Disallow execveat on interpreter scripts with fd opened with O_CLOEXEC.

When an interpreter script is opened with O_CLOEXEC and the resulting fd is
passed into execveat, an ENOENT error should occur (the script would otherwise
be inaccessible to the interpreter). This matches the actual behavior of
Linux's execveat.

PiperOrigin-RevId: 277306680

pkg/sentry/kernel/kernel.go

@@ -812,6 +812,7 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 		ResolveFinal:        true,
 		Filename:            args.Filename,
 		File:                args.File,
+		CloseOnExec:         false,
 		Argv:                args.Argv,
 		Envv:                args.Envv,
 		Features:            k.featureSet,

pkg/sentry/loader/loader.go

@@ -66,6 +66,12 @@ type LoadArgs struct {
 	// nil, then File will be loaded and Filename will be ignored.
 	File *fs.File
 
+	// CloseOnExec indicates that the executable (or one of its parent
+	// directories) was opened with O_CLOEXEC. If the executable is an
+	// interpreter script, then cause an ENOENT error to occur, since the
+	// script would otherwise be inaccessible to the interpreter.
+	CloseOnExec bool
+
 	// Argv is the vector of arguments to pass to the executable.
 	Argv []string
 
@@ -279,6 +285,9 @@ func loadExecutable(ctx context.Context, args LoadArgs) (loadedELF, arch.Context
 			d.IncRef()
 			return loaded, ac, d, args.Argv, err
 		case bytes.Equal(hdr[:2], []byte(interpreterScriptMagic)):
+			if args.CloseOnExec {
+				return loadedELF{}, nil, nil, nil, syserror.ENOENT
+			}
 			args.Filename, args.Argv, err = parseInterpreterScript(ctx, args.Filename, args.File, args.Argv)
 			if err != nil {
 				ctx.Infof("Error loading interpreter script: %v", err)

pkg/sentry/syscalls/linux/sys_thread.go

@@ -120,6 +120,7 @@ func execveat(t *kernel.Task, dirFD int32, pathnameAddr, argvAddr, envvAddr user
 
 	var wd *fs.Dirent
 	var executable *fs.File
+	var closeOnExec bool
 	if dirFD == linux.AT_FDCWD || path.IsAbs(pathname) {
 		// Even if the pathname is absolute, we may still need the wd
 		// for interpreter scripts if the path of the interpreter is
@@ -127,11 +128,12 @@ func execveat(t *kernel.Task, dirFD int32, pathnameAddr, argvAddr, envvAddr user
 		wd = t.FSContext().WorkingDirectory()
 	} else {
 		// Need to extract the given FD.
-		f := t.GetFile(dirFD)
+		f, fdFlags := t.FDTable().Get(dirFD)
 		if f == nil {
 			return 0, nil, syserror.EBADF
 		}
 		defer f.DecRef()
+		closeOnExec = fdFlags.CloseOnExec
 
 		if atEmptyPath && len(pathname) == 0 {
 			executable = f
@@ -157,6 +159,7 @@ func execveat(t *kernel.Task, dirFD int32, pathnameAddr, argvAddr, envvAddr user
 		ResolveFinal:        resolveFinal,
 		Filename:            pathname,
 		File:                executable,
+		CloseOnExec:         closeOnExec,
 		Argv:                argv,
 		Envv:                envv,
 		Features:            t.Arch().FeatureSet(),

test/syscalls/linux/exec.cc

@@ -681,6 +681,39 @@ TEST(ExecveatTest, SymlinkNoFollowWithNormalFile) {
                 ArgEnvExitStatus(0, 0), "");
 }
 
+TEST(ExecveatTest, BasicWithCloexecFD) {
+  std::string path = WorkloadPath(kBasicWorkload);
+  const FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(Open(path, O_CLOEXEC));
+
+  CheckExecveat(fd.get(), "", {path}, {}, AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH,
+                ArgEnvExitStatus(0, 0), absl::StrCat(path, "\n"));
+}
+
+TEST(ExecveatTest, InterpreterScriptWithCloexecFD) {
+  std::string path = WorkloadPath(kExitScript);
+  const FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(Open(path, O_CLOEXEC));
+
+  int execve_errno;
+  ASSERT_NO_ERRNO_AND_VALUE(ForkAndExecveat(fd.get(), "", {path}, {},
+                                            AT_EMPTY_PATH, /*child=*/nullptr,
+                                            &execve_errno));
+  EXPECT_EQ(execve_errno, ENOENT);
+}
+
+TEST(ExecveatTest, InterpreterScriptWithCloexecDirFD) {
+  std::string absolute_path = WorkloadPath(kExitScript);
+  std::string parent_dir = std::string(Dirname(absolute_path));
+  std::string base = std::string(Basename(absolute_path));
+  const FileDescriptor dirfd =
+      ASSERT_NO_ERRNO_AND_VALUE(Open(parent_dir, O_CLOEXEC | O_DIRECTORY));
+
+  int execve_errno;
+  ASSERT_NO_ERRNO_AND_VALUE(ForkAndExecveat(dirfd.get(), base, {base}, {},
+                                            /*flags=*/0, /*child=*/nullptr,
+                                            &execve_errno));
+  EXPECT_EQ(execve_errno, ENOENT);
+}
+
 TEST(ExecveatTest, InvalidFlags) {
   int execve_errno;
   ASSERT_NO_ERRNO_AND_VALUE(ForkAndExecveat(