[release-branch.go1.12] os: pass correct environment when creating Windows processes

This is CVE-2019-11888.

Previously, passing a nil environment but a non-nil token would result
in the new potentially unprivileged process inheriting the parent
potentially privileged environment, or would result in the new
potentially privileged process inheriting the parent potentially
unprivileged environment. Either way, it's bad. In the former case, it's
an infoleak. In the latter case, it's a possible EoP, since things like
PATH could be overwritten.

Not specifying an environment currently means, "use the existing
environment". This commit amends the behavior to be, "use the existing
environment of the token the process is being created for." The behavior
therefore stays the same when creating processes without specifying a
token. And it does the correct thing when creating processes when
specifying a token.

Updates #32000
Fixes #32081

Change-Id: Ib4a90cfffb6ba866c855f66f1313372fdd34ce41
Reviewed-on: https://go-review.googlesource.com/c/go/+/177538
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

5 files changed, 92 insertions, 1 deletions

diff --git a/src/internal/syscall/windows/syscall_windows.go b/src/internal/syscall/windows/syscall_windows.go
index 121132f6f7..099e91ed68 100644
--- a/src/internal/syscall/windows/syscall_windows.go
+++ b/src/internal/syscall/windows/syscall_windows.go
@@ -305,3 +305,6 @@ const (
 func LoadGetFinalPathNameByHandle() error {
 	return procGetFinalPathNameByHandleW.Find()
 }
+
+//sys	CreateEnvironmentBlock(block **uint16, token syscall.Token, inheritExisting bool) (err error) = userenv.CreateEnvironmentBlock
+//sys	DestroyEnvironmentBlock(block *uint16) (err error) = userenv.DestroyEnvironmentBlock
diff --git a/src/internal/syscall/windows/zsyscall_windows.go b/src/internal/syscall/windows/zsyscall_windows.go
index 9527a370a4..ca5b4e6f16 100644
--- a/src/internal/syscall/windows/zsyscall_windows.go
+++ b/src/internal/syscall/windows/zsyscall_windows.go
@@ -58,6 +58,8 @@ var (
 	procNetShareAdd                  = modnetapi32.NewProc("NetShareAdd")
 	procNetShareDel                  = modnetapi32.NewProc("NetShareDel")
 	procGetFinalPathNameByHandleW    = modkernel32.NewProc("GetFinalPathNameByHandleW")
+	procCreateEnvironmentBlock       = moduserenv.NewProc("CreateEnvironmentBlock")
+	procDestroyEnvironmentBlock      = moduserenv.NewProc("DestroyEnvironmentBlock")
 	procImpersonateSelf              = modadvapi32.NewProc("ImpersonateSelf")
 	procRevertToSelf                 = modadvapi32.NewProc("RevertToSelf")
 	procOpenThreadToken              = modadvapi32.NewProc("OpenThreadToken")
@@ -220,6 +222,36 @@ func GetFinalPathNameByHandle(file syscall.Handle, filePath *uint16, filePathSiz
 	return
 }
 
+func CreateEnvironmentBlock(block **uint16, token syscall.Token, inheritExisting bool) (err error) {
+	var _p0 uint32
+	if inheritExisting {
+		_p0 = 1
+	} else {
+		_p0 = 0
+	}
+	r1, _, e1 := syscall.Syscall(procCreateEnvironmentBlock.Addr(), 3, uintptr(unsafe.Pointer(block)), uintptr(token), uintptr(_p0))
+	if r1 == 0 {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}
+
+func DestroyEnvironmentBlock(block *uint16) (err error) {
+	r1, _, e1 := syscall.Syscall(procDestroyEnvironmentBlock.Addr(), 1, uintptr(unsafe.Pointer(block)), 0, 0)
+	if r1 == 0 {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}
+
 func ImpersonateSelf(impersonationlevel uint32) (err error) {
 	r1, _, e1 := syscall.Syscall(procImpersonateSelf.Addr(), 1, uintptr(impersonationlevel), 0, 0)
 	if r1 == 0 {
diff --git a/src/os/env_default.go b/src/os/env_default.go
new file mode 100644
index 0000000000..c11ccce7e3
--- /dev/null
+++ b/src/os/env_default.go
@@ -0,0 +1,13 @@
+// Copyright 2019 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.
+
+// +build !windows
+
+package os
+
+import "syscall"
+
+func environForSysProcAttr(sys *syscall.SysProcAttr) ([]string, error) {
+	return Environ(), nil
+}
diff --git a/src/os/env_windows.go b/src/os/env_windows.go
new file mode 100644
index 0000000000..e8f647e7ac
--- /dev/null
+++ b/src/os/env_windows.go
@@ -0,0 +1,40 @@
+// Copyright 2019 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 os
+
+import (
+	"internal/syscall/windows"
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
+
+func environForSysProcAttr(sys *syscall.SysProcAttr) (env []string, err error) {
+	if sys == nil || sys.Token == 0 {
+		return Environ(), nil
+	}
+	var block *uint16
+	err = windows.CreateEnvironmentBlock(&block, sys.Token, false)
+	if err != nil {
+		return nil, err
+	}
+	defer windows.DestroyEnvironmentBlock(block)
+	blockp := uintptr(unsafe.Pointer(block))
+	for {
+		entry := (*[(1 << 30) - 1]uint16)(unsafe.Pointer(blockp))[:]
+		for i, v := range entry {
+			if v == 0 {
+				entry = entry[:i]
+				break
+			}
+		}
+		if len(entry) == 0 {
+			break
+		}
+		env = append(env, string(utf16.Decode(entry)))
+		blockp += 2 * (uintptr(len(entry)) + 1)
+	}
+	return
+}
diff --git a/src/os/exec_posix.go b/src/os/exec_posix.go
index 4c8261295c..6f888b7e29 100644
--- a/src/os/exec_posix.go
+++ b/src/os/exec_posix.go
@@ -38,7 +38,10 @@ func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err e
 		Sys: attr.Sys,
 	}
 	if sysattr.Env == nil {
-		sysattr.Env = Environ()
+		sysattr.Env, err = environForSysProcAttr(sysattr.Sys)
+		if err != nil {
+			return nil, err
+		}
 	}
 	for _, f := range attr.Files {
 		sysattr.Files = append(sysattr.Files, f.Fd())