app: hardlink sockets to process-specific share local to XDG_RUNTIME_DIR

This avoids adding ACLs to the PulseAudio directory.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/app/share.display.go

@@ -35,9 +35,11 @@ func (seal *appSeal) shareDisplay() error {
 		if wd, ok := os.LookupEnv(waylandDisplay); !ok {
 			return (*ErrDisplayEnv)(wrapError(ErrWayland, "WAYLAND_DISPLAY is not set"))
 		} else {
-			// wayland socket path
+			// hardlink wayland socket
 			wp := path.Join(seal.RuntimePath, wd)
-			seal.appendEnv(waylandDisplay, wp)
+			wpi := path.Join(seal.shareLocal, "wayland")
+			seal.sys.link(wp, wpi)
+			seal.appendEnv(waylandDisplay, wpi)
 
 			// ensure Wayland socket ACL (e.g. `/run/user/%d/wayland-%d`)
 			seal.sys.updatePerm(wp, acl.Read, acl.Write, acl.Execute)

internal/app/share.pulse.go

@@ -7,7 +7,6 @@ import (
 	"os"
 	"path"
 
-	"git.ophivana.moe/cat/fortify/acl"
 	"git.ophivana.moe/cat/fortify/internal/state"
 )
 
@@ -35,7 +34,7 @@ func (seal *appSeal) sharePulse() error {
 		return nil
 	}
 
-	// ensure PulseAudio directory ACL (e.g. `/run/user/%d/pulse`)
+	// check PulseAudio directory presence (e.g. `/run/user/%d/pulse`)
 	pd := path.Join(seal.RuntimePath, "pulse")
 	ps := path.Join(pd, "native")
 	if _, err := os.Stat(pd); err != nil {
@@ -47,10 +46,7 @@ func (seal *appSeal) sharePulse() error {
 			fmt.Sprintf("PulseAudio directory '%s' not found", pd)))
 	}
 
-	seal.appendEnv(pulseServer, "unix:"+ps)
+	// check PulseAudio socket permission (e.g. `/run/user/%d/pulse/native`)
-	seal.sys.updatePerm(pd, acl.Execute)
-
-	// ensure PulseAudio socket permission (e.g. `/run/user/%d/pulse/native`)
 	if s, err := os.Stat(ps); err != nil {
 		if !errors.Is(err, fs.ErrNotExist) {
 			return (*PulseSocketAccessError)(wrapError(err,
@@ -65,6 +61,11 @@ func (seal *appSeal) sharePulse() error {
 		}
 	}
 
+	// hard link pulse socket into target-executable share
+	psi := path.Join(seal.shareLocal, "pulse")
+	seal.sys.link(ps, psi)
+	seal.appendEnv(pulseServer, "unix:"+psi)
+
 	// publish current user's pulse cookie for target user
 	if src, err := discoverPulseCookie(); err != nil {
 		return err

internal/app/share.runtime.go

@@ -16,6 +16,7 @@ const (
 func (seal *appSeal) shareRuntime() {
 	// ensure RunDir (e.g. `/run/user/%d/fortify`)
 	seal.sys.ensure(seal.RunDirPath, 0700)
+	seal.sys.updatePerm(seal.RunDirPath, acl.Execute)
 
 	// ensure runtime directory ACL (e.g. `/run/user/%d`)
 	seal.sys.updatePerm(seal.RuntimePath, acl.Execute)
@@ -28,6 +29,11 @@ func (seal *appSeal) shareRuntime() {
 	// acl is unnecessary as this directory is world executable
 	seal.share = path.Join(seal.SharePath, seal.id.String())
 	seal.sys.ensureEphemeral(seal.share, 0701)
+
+	// ensure process-specific share local to XDG_RUNTIME_DIR (e.g. `/run/user/%d/fortify/%s`)
+	seal.shareLocal = path.Join(seal.RunDirPath, seal.id.String())
+	seal.sys.ensureEphemeral(seal.shareLocal, 0700)
+	seal.sys.updatePerm(seal.shareLocal, acl.Execute)
 }
 
 func (seal *appSeal) shareRuntimeChild() string {

internal/app/system.go

@@ -33,6 +33,8 @@ type appSeal struct {
 	launchOption uint8
 	// process-specific share directory path
 	share string
+	// process-specific share directory path local to XDG_RUNTIME_DIR
+	shareLocal string
 
 	// path to launcher program
 	toolPath string
@@ -74,6 +76,8 @@ type appSealTx struct {
 	mkdir []appEnsureEntry
 	// dst, src pairs of temporarily shared files
 	tmpfiles [][2]string
+	// dst, src pairs of temporarily hard linked files
+	hardlinks [][2]string
 
 	// sealed path to fortify executable, used by shim
 	executable string
@@ -143,6 +147,11 @@ func (tx *appSealTx) copyFile(dst, src string) {
 	tx.updatePerm(dst, acl.Read)
 }
 
+// link appends a hardlink action
+func (tx *appSealTx) link(oldname, newname string) {
+	tx.hardlinks = append(tx.hardlinks, [2]string{oldname, newname})
+}
+
 type (
 	ChangeHostsError BaseError
 	EnsureDirError   BaseError
@@ -152,7 +161,7 @@ type (
 )
 
 // commit applies recorded actions
-// order: xhost, mkdir, tmpfiles, dbus, acl
+// order: xhost, mkdir, tmpfiles, hardlinks, dbus, acl
 func (tx *appSealTx) commit() error {
 	if tx.complete {
 		panic("seal transaction committed twice")
@@ -211,6 +220,18 @@ func (tx *appSealTx) commit() error {
 		}
 	}
 
+	// create hardlinks
+	for _, link := range tx.hardlinks {
+		verbose.Println("creating hardlink", link[1], "from", link[0])
+		if err := os.Link(link[0], link[1]); err != nil {
+			return (*TmpfileError)(wrapError(err,
+				fmt.Sprintf("cannot create hardlink '%s' from '%s': %s", link[1], link[0], err)))
+		} else {
+			// register partial commit
+			txp.link(link[0], link[1])
+		}
+	}
+
 	if tx.dbus != nil {
 		// start dbus proxy
 		verbose.Printf("session bus proxy on '%s' for upstream '%s'\n", tx.dbusAddr[0][1], tx.dbusAddr[0][0])
@@ -245,7 +266,7 @@ func (tx *appSealTx) commit() error {
 }
 
 // revert rolls back recorded actions
-// order: acl, dbus, tmpfiles, mkdir, xhost
+// order: acl, dbus, hardlinks, tmpfiles, mkdir, xhost
 // errors are printed but not treated as fatal
 func (tx *appSealTx) revert(global bool) error {
 	if tx.closed {
@@ -279,6 +300,13 @@ func (tx *appSealTx) revert(global bool) error {
 		joinError(err, "cannot stop message bus proxy:", err)
 	}
 
+	// remove hardlinks
+	for _, link := range tx.hardlinks {
+		verbose.Println("removing hardlink", link[1])
+		err := os.Remove(link[1])
+		joinError(err, fmt.Sprintf("cannot remove hardlink '%s': %s", link[1], err))
+	}
+
 	// remove tmpfiles
 	for _, tmpfile := range tx.tmpfiles {
 		verbose.Println("removing tmpfile", tmpfile[0])