os, net/http: avoid escapes from os.DirFS and http.Dir on Windows

Do not permit access to Windows reserved device names (NUL, COM1, etc.)
via os.DirFS and http.Dir filesystems.

Avoid escapes from os.DirFS(`\`) on Windows. DirFS would join the
the root to the relative path with a path separator, making
os.DirFS(`\`).Open(`/foo/bar`) open the path `\\foo\bar`, which is
a UNC name. Not only does this not open the intended file, but permits
reference to any file on the system rather than only files on the
current drive.

Make os.DirFS("") invalid, with all file access failing. Previously,
a root of "" was interpreted as "/", which is surprising and probably
unintentional.

Fixes CVE-2022-41720
Fixes #56694

Change-Id: I275b5fa391e6ad7404309ea98ccc97405942e0f0
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1663834
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Reviewed-by: Julie Qiu <julieqiu@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/455362
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jenny Rakoczy <jenny@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/455716
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>

src/go/build/deps_test.go

@@ -144,6 +144,7 @@ var depsRules = `
 	io/fs
 	< internal/testlog
 	< internal/poll
+	< internal/safefilepath
 	< os
 	< os/signal;
 

src/internal/safefilepath/path.go

@@ -0,0 +1,21 @@
+// Copyright 2022 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 safefilepath manipulates operating-system file paths.
+package safefilepath
+
+import (
+	"errors"
+)
+
+var errInvalidPath = errors.New("invalid path")
+
+// FromFS converts a slash-separated path into an operating-system path.
+//
+// FromFS returns an error if the path cannot be represented by the operating
+// system. For example, paths containing '\' and ':' characters are rejected
+// on Windows.
+func FromFS(path string) (string, error) {
+	return fromFS(path)
+}

src/internal/safefilepath/path_other.go

@@ -0,0 +1,23 @@
+// Copyright 2022 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 !windows
+
+package safefilepath
+
+import "runtime"
+
+func fromFS(path string) (string, error) {
+	if runtime.GOOS == "plan9" {
+		if len(path) > 0 && path[0] == '#' {
+			return path, errInvalidPath
+		}
+	}
+	for i := range path {
+		if path[i] == 0 {
+			return "", errInvalidPath
+		}
+	}
+	return path, nil
+}

src/internal/safefilepath/path_test.go

@@ -0,0 +1,88 @@
+// Copyright 2022 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 safefilepath_test
+
+import (
+	"internal/safefilepath"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+type PathTest struct {
+	path, result string
+}
+
+const invalid = ""
+
+var fspathtests = []PathTest{
+	{".", "."},
+	{"/a/b/c", "/a/b/c"},
+	{"a\x00b", invalid},
+}
+
+var winreservedpathtests = []PathTest{
+	{`a\b`, `a\b`},
+	{`a:b`, `a:b`},
+	{`a/b:c`, `a/b:c`},
+	{`NUL`, `NUL`},
+	{`./com1`, `./com1`},
+	{`a/nul/b`, `a/nul/b`},
+}
+
+// Whether a reserved name with an extension is reserved or not varies by
+// Windows version.
+var winreservedextpathtests = []PathTest{
+	{"nul.txt", "nul.txt"},
+	{"a/nul.txt/b", "a/nul.txt/b"},
+}
+
+var plan9reservedpathtests = []PathTest{
+	{`#c`, `#c`},
+}
+
+func TestFromFS(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows":
+		if canWriteFile(t, "NUL") {
+			t.Errorf("can unexpectedly write a file named NUL on Windows")
+		}
+		if canWriteFile(t, "nul.txt") {
+			fspathtests = append(fspathtests, winreservedextpathtests...)
+		} else {
+			winreservedpathtests = append(winreservedpathtests, winreservedextpathtests...)
+		}
+		for i := range winreservedpathtests {
+			winreservedpathtests[i].result = invalid
+		}
+		for i := range fspathtests {
+			fspathtests[i].result = filepath.FromSlash(fspathtests[i].result)
+		}
+	case "plan9":
+		for i := range plan9reservedpathtests {
+			plan9reservedpathtests[i].result = invalid
+		}
+	}
+	tests := fspathtests
+	tests = append(tests, winreservedpathtests...)
+	tests = append(tests, plan9reservedpathtests...)
+	for _, test := range tests {
+		got, err := safefilepath.FromFS(test.path)
+		if (got == "") != (err != nil) {
+			t.Errorf(`FromFS(%q) = %q, %v; want "" only if err != nil`, test.path, got, err)
+		}
+		if got != test.result {
+			t.Errorf("FromFS(%q) = %q, %v; want %q", test.path, got, err, test.result)
+		}
+	}
+}
+
+func canWriteFile(t *testing.T, name string) bool {
+	path := filepath.Join(t.TempDir(), name)
+	os.WriteFile(path, []byte("ok"), 0666)
+	b, _ := os.ReadFile(path)
+	return string(b) == "ok"
+}

src/internal/safefilepath/path_windows.go

@@ -0,0 +1,95 @@
+// Copyright 2022 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 safefilepath
+
+import (
+	"syscall"
+	"unicode/utf8"
+)
+
+func fromFS(path string) (string, error) {
+	if !utf8.ValidString(path) {
+		return "", errInvalidPath
+	}
+	for len(path) > 1 && path[0] == '/' && path[1] == '/' {
+		path = path[1:]
+	}
+	containsSlash := false
+	for p := path; p != ""; {
+		// Find the next path element.
+		i := 0
+		dot := -1
+		for i < len(p) && p[i] != '/' {
+			switch p[i] {
+			case 0, '\\', ':':
+				return "", errInvalidPath
+			case '.':
+				if dot < 0 {
+					dot = i
+				}
+			}
+			i++
+		}
+		part := p[:i]
+		if i < len(p) {
+			containsSlash = true
+			p = p[i+1:]
+		} else {
+			p = ""
+		}
+		// Trim the extension and look for a reserved name.
+		base := part
+		if dot >= 0 {
+			base = part[:dot]
+		}
+		if isReservedName(base) {
+			if dot < 0 {
+				return "", errInvalidPath
+			}
+			// The path element is a reserved name with an extension.
+			// Some Windows versions consider this a reserved name,
+			// while others do not. Use FullPath to see if the name is
+			// reserved.
+			if p, _ := syscall.FullPath(part); len(p) >= 4 && p[:4] == `\\.\` {
+				return "", errInvalidPath
+			}
+		}
+	}
+	if containsSlash {
+		// We can't depend on strings, so substitute \ for / manually.
+		buf := []byte(path)
+		for i, b := range buf {
+			if b == '/' {
+				buf[i] = '\\'
+			}
+		}
+		path = string(buf)
+	}
+	return path, nil
+}
+
+// isReservedName reports if name is a Windows reserved device name.
+// It does not detect names with an extension, which are also reserved on some Windows versions.
+//
+// For details, search for PRN in
+// https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file.
+func isReservedName(name string) bool {
+	if 3 <= len(name) && len(name) <= 4 {
+		switch string([]byte{toUpper(name[0]), toUpper(name[1]), toUpper(name[2])}) {
+		case "CON", "PRN", "AUX", "NUL":
+			return len(name) == 3
+		case "COM", "LPT":
+			return len(name) == 4 && '1' <= name[3] && name[3] <= '9'
+		}
+	}
+	return false
+}
+
+func toUpper(c byte) byte {
+	if 'a' <= c && c <= 'z' {
+		return c - ('a' - 'A')
+	}
+	return c
+}

src/net/http/fs.go

@@ -9,6 +9,7 @@ package http
 import (
 	"errors"
 	"fmt"
+	"internal/safefilepath"
 	"io"
 	"io/fs"
 	"mime"
@@ -69,14 +70,15 @@ func mapOpenError(originalErr error, name string, sep rune, stat func(string) (f
 // Open implements FileSystem using os.Open, opening files for reading rooted
 // and relative to the directory d.
 func (d Dir) Open(name string) (File, error) {
-	if filepath.Separator != '/' && strings.ContainsRune(name, filepath.Separator) {
-		return nil, errors.New("http: invalid character in file path")
+	path, err := safefilepath.FromFS(path.Clean("/" + name))
+	if err != nil {
+		return nil, errors.New("http: invalid or unsafe file path")
 	}
 	dir := string(d)
 	if dir == "" {
 		dir = "."
 	}
-	fullName := filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name)))
+	fullName := filepath.Join(dir, path)
 	f, err := os.Open(fullName)
 	if err != nil {
 		return nil, mapOpenError(err, fullName, filepath.Separator, os.Stat)

src/net/http/fs_test.go

@@ -745,6 +745,25 @@ func testFileServerZeroByte(t *testing.T, mode testMode) {
 	}
 }
 
+func TestFileServerNamesEscape(t *testing.T) { run(t, testFileServerNamesEscape) }
+func testFileServerNamesEscape(t *testing.T, mode testMode) {
+	ts := newClientServerTest(t, mode, FileServer(Dir("testdata"))).ts
+	for _, path := range []string{
+		"/../testdata/file",
+		"/NUL", // don't read from device files on Windows
+	} {
+		res, err := ts.Client().Get(ts.URL + path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		res.Body.Close()
+		if res.StatusCode < 400 || res.StatusCode > 599 {
+			t.Errorf("Get(%q): got status %v, want 4xx or 5xx", path, res.StatusCode)
+		}
+
+	}
+}
+
 type fakeFileInfo struct {
 	dir      bool
 	basename string

src/os/file.go

@@ -42,6 +42,7 @@ package os
 import (
 	"errors"
 	"internal/poll"
+	"internal/safefilepath"
 	"internal/testlog"
 	"io"
 	"io/fs"
@@ -595,6 +596,8 @@ func (f *File) SyscallConn() (syscall.RawConn, error) {
 // a general substitute for a chroot-style security mechanism when the directory tree
 // contains arbitrary content.
 //
+// The directory dir must not be "".
+//
 // The result implements fs.StatFS.
 func DirFS(dir string) fs.FS {
 	return dirFS(dir)
@@ -615,10 +618,11 @@ func containsAny(s, chars string) bool {
 type dirFS string
 
 func (dir dirFS) Open(name string) (fs.File, error) {
-	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
-		return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
+	fullname, err := dir.join(name)
+	if err != nil {
+		return nil, &PathError{Op: "stat", Path: name, Err: err}
 	}
-	f, err := Open(dir.join(name))
+	f, err := Open(fullname)
 	if err != nil {
 		// DirFS takes a string appropriate for GOOS,
 		// while the name argument here is always slash separated.
@@ -631,10 +635,11 @@ func (dir dirFS) Open(name string) (fs.File, error) {
 }
 
 func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
-	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
-		return nil, &PathError{Op: "stat", Path: name, Err: ErrInvalid}
+	fullname, err := dir.join(name)
+	if err != nil {
+		return nil, &PathError{Op: "stat", Path: name, Err: err}
 	}
-	f, err := Stat(dir.join(name))
+	f, err := Stat(fullname)
 	if err != nil {
 		// See comment in dirFS.Open.
 		err.(*PathError).Path = name
@@ -643,19 +648,22 @@ func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
 	return f, nil
 }
 
-// join returns the path for name in dir. We can't always use "/"
-// because that fails on Windows for UNC paths.
-func (dir dirFS) join(name string) string {
-	if runtime.GOOS == "windows" && containsAny(name, "/") {
-		buf := []byte(name)
-		for i, b := range buf {
-			if b == '/' {
-				buf[i] = '\\'
-			}
-		}
-		name = string(buf)
+// join returns the path for name in dir.
+func (dir dirFS) join(name string) (string, error) {
+	if dir == "" {
+		return "", errors.New("os: DirFS with empty root")
 	}
-	return string(dir) + string(PathSeparator) + name
+	if !fs.ValidPath(name) {
+		return "", ErrInvalid
+	}
+	name, err := safefilepath.FromFS(name)
+	if err != nil {
+		return "", ErrInvalid
+	}
+	if IsPathSeparator(dir[len(dir)-1]) {
+		return string(dir) + name, nil
+	}
+	return string(dir) + string(PathSeparator) + name, nil
 }
 
 // ReadFile reads the named file and returns the contents.

src/os/os_test.go

@@ -2729,6 +2729,44 @@ func TestDirFS(t *testing.T) {
 	if err == nil {
 		t.Fatalf(`Open testdata\dirfs succeeded`)
 	}
+
+	// Test that Open does not open Windows device files.
+	_, err = d.Open(`NUL`)
+	if err == nil {
+		t.Errorf(`Open NUL succeeded`)
+	}
+}
+
+func TestDirFSRootDir(t *testing.T) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cwd = cwd[len(filepath.VolumeName(cwd)):] // trim volume prefix (C:) on Windows
+	cwd = filepath.ToSlash(cwd)               // convert \ to /
+	cwd = strings.TrimPrefix(cwd, "/")        // trim leading /
+
+	// Test that Open can open a path starting at /.
+	d := DirFS("/")
+	f, err := d.Open(cwd + "/testdata/dirfs/a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+}
+
+func TestDirFSEmptyDir(t *testing.T) {
+	d := DirFS("")
+	cwd, _ := os.Getwd()
+	for _, path := range []string{
+		"testdata/dirfs/a",                          // not DirFS(".")
+		filepath.ToSlash(cwd) + "/testdata/dirfs/a", // not DirFS("/")
+	} {
+		_, err := d.Open(path)
+		if err == nil {
+			t.Fatalf(`DirFS("").Open(%q) succeeded`, path)
+		}
+	}
 }
 
 func TestDirFSPathsValid(t *testing.T) {