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From 3746d930467a486fb714a885ad92ebe16cf33c2a Mon Sep 17 00:00:00 2001
From: Jason A. Donenfeld <Jason@zx2c4.com>
Date: Sun, 12 May 2019 14:34:30 +0200
Subject: [PATCH] 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.

Change-Id: Ia57f6e89b97bdbaf7274d6a89c1d9948b6d40ef5
---

diff --git a/src/internal/syscall/windows/syscall_windows.go b/src/internal/syscall/windows/syscall_windows.go
index 121132f..099e91e 100644
--- a/src/internal/syscall/windows/syscall_windows.go
+++ b/src/internal/syscall/windows/syscall_windows.go
@@ -305,3 +305,6 @@
 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 9527a37..ca5b4e6 100644
--- a/src/internal/syscall/windows/zsyscall_windows.go
+++ b/src/internal/syscall/windows/zsyscall_windows.go
@@ -58,6 +58,8 @@
 	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 @@
 	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 0000000..91c5e9a
--- /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) (env []string) {
+	return Environ()
+}
diff --git a/src/os/env_windows.go b/src/os/env_windows.go
new file mode 100644
index 0000000..76cf011
--- /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) {
+	if sys == nil || sys.Token == 0 {
+		return Environ()
+	}
+	var block *uint16
+	err := windows.CreateEnvironmentBlock(&block, sys.Token, false)
+	if err != nil {
+		return nil
+	}
+	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)
+	}
+	windows.DestroyEnvironmentBlock(block)
+	return
+}
diff --git a/src/os/exec_posix.go b/src/os/exec_posix.go
index 7b1ef67..651d24d 100644
--- a/src/os/exec_posix.go
+++ b/src/os/exec_posix.go
@@ -38,7 +38,7 @@
 		Sys: attr.Sys,
 	}
 	if sysattr.Env == nil {
-		sysattr.Env = Environ()
+		sysattr.Env = environForSysProcAttr(sysattr.Sys)
 	}
 	sysattr.Files = make([]uintptr, 0, len(attr.Files))
 	for _, f := range attr.Files {