Use securejoin.SecureJoin when forming userns paths

We need to read /etc/passwd and /etc/group in the container to
get an idea of how many UIDs and GIDs we need to allocate for a
user namespace when `--userns=auto` is specified. We were forming
paths for these using filepath.Join, which is not safe for paths
within a container, resulting in this CVE allowing crafted
symlinks in the container to access paths on the host instead.

Addresses CVE-2024-9676

Signed-off-by: Matt Heon <mheon@redhat.com>

userns.go

@@ -1,18 +1,21 @@
+//go:build linux
+
 package storage
 
 import (
 	"fmt"
 	"os"
 	"os/user"
-	"path/filepath"
 	"strconv"
 
 	drivers "github.com/containers/storage/drivers"
 	"github.com/containers/storage/pkg/idtools"
 	"github.com/containers/storage/pkg/unshare"
 	"github.com/containers/storage/types"
+	securejoin "github.com/cyphar/filepath-securejoin"
 	libcontainerUser "github.com/moby/sys/user"
 	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
 )
 
 // getAdditionalSubIDs looks up the additional IDs configured for
@@ -85,40 +88,59 @@ const nobodyUser = 65534
 // parseMountedFiles returns the maximum UID and GID found in the /etc/passwd and
 // /etc/group files.
 func parseMountedFiles(containerMount, passwdFile, groupFile string) uint32 {
+	var (
+		passwd *os.File
+		group  *os.File
+		size   int
+		err    error
+	)
 	if passwdFile == "" {
-		passwdFile = filepath.Join(containerMount, "etc/passwd")
+		passwd, err = secureOpen(containerMount, "/etc/passwd")
+	} else {
+		// User-specified override from a volume. Will not be in
+		// container root.
+		passwd, err = os.Open(passwdFile)
 	}
-	if groupFile == "" {
-		groupFile = filepath.Join(containerMount, "etc/group")
-	}
-
-	size := 0
-
-	users, err := libcontainerUser.ParsePasswdFile(passwdFile)
 	if err == nil {
-		for _, u := range users {
-			// Skip the "nobody" user otherwise we end up with 65536
-			// ids with most images
-			if u.Name == "nobody" || u.Name == "nogroup" {
-				continue
-			}
-			if u.Uid > size && u.Uid != nobodyUser {
-				size = u.Uid + 1
-			}
-			if u.Gid > size && u.Gid != nobodyUser {
-				size = u.Gid + 1
+		defer passwd.Close()
+
+		users, err := libcontainerUser.ParsePasswd(passwd)
+		if err == nil {
+			for _, u := range users {
+				// Skip the "nobody" user otherwise we end up with 65536
+				// ids with most images
+				if u.Name == "nobody" || u.Name == "nogroup" {
+					continue
+				}
+				if u.Uid > size && u.Uid != nobodyUser {
+					size = u.Uid + 1
+				}
+				if u.Gid > size && u.Gid != nobodyUser {
+					size = u.Gid + 1
+				}
 			}
 		}
 	}
 
-	groups, err := libcontainerUser.ParseGroupFile(groupFile)
+	if groupFile == "" {
+		group, err = secureOpen(containerMount, "/etc/group")
+	} else {
+		// User-specified override from a volume. Will not be in
+		// container root.
+		group, err = os.Open(groupFile)
+	}
 	if err == nil {
-		for _, g := range groups {
-			if g.Name == "nobody" || g.Name == "nogroup" {
-				continue
-			}
-			if g.Gid > size && g.Gid != nobodyUser {
-				size = g.Gid + 1
+		defer group.Close()
+
+		groups, err := libcontainerUser.ParseGroup(group)
+		if err == nil {
+			for _, g := range groups {
+				if g.Name == "nobody" || g.Name == "nogroup" {
+					continue
+				}
+				if g.Gid > size && g.Gid != nobodyUser {
+					size = g.Gid + 1
+				}
 			}
 		}
 	}
@@ -309,3 +331,14 @@ func getAutoUserNSIDMappings(
 	gidMap := append(availableGIDs.zip(requestedContainerGIDs), additionalGIDMappings...)
 	return uidMap, gidMap, nil
 }
+
+// Securely open (read-only) a file in a container mount.
+func secureOpen(containerMount, file string) (*os.File, error) {
+	tmpFile, err := securejoin.OpenInRoot(containerMount, file)
+	if err != nil {
+		return nil, err
+	}
+	defer tmpFile.Close()
+
+	return securejoin.Reopen(tmpFile, unix.O_RDONLY)
+}

userns_test.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package storage
 
 import (

userns_unsupported.go

@@ -0,0 +1,14 @@
+//go:build !linux
+
+package storage
+
+import (
+	"errors"
+
+	"github.com/containers/storage/pkg/idtools"
+	"github.com/containers/storage/types"
+)
+
+func (s *store) getAutoUserNS(_ *types.AutoUserNsOptions, _ *Image, _ rwLayerStore, _ []roLayerStore) ([]idtools.IDMap, []idtools.IDMap, error) {
+	return nil, nil, errors.New("user namespaces are not supported on this platform")
+}