runtime: enforce standard file descriptors open on init on unix

On Unix-like platforms, enforce that the standard file descriptions (0,
1, 2) are always open during initialization. If any of the FDs are
closed, we open them pointing at /dev/null, or fail.

Fixes #60641

Change-Id: Iaab6b3f3e5ca44006ae3ba3544d47da9a613f58f
Reviewed-on: https://go-review.googlesource.com/c/go/+/509020
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Roland Shoemaker <[email protected]>
Auto-Submit: Roland Shoemaker <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rolandshoemaker

src/runtime/fds_nonunix.go

@@ -0,0 +1,11 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !unix
+
+package runtime
+
+func checkfds() {
+	// Nothing to do on non-Unix platforms.
+}

src/runtime/fds_test.go

@@ -0,0 +1,78 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package runtime_test
+
+import (
+	"internal/testenv"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestCheckFDs(t *testing.T) {
+	if *flagQuick {
+		t.Skip("-quick")
+	}
+
+	testenv.MustHaveGoBuild(t)
+
+	fdsBin, err := buildTestProg(t, "testfds")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	i, err := os.CreateTemp(t.TempDir(), "fds-input")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := i.Write([]byte("stdin")); err != nil {
+		t.Fatal(err)
+	}
+	if err := i.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	o, err := os.CreateTemp(t.TempDir(), "fds-output")
+	if err != nil {
+		t.Fatal(err)
+	}
+	outputPath := o.Name()
+	if err := o.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	env := []string{"TEST_OUTPUT=" + outputPath}
+	for _, e := range os.Environ() {
+		if strings.HasPrefix(e, "GODEBUG=") || strings.HasPrefix(e, "GOTRACEBACK=") {
+			continue
+		}
+		env = append(env, e)
+	}
+
+	proc, err := os.StartProcess(fdsBin, []string{fdsBin}, &os.ProcAttr{
+		Env:   env,
+		Files: []*os.File{},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	ps, err := proc.Wait()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ps.ExitCode() != 0 {
+		t.Fatalf("testfds failed: %d", ps.ExitCode())
+	}
+
+	fc, err := os.ReadFile(outputPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(fc) != "" {
+		t.Errorf("unexpected file content, got: %q", string(fc))
+	}
+}

src/runtime/fds_unix.go

@@ -0,0 +1,44 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package runtime
+
+func checkfds() {
+	if islibrary || isarchive {
+		// If the program is actually a library, presumably being consumed by
+		// another program, we don't want to mess around with the file
+		// descriptors.
+		return
+	}
+
+	const (
+		// F_GETFD, EBADF, O_RDWR are standard across all unixes we support, so
+		// we define them here rather than in each of the OS specific files.
+		F_GETFD = 0x01
+		EBADF   = 0x09
+		O_RDWR  = 0x02
+	)
+
+	devNull := []byte("/dev/null\x00")
+	for i := 0; i < 3; i++ {
+		ret, errno := fcntl(int32(i), F_GETFD, 0)
+		if ret >= 0 {
+			continue
+		}
+		if errno != EBADF {
+			print("runtime: unexpected error while checking standard file descriptor ", i, ", errno=", errno, "\n")
+			throw("cannot open standard fds")
+		}
+
+		if ret := open(&devNull[0], O_RDWR, 0); ret < 0 {
+			print("runtime: standard file descriptor ", i, " closed, unable to open /dev/null, errno=", errno, "\n")
+			throw("cannot open standard fds")
+		} else if ret != int32(i) {
+			print("runtime: opened unexpected file descriptor ", ret, " when attempting to open ", i, "\n")
+			throw("cannot open standard fds")
+		}
+	}
+}

src/runtime/proc.go

@@ -741,6 +741,7 @@ func schedinit() {
 	goargs()
 	goenvs()
 	secure()
+	checkfds()
 	parsedebugvars()
 	gcinit()
 

src/runtime/security_unix.go

@@ -13,19 +13,12 @@ func secure() {
 		return
 	}
 
-	// When secure mode is enabled, we do two things:
-	//   1. ensure the file descriptors 0, 1, and 2 are open, and if not open them,
-	//      pointing at /dev/null (or fail)
-	//   2. enforce specific environment variable values (currently we only force
-	//		GOTRACEBACK=none)
+	// When secure mode is enabled, we do one thing: enforce specific
+	// environment variable values (currently we only force GOTRACEBACK=none)
 	//
 	// Other packages may also disable specific functionality when secure mode
 	// is enabled (determined by using linkname to call isSecureMode).
-	//
-	// NOTE: we may eventually want to enforce (1) regardless of whether secure
-	// mode is enabled or not.
 
-	secureFDs()
 	secureEnv()
 }
 
@@ -41,32 +34,3 @@ func secureEnv() {
 		envs = append(envs, "GOTRACEBACK=none")
 	}
 }
-
-func secureFDs() {
-	const (
-		// F_GETFD and EBADF are standard across all unixes, define
-		// them here rather than in each of the OS specific files
-		F_GETFD = 0x01
-		EBADF   = 0x09
-	)
-
-	devNull := []byte("/dev/null\x00")
-	for i := 0; i < 3; i++ {
-		ret, errno := fcntl(int32(i), F_GETFD, 0)
-		if ret >= 0 {
-			continue
-		}
-		if errno != EBADF {
-			print("runtime: unexpected error while checking standard file descriptor ", i, ", errno=", errno, "\n")
-			throw("cannot secure fds")
-		}
-
-		if ret := open(&devNull[0], 2 /* O_RDWR */, 0); ret < 0 {
-			print("runtime: standard file descriptor ", i, " closed, unable to open /dev/null, errno=", errno, "\n")
-			throw("cannot secure fds")
-		} else if ret != int32(i) {
-			print("runtime: opened unexpected file descriptor ", ret, " when attempting to open ", i, "\n")
-			throw("cannot secure fds")
-		}
-	}
-}

src/runtime/testdata/testfds/main.go

@@ -0,0 +1,29 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+)
+
+func main() {
+	f, err := os.OpenFile(os.Getenv("TEST_OUTPUT"), os.O_CREATE|os.O_RDWR, 0600)
+	if err != nil {
+		log.Fatalf("os.Open failed: %s", err)
+	}
+	defer f.Close()
+	b, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		log.Fatalf("io.ReadAll(os.Stdin) failed: %s", err)
+	}
+	if len(b) != 0 {
+		log.Fatalf("io.ReadAll(os.Stdin) returned non-nil: %x", b)
+	}
+	fmt.Fprintf(os.Stdout, "stdout\n")
+	fmt.Fprintf(os.Stderr, "stderr\n")
+}