app: set up acl on X11 socket

The socket is typically owned by the priv-user, and inaccessible by the target user, so just allowing access to the directory is not enough. This change fixes this oversight and add checks that will also be useful for merging https://git.gensokyo.uk/security/hakurei/pulls/1.

Signed-off-by: Ophestra <cat@gensokyo.uk>

8 files changed, 111 insertions, 3 deletions

diff --git a/internal/app/seal_linux.go b/internal/app/seal_linux.go
index 49ef78f7..c7293720 100644
--- a/internal/app/seal_linux.go
+++ b/internal/app/seal_linux.go
@@ -12,6 +12,7 @@ import (
 	"path"
 	"regexp"
 	"slices"
+	"strconv"
 	"strings"
 	"sync/atomic"
 	"syscall"
@@ -386,9 +387,34 @@ func (seal *outcome) finalise(ctx context.Context, sys sys.State, config *hst.Co
 			return hlog.WrapErr(ErrXDisplay,
 				"DISPLAY is not set")
 		} else {
+			socketDir := container.AbsFHSTmp.Append(".X11-unix")
+
+			// the socket file at `/tmp/.X11-unix/X%d` is typically owned by the priv user
+			// and not accessible by the target user
+			var socketPath *container.Absolute
+			if len(d) > 1 && d[0] == ':' { // `:%d`
+				if n, err := strconv.Atoi(d[1:]); err == nil && n >= 0 {
+					socketPath = socketDir.Append("X" + strconv.Itoa(n))
+				}
+			} else if len(d) > 5 && strings.HasPrefix(d, "unix:") { // `unix:%s`
+				if a, err := container.NewAbs(d[5:]); err == nil {
+					socketPath = a
+				}
+			}
+			if socketPath != nil {
+				if _, err := sys.Stat(socketPath.String()); err != nil {
+					if !errors.Is(err, fs.ErrNotExist) {
+						return hlog.WrapErrSuffix(err,
+							fmt.Sprintf("cannot access X11 socket %q:", socketPath))
+					}
+				} else {
+					seal.sys.UpdatePermType(system.EX11, socketPath.String(), acl.Read, acl.Write, acl.Execute)
+					d = "unix:" + socketPath.String()
+				}
+			}
+
 			seal.sys.ChangeHosts("#" + seal.user.uid.String())
 			seal.env[display] = d
-			socketDir := container.AbsFHSTmp.Append(".X11-unix")
 			seal.container.Bind(socketDir, socketDir, 0)
 		}
 	}
diff --git a/test/sandbox/assert.go b/test/sandbox/assert.go
index 57d17039..2c2c9f20 100644
--- a/test/sandbox/assert.go
+++ b/test/sandbox/assert.go
@@ -15,6 +15,7 @@ import (
 	"errors"
 	"io/fs"
 	"log"
+	"net"
 	"os"
 	"syscall"
 )
@@ -33,6 +34,10 @@ type TestCase struct {
 	FS      *FS               `json:"fs"`
 	Mount   []*MountinfoEntry `json:"mount"`
 	Seccomp bool              `json:"seccomp"`
+
+	TrySocket      string `json:"try_socket,omitempty"`
+	SocketAbstract bool   `json:"socket_abstract,omitempty"`
+	SocketPathname bool   `json:"socket_pathname,omitempty"`
 }
 
 type T struct {
@@ -125,6 +130,47 @@ func (t *T) MustCheck(want *TestCase) {
 	} else {
 		printf("[SKIP] skipping seccomp check")
 	}
+
+	if want.TrySocket != "" {
+		abstractConn, abstractErr := net.Dial("unix", "@"+want.TrySocket)
+		pathnameConn, pathnameErr := net.Dial("unix", want.TrySocket)
+		ok := true
+
+		if abstractErr == nil {
+			if err := abstractConn.Close(); err != nil {
+				ok = false
+				log.Printf("Close: %v", err)
+			}
+		}
+		if pathnameErr == nil {
+			if err := pathnameConn.Close(); err != nil {
+				ok = false
+				log.Printf("Close: %v", err)
+			}
+		}
+
+		abstractWantErr := error(syscall.EPERM)
+		pathnameWantErr := error(syscall.ENOENT)
+		if want.SocketAbstract {
+			abstractWantErr = nil
+		}
+		if want.SocketPathname {
+			pathnameWantErr = nil
+		}
+
+		if !errors.Is(abstractErr, abstractWantErr) {
+			ok = false
+			log.Printf("abstractErr: %v, want %v", abstractErr, abstractWantErr)
+		}
+		if !errors.Is(pathnameErr, pathnameWantErr) {
+			ok = false
+			log.Printf("pathnameErr: %v, want %v", pathnameErr, pathnameWantErr)
+		}
+
+		if !ok {
+			os.Exit(1)
+		}
+	}
 }
 
 func MustCheckFilter(pid int, want string) {
diff --git a/test/sandbox/case/default.nix b/test/sandbox/case/default.nix
index 6db1ff45..ec814aab 100644
--- a/test/sandbox/case/default.nix
+++ b/test/sandbox/case/default.nix
@@ -50,6 +50,9 @@ let
         useCommonPaths
         userns
         ;
+      enablements = {
+        inherit (tc) x11;
+      };
       share = testProgram;
       packages = [ ];
       path = "${testProgram}/bin/hakurei-test";
diff --git a/test/sandbox/case/device.nix b/test/sandbox/case/device.nix
index fb2a54f2..e9cf724b 100644
--- a/test/sandbox/case/device.nix
+++ b/test/sandbox/case/device.nix
@@ -25,6 +25,7 @@ in
   mapRealUid = false;
   useCommonPaths = true;
   userns = false;
+  x11 = true;
 
   # 0, PresetStrict
   expectedFilter = {
@@ -35,6 +36,7 @@ in
   want = {
     env = [
       "DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/65534/bus"
+      "DISPLAY=unix:/tmp/.X11-unix/X0"
       "HOME=/var/lib/hakurei/u0/a4"
       "PULSE_SERVER=unix:/run/user/65534/pulse/native"
       "SHELL=/run/current-system/sw/bin/bash"
@@ -161,7 +163,9 @@ in
         } null;
         devices = fs "800001ed" null null;
       } null;
-      tmp = fs "800001f8" { } null;
+      tmp = fs "800001f8" {
+        ".X11-unix" = fs "801001ff" { X0 = fs "10001fd" null null; } null;
+      } null;
       usr = fs "800001c0" { bin = fs "800001ed" { env = fs "80001ff" null null; } null; } null;
       var = fs "800001c0" {
         lib = fs "800001c0" {
@@ -231,10 +235,15 @@ in
       (ent ignore "/etc/passwd" "ro,nosuid,nodev,relatime" "tmpfs" "rootfs" "rw,uid=1000004,gid=1000004")
       (ent ignore "/etc/group" "ro,nosuid,nodev,relatime" "tmpfs" "rootfs" "rw,uid=1000004,gid=1000004")
       (ent ignore "/run/user/65534/wayland-0" "ro,nosuid,nodev,relatime" "ext4" "/dev/disk/by-label/nixos" "rw")
+      (ent "/tmp/.X11-unix" "/tmp/.X11-unix" "ro,nosuid,nodev,relatime" "ext4" "/dev/disk/by-label/nixos" "rw")
       (ent ignore "/run/user/65534/pulse/native" "ro,nosuid,nodev,relatime" "tmpfs" "tmpfs" ignore)
       (ent ignore "/run/user/65534/bus" "ro,nosuid,nodev,relatime" "ext4" "/dev/disk/by-label/nixos" "rw")
     ];
 
     seccomp = true;
+
+    try_socket = "/tmp/.X11-unix/X0";
+    socket_abstract = true;
+    socket_pathname = true;
   };
 }
diff --git a/test/sandbox/case/mapuid.nix b/test/sandbox/case/mapuid.nix
index 1069f9fa..18d82e77 100644
--- a/test/sandbox/case/mapuid.nix
+++ b/test/sandbox/case/mapuid.nix
@@ -34,6 +34,7 @@ in
   mapRealUid = true;
   useCommonPaths = true;
   userns = false;
+  x11 = false;
 
   # 0, PresetStrict
   expectedFilter = {
@@ -266,5 +267,9 @@ in
     ];
 
     seccomp = true;
+
+    try_socket = "/tmp/.X11-unix/X0";
+    socket_abstract = true;
+    socket_pathname = false;
   };
 }
diff --git a/test/sandbox/case/pdlike.nix b/test/sandbox/case/pdlike.nix
index 4f0e71c4..da6753f8 100644
--- a/test/sandbox/case/pdlike.nix
+++ b/test/sandbox/case/pdlike.nix
@@ -34,6 +34,7 @@ in
   mapRealUid = false;
   useCommonPaths = false;
   userns = true;
+  x11 = false;
 
   # 0, PresetExt | PresetDenyDevel
   expectedFilter = {
@@ -261,5 +262,9 @@ in
     ];
 
     seccomp = true;
+
+    try_socket = "/tmp/.X11-unix/X0";
+    socket_abstract = true;
+    socket_pathname = false;
   };
 }
diff --git a/test/sandbox/case/preset.nix b/test/sandbox/case/preset.nix
index 60257ad0..be376a17 100644
--- a/test/sandbox/case/preset.nix
+++ b/test/sandbox/case/preset.nix
@@ -34,6 +34,7 @@ in
   mapRealUid = false;
   useCommonPaths = false;
   userns = false;
+  x11 = false;
 
   # 0, PresetStrict
   expectedFilter = {
@@ -259,5 +260,9 @@ in
     ];
 
     seccomp = true;
+
+    try_socket = "/tmp/.X11-unix/X0";
+    socket_abstract = true;
+    socket_pathname = false;
   };
 }
diff --git a/test/sandbox/case/tty.nix b/test/sandbox/case/tty.nix
index 267ac661..ff7bf035 100644
--- a/test/sandbox/case/tty.nix
+++ b/test/sandbox/case/tty.nix
@@ -34,6 +34,7 @@ in
   mapRealUid = false;
   useCommonPaths = true;
   userns = false;
+  x11 = true;
 
   # 0, PresetExt | PresetDenyNS | PresetDenyDevel
   expectedFilter = {
@@ -44,6 +45,7 @@ in
   want = {
     env = [
       "DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/65534/bus"
+      "DISPLAY=unix:/tmp/.X11-unix/X0"
       "HOME=/var/lib/hakurei/u0/a2"
       "PULSE_SERVER=unix:/run/user/65534/pulse/native"
       "SHELL=/run/current-system/sw/bin/bash"
@@ -188,7 +190,9 @@ in
         } null;
         devices = fs "800001ed" null null;
       } null;
-      tmp = fs "800001f8" { } null;
+      tmp = fs "800001f8" {
+        ".X11-unix" = fs "801001ff" { X0 = fs "10001fd" null null; } null;
+      } null;
       usr = fs "800001c0" { bin = fs "800001ed" { env = fs "80001ff" null null; } null; } null;
       var = fs "800001c0" {
         lib = fs "800001c0" {
@@ -263,10 +267,15 @@ in
       (ent ignore "/etc/passwd" "ro,nosuid,nodev,relatime" "tmpfs" "rootfs" "rw,uid=1000002,gid=1000002")
       (ent ignore "/etc/group" "ro,nosuid,nodev,relatime" "tmpfs" "rootfs" "rw,uid=1000002,gid=1000002")
       (ent ignore "/run/user/65534/wayland-0" "ro,nosuid,nodev,relatime" "ext4" "/dev/disk/by-label/nixos" "rw")
+      (ent "/tmp/.X11-unix" "/tmp/.X11-unix" "ro,nosuid,nodev,relatime" "ext4" "/dev/disk/by-label/nixos" "rw")
       (ent ignore "/run/user/65534/pulse/native" "ro,nosuid,nodev,relatime" "tmpfs" "tmpfs" ignore)
       (ent ignore "/run/user/65534/bus" "ro,nosuid,nodev,relatime" "ext4" "/dev/disk/by-label/nixos" "rw")
     ];
 
     seccomp = true;
+
+    try_socket = "/tmp/.X11-unix/X0";
+    socket_abstract = true;
+    socket_pathname = true;
   };
 }