Refuse launch dirs overlapping sensitive dirs under every root

main.go

@@ -339,6 +339,16 @@ func sensitiveDirs(home string) []string {
 	return dirs
 }
 
+// overlapsSensitive returns the sensitive dir that p is, sits under or contains.
+func overlapsSensitive(p, home string) string {
+	for _, s := range sensitiveDirs(home) {
+		if s = realpath(s); inside(p, s) || inside(s, p) {
+			return s
+		}
+	}
+	return ""
+}
+
 // allowedRoots reads the launch-dir roots; without the file the only root is
 // $HOME and def is true.
 func allowedRoots(home string) (roots []string, def bool, err error) {
@@ -378,10 +388,8 @@ func allowedRoots(home string) (roots []string, def bool, err error) {
 				return nil, false, fmt.Errorf("%s: %q is or contains $PATH entry %s", allowedPathsFile(), l, d)
 			}
 		}
-		for _, s := range sensitiveDirs(home) {
-			if s = realpath(s); inside(r, s) || inside(s, r) {
-				return nil, false, fmt.Errorf("%s: %q overlaps %s", allowedPathsFile(), l, s)
-			}
+		if s := overlapsSensitive(r, home); s != "" {
+			return nil, false, fmt.Errorf("%s: %q overlaps %s", allowedPathsFile(), l, s)
 		}
 		roots = append(roots, r)
 	}
@@ -399,7 +407,7 @@ func pathDirs() []string {
 
 // checkLaunchDir returns pwd resolved if it is strictly under an allowed root,
 // never $HOME itself, anything holding the sandbox's config, the API socket or
-// a $PATH entry, nor anything overlapping a forbidden system dir. Under the
+// a $PATH entry, nor anything overlapping a forbidden system or sensitive dir. Under the
 // default $HOME root, dot-dirs of $HOME are refused too.
 func checkLaunchDir(pwd, allowlist string) (string, error) {
 	pwd = realpath(pwd)
@@ -426,6 +434,9 @@ func checkLaunchDir(pwd, allowlist string) (string, error) {
 			return "", fmt.Errorf("refusing to mount %s: it overlaps %s", pwd, bad)
 		}
 	}
+	if s := overlapsSensitive(pwd, home); s != "" {
+		return "", fmt.Errorf("refusing to mount %s: it overlaps %s", pwd, s)
+	}
 	for _, r := range roots {
 		if pwd != r && inside(pwd, r) {
 			return pwd, nil

main_test.go

@@ -326,6 +326,18 @@ func TestLaunchRefusals(t *testing.T) {
 		}},
 		{"invalid-root", "home/proj", func(t *testing.T, e *env) { writePaths(t, e, "/run/x") }},
 		{"var-tmp-under-var-root", "/var/tmp", func(t *testing.T, e *env) { writePaths(t, e, "/var") }},
+		{"xdg-config-subdir", "home/cfg/systemd", func(t *testing.T, e *env) {
+			_ = os.MkdirAll(e.work+"/home/cfg/systemd", 0o755)
+			t.Setenv("XDG_CONFIG_HOME", e.work+"/home/cfg")
+		}},
+		{"xdg-data-home", "home/data", func(t *testing.T, e *env) {
+			_ = os.MkdirAll(e.work+"/home/data", 0o755)
+			t.Setenv("XDG_DATA_HOME", e.work+"/home/data")
+		}},
+		{"xdg-data-containers", "home/data/containers", func(t *testing.T, e *env) {
+			_ = os.MkdirAll(e.work+"/home/data/containers", 0o755)
+			t.Setenv("XDG_DATA_HOME", e.work+"/home/data")
+		}},
 		{"default-root-dot-dir", "home/.local/src/x", func(t *testing.T, e *env) {
 			_ = os.MkdirAll(e.work+"/home/.local/src/x", 0o755)
 		}},
@@ -365,6 +377,9 @@ func TestCheckLaunchDirRuntimeFallback(t *testing.T) {
 	forbiddenRoots = []string{"/run"}
 	t.Cleanup(func() { forbiddenRoots = origRoots })
 	_, err = checkLaunchDir("/tmp/home/proj", "/nonexistent/hosts")
+	check(t, "under runtime dir", true, err != nil && strings.Contains(err.Error(), "overlaps "+realpath("/tmp")))
+	t.Setenv("XDG_RUNTIME_DIR", "/nonexistent/rt")
+	_, err = checkLaunchDir("/tmp/home/proj", "/nonexistent/hosts")
 	check(t, "subdir", nil, err)
 }
 
@@ -579,10 +594,11 @@ func TestLaunchDotDirRoot(t *testing.T) {
 }
 
 func TestCheckLaunchDirRoots(t *testing.T) {
-	cfg := t.TempDir()
+	cfg, _ := filepath.EvalSymlinks(t.TempDir())
 	t.Setenv("HOME", "/home/u")
 	t.Setenv("XDG_CONFIG_HOME", cfg)
-	t.Setenv("XDG_DATA_HOME", "")
+	t.Setenv("XDG_DATA_HOME", "/srv/data")
+	t.Setenv("XDG_RUNTIME_DIR", "/srv/rt")
 	t.Setenv("PATH", "/usr/bin:/srv/tools/bin")
 	f := filepath.Join(cfg, "agent-sandbox", "allowed-paths")
 	_ = os.MkdirAll(filepath.Dir(f), 0o755)
@@ -596,7 +612,7 @@ func TestCheckLaunchDirRoots(t *testing.T) {
 		check(t, dir, ok, err == nil)
 	}
 	for _, bad := range []string{"/", "/home", "~", "~/", "~/.local", "~/.local/share", "~/.ssh", "~/.ssh/x",
-		"~/.gnupg", "~/.config/foo", cfg, cfg + "/agent-sandbox/x", "~/.local/share/containers/s", "relative/x", "/run/user/1000", "/tmp/x", "/var/tmp", "/proc/1",
+		"~/.gnupg", "~/.config/foo", "~/.local/share/containers/s", "relative/x", "/run/user/1000", "/tmp/x", "/var/tmp", "/proc/1",
 		"/sys", "/dev/shm", "/usr", "/srv/tools", "/var"} {
 		write("/srv/work\n" + bad)
 		_, _, err := allowedRoots("/home/u")
@@ -605,6 +621,18 @@ func TestCheckLaunchDirRoots(t *testing.T) {
 	write("/var")
 	_, err := checkLaunchDir("/var/tmp/x", "/nowhere/hosts")
 	check(t, "/var/tmp under /var root", true, err != nil)
+
+	// cfg lives under /tmp, so drop that rule to leave only the sensitive one.
+	origRoots := forbiddenRoots
+	forbiddenRoots = []string{"/run"}
+	t.Cleanup(func() { forbiddenRoots = origRoots })
+	for bad, want := range map[string]string{cfg: cfg, cfg + "/agent-sandbox/x": cfg, "/srv/data/containers/x": "/srv/data/containers",
+		"/srv/data": "/srv/data/containers", "~/.ssh/x": "/home/u/.ssh", "~/.gnupg/x": "/home/u/.gnupg",
+		"~/.config/foo": "/home/u/.config", "~/.local/share/containers/s": "/home/u/.local/share/containers", "/srv/rt/x": "/srv/rt"} {
+		write("/srv/work\n" + bad)
+		_, _, err := allowedRoots("/home/u")
+		check(t, "sensitive root "+bad, true, strings.Contains(fmt.Sprint(err), "overlaps "+want))
+	}
 }
 
 func TestLaunchPassesArgsVerbatim(t *testing.T) {