rootfs string, f *os.File, flags int) (_ *os.File, Err err
 	if !pathrs.IsLexicallyInRoot(rootfs, fullPath) {
 		return nil, fmt.Errorf("mountpoint %q is outside of rootfs %q", fullPath, rootfs)
 	}
-	unsafePath := utils.StripRoot(rootfs, fullPath)
+	unsafePath := pathrs.LexicallyStripRoot(rootfs, fullPath)
 	reopened, err := pathrs.OpenInRoot(rootfs, unsafePath, flags)
 	if err != nil {
 		return nil, fmt.Errorf("re-open mountpoint %q: %w", unsafePath, err)
@@ -1432,7 +1412,7 @@ func (m *mountEntry) mountPropagate(rootfs string, mountLabel string) error {
 	// operations on it. We need to set up files in "/dev", and other tmpfs
 	// mounts may need to be chmod-ed after mounting. These mounts will be
 	// remounted ro later in finalizeRootfs(), if necessary.
-	if m.Device == "tmpfs" || utils.CleanPath(m.Destination) == "/dev" {
+	if m.Device == "tmpfs" || pathrs.LexicallyCleanPath(m.Destination) == "/dev" {
 		flags &= ^unix.MS_RDONLY
 	}
 
@@ -1456,9 +1436,7 @@ func (m *mountEntry) mountPropagate(rootfs string, mountLabel string) error {
 	_ = m.dstFile.Close()
 	m.dstFile = newDstFile
 
-	// We have to apply mount propagation flags in a separate WithProcfd() call
-	// because the previous call invalidates the passed procfd -- the mount
-	// target needs to be re-opened.
+	// Apply the propagation flags on the new mount.
 	if err := utils.WithProcfdFile(m.dstFile, func(dstFd string) error {
 		for _, pflag := range m.PropagationFlags {
 			if err := mountViaFds("", nil, m.Destination, dstFd, "", uintptr(pflag), ""); err != nil {

libcontainer/specconv/spec_linux.go

@@ -16,17 +16,17 @@ import (
 
 	systemdDbus "github.com/coreos/go-systemd/v22/dbus"
 	dbus "github.com/godbus/dbus/v5"
+	"github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
+
 	"github.com/opencontainers/cgroups"
 	devices "github.com/opencontainers/cgroups/devices/config"
 	"github.com/opencontainers/runc/internal/linux"
+	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/internal/userns"
 	"github.com/opencontainers/runc/libcontainer/seccomp"
-	libcontainerUtils "github.com/opencontainers/runc/libcontainer/utils"
-	"github.com/opencontainers/runtime-spec/specs-go"
-	"github.com/sirupsen/logrus"
-
-	"golang.org/x/sys/unix"
 )
 
 var (
@@ -802,7 +802,7 @@ func CreateCgroupConfig(opts *CreateOpts, defaultDevs []*devices.Device) (*cgrou
 		if useSystemdCgroup {
 			myCgroupPath = spec.Linux.CgroupsPath
 		} else {
-			myCgroupPath = libcontainerUtils.CleanPath(spec.Linux.CgroupsPath)
+			myCgroupPath = pathrs.LexicallyCleanPath(spec.Linux.CgroupsPath)
 		}
 	}
 

libcontainer/utils/utils.go

@@ -4,11 +4,11 @@ package utils
 import (
 	"encoding/json"
 	"io"
-	"os"
-	"path/filepath"
 	"strings"
 
 	"golang.org/x/sys/unix"
+
+	"github.com/opencontainers/runc/internal/pathrs"
 )
 
 const (
@@ -37,53 +37,6 @@ func WriteJSON(w io.Writer, v any) error {
 	return err
 }
 
-// CleanPath makes a path safe for use with filepath.Join. This is done by not
-// only cleaning the path, but also (if the path is relative) adding a leading
-// '/' and cleaning it (then removing the leading '/'). This ensures that a
-// path resulting from prepending another path will always resolve to lexically
-// be a subdirectory of the prefixed path. This is all done lexically, so paths
-// that include symlinks won't be safe as a result of using CleanPath.
-func CleanPath(path string) string {
-	// Deal with empty strings nicely.
-	if path == "" {
-		return ""
-	}
-
-	// Ensure that all paths are cleaned (especially problematic ones like
-	// "/../../../../../" which can cause lots of issues).
-
-	if filepath.IsAbs(path) {
-		return filepath.Clean(path)
-	}
-
-	// If the path isn't absolute, we need to do more processing to fix paths
-	// such as "../../../../<etc>/some/path". We also shouldn't convert absolute
-	// paths to relative ones.
-	path = filepath.Clean(string(os.PathSeparator) + path)
-	// This can't fail, as (by definition) all paths are relative to root.
-	path, _ = filepath.Rel(string(os.PathSeparator), path)
-
-	return path
-}
-
-// StripRoot returns the passed path, stripping the root path if it was
-// (lexicially) inside it. Note that both passed paths will always be treated
-// as absolute, and the returned path will also always be absolute. In
-// addition, the paths are cleaned before stripping the root.
-func StripRoot(root, path string) string {
-	// Make the paths clean and absolute.
-	root, path = CleanPath("/"+root), CleanPath("/"+path)
-	switch {
-	case path == root:
-		path = "/"
-	case root == "/":
-		// do nothing
-	default:
-		path = strings.TrimPrefix(path, root+"/")
-	}
-	return CleanPath("/" + path)
-}
-
 // SearchLabels searches through a list of key=value pairs for a given key,
 // returning its value, and the binary flag telling whether the key exist.
 func SearchLabels(labels []string, key string) (string, bool) {
@@ -114,3 +67,23 @@ func Annotations(labels []string) (bundle string, userAnnotations map[string]str
 	}
 	return bundle, userAnnotations
 }
+
+// CleanPath makes a path safe for use with filepath.Join. This is done by not
+// only cleaning the path, but also (if the path is relative) adding a leading
+// '/' and cleaning it (then removing the leading '/'). This ensures that a
+// path resulting from prepending another path will always resolve to lexically
+// be a subdirectory of the prefixed path. This is all done lexically, so paths
+// that include symlinks won't be safe as a result of using CleanPath.
+//
+// Deprecated: This function has been moved to internal/pathrs and this wrapper
+// will be removed in runc 1.5.
+var CleanPath = pathrs.LexicallyCleanPath
+
+// StripRoot returns the passed path, stripping the root path if it was
+// (lexicially) inside it. Note that both passed paths will always be treated
+// as absolute, and the returned path will also always be absolute. In
+// addition, the paths are cleaned before stripping the root.
+//
+// Deprecated: This function has been moved to internal/pathrs and this wrapper
+// will be removed in runc 1.5.
+var StripRoot = pathrs.LexicallyStripRoot

libcontainer/utils/utils_test.go

@@ -70,70 +70,3 @@ func TestWriteJSON(t *testing.T) {
 		t.Errorf("expected to write %s but was %s", expected, b.String())
 	}
 }
-
-func TestCleanPath(t *testing.T) {
-	path := CleanPath("")
-	if path != "" {
-		t.Errorf("expected to receive empty string and received %s", path)
-	}
-
-	path = CleanPath("rootfs")
-	if path != "rootfs" {
-		t.Errorf("expected to receive 'rootfs' and received %s", path)
-	}
-
-	path = CleanPath("../../../var")
-	if path != "var" {
-		t.Errorf("expected to receive 'var' and received %s", path)
-	}
-
-	path = CleanPath("/../../../var")
-	if path != "/var" {
-		t.Errorf("expected to receive '/var' and received %s", path)
-	}
-
-	path = CleanPath("/foo/bar/")
-	if path != "/foo/bar" {
-		t.Errorf("expected to receive '/foo/bar' and received %s", path)
-	}
-
-	path = CleanPath("/foo/bar/../")
-	if path != "/foo" {
-		t.Errorf("expected to receive '/foo' and received %s", path)
-	}
-}
-
-func TestStripRoot(t *testing.T) {
-	for _, test := range []struct {
-		root, path, out string
-	}{
-		// Works with multiple components.
-		{"/a/b", "/a/b/c", "/c"},
-		{"/hello/world", "/hello/world/the/quick-brown/fox", "/the/quick-brown/fox"},
-		// '/' must be a no-op.
-		{"/", "/a/b/c", "/a/b/c"},
-		// Must be the correct order.
-		{"/a/b", "/a/c/b", "/a/c/b"},
-		// Must be at start.
-		{"/abc/def", "/foo/abc/def/bar", "/foo/abc/def/bar"},
-		// Must be a lexical parent.
-		{"/foo/bar", "/foo/barSAMECOMPONENT", "/foo/barSAMECOMPONENT"},
-		// Must only strip the root once.
-		{"/foo/bar", "/foo/bar/foo/bar/baz", "/foo/bar/baz"},
-		// Deal with .. in a fairly sane way.
-		{"/foo/bar", "/foo/bar/../baz", "/foo/baz"},
-		{"/foo/bar", "../../../../../../foo/bar/baz", "/baz"},
-		{"/foo/bar", "/../../../../../../foo/bar/baz", "/baz"},
-		{"/foo/bar/../baz", "/foo/baz/bar", "/bar"},
-		{"/foo/bar/../baz", "/foo/baz/../bar/../baz/./foo", "/foo"},
-		// All paths are made absolute before stripping.
-		{"foo/bar", "/foo/bar/baz/bee", "/baz/bee"},
-		{"/foo/bar", "foo/bar/baz/beef", "/baz/beef"},
-		{"foo/bar", "foo/bar/baz/beets", "/baz/beets"},
-	} {
-		got := StripRoot(test.root, test.path)
-		if got != test.out {
-			t.Errorf("StripRoot(%q, %q) -- got %q, expected %q", test.root, test.path, got, test.out)
-		}
-	}
-}

libcontainer/utils/utils_unix.go

@@ -13,10 +13,11 @@ import (
 	_ "unsafe" // for go:linkname
 
 	securejoin "github.com/cyphar/filepath-securejoin"
-	"github.com/opencontainers/runc/internal/linux"
-	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
+
+	"github.com/opencontainers/runc/internal/linux"
+	"github.com/opencontainers/runc/internal/pathrs"
 )
 
 var (
@@ -151,9 +152,12 @@ func NewSockPair(name string) (parent, child *os.File, err error) {
 // through the passed fdpath should be safe. Do not access this path through
 // the original path strings, and do not attempt to use the pathname outside of
 // the passed closure (the file handle will be freed once the closure returns).
+//
+// Deprecated: This function is an internal implementation detail of runc and
+// is no longer used. It will be removed in runc 1.5.
 func WithProcfd(root, unsafePath string, fn func(procfd string) error) error {
 	// Remove the root then forcefully resolve inside the root.
-	unsafePath = StripRoot(root, unsafePath)
+	unsafePath = pathrs.LexicallyStripRoot(root, unsafePath)
 	fullPath, err := securejoin.SecureJoin(root, unsafePath)
 	if err != nil {
 		return fmt.Errorf("resolving path inside rootfs failed: %w", err)
@@ -180,10 +184,15 @@ func WithProcfd(root, unsafePath string, fn func(procfd string) error) error {
 	return fn(procfd)
 }
 
-// WithProcfdFile is a very minimal wrapper around [ProcThreadSelfFd], intended
-// to make migrating from [WithProcfd] and [WithProcfdPath] usage easier. The
+// WithProcfdFile is a very minimal wrapper around [ProcThreadSelfFd]. The
 // caller is responsible for making sure that the provided file handle is
 // actually safe to operate on.
+//
+// NOTE: THIS FUNCTION IS INTERNAL TO RUNC, DO NOT USE IT.
+//
+// TODO: Migrate the mount logic towards a more move_mount(2)-friendly design
+// where this is kind of /proc/self/... tomfoolery is only done in a fallback
+// path for old kernels.
 func WithProcfdFile(file *os.File, fn func(procfd string) error) error {
 	fdpath, closer := ProcThreadSelfFd(file.Fd())
 	defer closer()

tests/integration/mounts.bats

@@ -127,6 +127,42 @@ function test_mount_order() {
 	[[ "$output" == *"a/x"* ]] # the final "file" was from a/x.
 }
 
+# This needs to be placed at the top of the bats file to work around
+# a shellcheck bug. See <https://github.com/koalaman/shellcheck/issues/2873>.
+test_mount_target() {
+	src="$1"
+	dst="$2"
+	real_dst="${3:-$dst}"
+
+	echo "== $src -> $dst (=> $real_dst) =="
+
+	old_config="$(mktemp ./config.json.bak.XXXXXX)"
+	cp ./config.json "$old_config"
+
+	update_config '.mounts += [{
+		source: "'"$src"'",
+		destination: "'"$dst"'",
+		options: ["bind"]
+	}]'
+
+	# Make sure the target path is at the right spot and is actually a
+	# bind-mount of the correct inode.
+	update_config '.process.args = ["stat", "-c", "%n %d:%i", "--", "'"$real_dst"'"]'
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+	[[ "$output" == "$real_dst $(stat -c "%d:%i" -- "$src")" ]]
+
+	# Make sure there is a mount entry for the target path.
+	# shellcheck disable=SC2016
+	update_config '.process.args = ["awk", "-F", "PATH='"$real_dst"'", "$2 == PATH", "/proc/self/mounts"]'
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+	[[ "$output" == *"$real_dst"* ]]
+
+	# Switch back the old config so this function can be called multiple times.
+	mv "$old_config" "./config.json"
+}
+
 # https://github.com/opencontainers/runc/issues/3991
 @test "runc run [tmpcopyup]" {
 	mkdir -p rootfs/dir1/dir2
@@ -343,3 +379,25 @@ function test_mount_order() {
 @test "runc run [mount order, container idmap source] (userns)" {
 	test_mount_order userns,idmap
 }
+
+@test "runc run [bind mount through a dangling symlink component]" {
+	rm -rf rootfs/etc/hosts
+	ln -s /tmp/foo/bar rootfs/jump
+	ln -s /jump/baz/hosts rootfs/etc/hosts
+
+	rm -rf rootfs/tmp/foo
+	test_mount_target ./config.json /etc/hosts /tmp/foo/bar/baz/hosts
+}
+
+@test "runc run [bind mount through a trailing dangling symlink]" {
+	rm -rf rootfs/etc/hosts
+	ln -s /tmp/hosts rootfs/etc/hosts
+
+	# File.
+	rm -rf rootfs/tmp/hosts
+	test_mount_target ./config.json /etc/hosts /tmp/hosts
+
+	# Directory.
+	rm -rf rootfs/tmp/hosts
+	test_mount_target . /etc/hosts /tmp/hosts
+}