runtime: safely load DLLs

While many other call sites have been moved to using the proper
higher-level system loading, these areas were left out. This prevents
DLL directory injection attacks. This includes both the runtime load
calls (using LoadLibrary prior) and the implicitly linked ones via
cgo_import_dynamic, which we move to our LoadLibraryEx. The goal is to
only loosely load kernel32.dll and strictly load all others.

Meanwhile we make sure that we never fallback to insecure loading on
older or unpatched systems.

This is CVE-2019-9634.

Fixes #14959
Fixes #28978
Fixes #30642

Change-Id: I401a13ed8db248ab1bb5039bf2d31915cac72b93
Reviewed-on: https://go-review.googlesource.com/c/go/+/165798
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/runtime/os_windows.go

@@ -29,6 +29,7 @@ const (
 //go:cgo_import_dynamic runtime._GetProcessAffinityMask GetProcessAffinityMask%3 "kernel32.dll"
 //go:cgo_import_dynamic runtime._GetQueuedCompletionStatus GetQueuedCompletionStatus%5 "kernel32.dll"
 //go:cgo_import_dynamic runtime._GetStdHandle GetStdHandle%1 "kernel32.dll"
+//go:cgo_import_dynamic runtime._GetSystemDirectoryA GetSystemDirectoryA%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._GetSystemInfo GetSystemInfo%1 "kernel32.dll"
 //go:cgo_import_dynamic runtime._GetThreadContext GetThreadContext%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._LoadLibraryW LoadLibraryW%1 "kernel32.dll"
@@ -47,12 +48,9 @@ const (
 //go:cgo_import_dynamic runtime._VirtualAlloc VirtualAlloc%4 "kernel32.dll"
 //go:cgo_import_dynamic runtime._VirtualFree VirtualFree%3 "kernel32.dll"
 //go:cgo_import_dynamic runtime._VirtualQuery VirtualQuery%3 "kernel32.dll"
-//go:cgo_import_dynamic runtime._WSAGetOverlappedResult WSAGetOverlappedResult%5 "ws2_32.dll"
 //go:cgo_import_dynamic runtime._WaitForSingleObject WaitForSingleObject%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._WriteConsoleW WriteConsoleW%5 "kernel32.dll"
 //go:cgo_import_dynamic runtime._WriteFile WriteFile%5 "kernel32.dll"
-//go:cgo_import_dynamic runtime._timeBeginPeriod timeBeginPeriod%1 "winmm.dll"
-//go:cgo_import_dynamic runtime._timeEndPeriod timeEndPeriod%1 "winmm.dll"
 
 type stdFunction unsafe.Pointer
 
@@ -75,6 +73,7 @@ var (
 	_GetProcessAffinityMask,
 	_GetQueuedCompletionStatus,
 	_GetStdHandle,
+	_GetSystemDirectoryA,
 	_GetSystemInfo,
 	_GetSystemTimeAsFileTime,
 	_GetThreadContext,
@@ -96,19 +95,17 @@ var (
 	_VirtualAlloc,
 	_VirtualFree,
 	_VirtualQuery,
-	_WSAGetOverlappedResult,
 	_WaitForSingleObject,
 	_WriteConsoleW,
 	_WriteFile,
-	_timeBeginPeriod,
-	_timeEndPeriod,
 	_ stdFunction
 
 	// Following syscalls are only available on some Windows PCs.
 	// We will load syscalls, if available, before using them.
 	_AddDllDirectory,
 	_AddVectoredContinueHandler,
 	_GetQueuedCompletionStatusEx,
+	_LoadLibraryExA,
 	_LoadLibraryExW,
 	_ stdFunction
 
@@ -126,6 +123,12 @@ var (
 	// links wrong printf function to cgo executable (see issue
 	// 12030 for details).
 	_NtWaitForSingleObject stdFunction
+
+	// These are from non-kernel32.dll, so we prefer to LoadLibraryEx them.
+	_timeBeginPeriod,
+	_timeEndPeriod,
+	_WSAGetOverlappedResult,
+	_ stdFunction
 )
 
 // Function to be called by windows CreateThread
@@ -173,6 +176,26 @@ func windowsFindfunc(lib uintptr, name []byte) stdFunction {
 	return stdFunction(unsafe.Pointer(f))
 }
 
+var sysDirectory [521]byte
+var sysDirectoryLen uintptr
+
+func windowsLoadSystemLib(name []byte) uintptr {
+	if useLoadLibraryEx {
+		return stdcall3(_LoadLibraryExA, uintptr(unsafe.Pointer(&name[0])), 0, _LOAD_LIBRARY_SEARCH_SYSTEM32)
+	} else {
+		if sysDirectoryLen == 0 {
+			l := stdcall2(_GetSystemDirectoryA, uintptr(unsafe.Pointer(&sysDirectory[0])), uintptr(len(sysDirectory)-1))
+			if l == 0 || l > uintptr(len(sysDirectory)-1) {
+				throw("Unable to determine system directory")
+			}
+			sysDirectory[l] = '\\'
+			sysDirectoryLen = l + 1
+		}
+		absName := append(sysDirectory[:sysDirectoryLen], name...)
+		return stdcall1(_LoadLibraryA, uintptr(unsafe.Pointer(&absName[0])))
+	}
+}
+
 func loadOptionalSyscalls() {
 	var kernel32dll = []byte("kernel32.dll\000")
 	k32 := stdcall1(_LoadLibraryA, uintptr(unsafe.Pointer(&kernel32dll[0])))
@@ -182,17 +205,19 @@ func loadOptionalSyscalls() {
 	_AddDllDirectory = windowsFindfunc(k32, []byte("AddDllDirectory\000"))
 	_AddVectoredContinueHandler = windowsFindfunc(k32, []byte("AddVectoredContinueHandler\000"))
 	_GetQueuedCompletionStatusEx = windowsFindfunc(k32, []byte("GetQueuedCompletionStatusEx\000"))
+	_LoadLibraryExA = windowsFindfunc(k32, []byte("LoadLibraryExA\000"))
 	_LoadLibraryExW = windowsFindfunc(k32, []byte("LoadLibraryExW\000"))
+	useLoadLibraryEx = (_LoadLibraryExW != nil && _LoadLibraryExA != nil && _AddDllDirectory != nil)
 
 	var advapi32dll = []byte("advapi32.dll\000")
-	a32 := stdcall1(_LoadLibraryA, uintptr(unsafe.Pointer(&advapi32dll[0])))
+	a32 := windowsLoadSystemLib(advapi32dll)
 	if a32 == 0 {
 		throw("advapi32.dll not found")
 	}
 	_RtlGenRandom = windowsFindfunc(a32, []byte("SystemFunction036\000"))
 
 	var ntdll = []byte("ntdll.dll\000")
-	n32 := stdcall1(_LoadLibraryA, uintptr(unsafe.Pointer(&ntdll[0])))
+	n32 := windowsLoadSystemLib(ntdll)
 	if n32 == 0 {
 		throw("ntdll.dll not found")
 	}
@@ -205,6 +230,27 @@ func loadOptionalSyscalls() {
 		}
 	}
 
+	var winmmdll = []byte("winmm.dll\000")
+	m32 := windowsLoadSystemLib(winmmdll)
+	if m32 == 0 {
+		throw("winmm.dll not found")
+	}
+	_timeBeginPeriod = windowsFindfunc(m32, []byte("timeBeginPeriod\000"))
+	_timeEndPeriod = windowsFindfunc(m32, []byte("timeEndPeriod\000"))
+	if _timeBeginPeriod == nil || _timeEndPeriod == nil {
+		throw("timeBegin/EndPeriod not found")
+	}
+
+	var ws232dll = []byte("ws2_32.dll\000")
+	ws232 := windowsLoadSystemLib(ws232dll)
+	if ws232 == 0 {
+		throw("ws2_32.dll not found")
+	}
+	_WSAGetOverlappedResult = windowsFindfunc(ws232, []byte("WSAGetOverlappedResult\000"))
+	if _WSAGetOverlappedResult == nil {
+		throw("WSAGetOverlappedResult not found")
+	}
+
 	if windowsFindfunc(n32, []byte("wine_get_version\000")) != nil {
 		// running on Wine
 		initWine(k32)
@@ -311,8 +357,6 @@ func osinit() {
 
 	loadOptionalSyscalls()
 
-	useLoadLibraryEx = (_LoadLibraryExW != nil && _AddDllDirectory != nil)
-
 	disableWER()
 
 	initExceptionHandler()

src/runtime/syscall_windows.go

@@ -104,9 +104,13 @@ func compileCallback(fn eface, cleanstack bool) (code uintptr) {
 
 const _LOAD_LIBRARY_SEARCH_SYSTEM32 = 0x00000800
 
+// When available, this function will use LoadLibraryEx with the filename
+// parameter and the important SEARCH_SYSTEM32 argument. But on systems that
+// do not have that option, absoluteFilepath should contain a fallback
+// to the full path inside of system32 for use with vanilla LoadLibrary.
 //go:linkname syscall_loadsystemlibrary syscall.loadsystemlibrary
 //go:nosplit
-func syscall_loadsystemlibrary(filename *uint16) (handle, err uintptr) {
+func syscall_loadsystemlibrary(filename *uint16, absoluteFilepath *uint16) (handle, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
 	c := &getg().m.syscall
@@ -121,15 +125,9 @@ func syscall_loadsystemlibrary(filename *uint16) (handle, err uintptr) {
 		}{filename, 0, _LOAD_LIBRARY_SEARCH_SYSTEM32}
 		c.args = uintptr(noescape(unsafe.Pointer(&args)))
 	} else {
-		// User doesn't have KB2533623 installed. The caller
-		// wanted to only load the filename DLL from the
-		// System32 directory but that facility doesn't exist,
-		// so just load it the normal way. This is a potential
-		// security risk, but so is not installing security
-		// updates.
 		c.fn = getLoadLibrary()
 		c.n = 1
-		c.args = uintptr(noescape(unsafe.Pointer(&filename)))
+		c.args = uintptr(noescape(unsafe.Pointer(&absoluteFilepath)))
 	}
 
 	cgocall(asmstdcallAddr, unsafe.Pointer(c))

src/syscall/dll_windows.go

@@ -28,7 +28,7 @@ func Syscall12(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12 ui
 func Syscall15(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 uintptr) (r1, r2 uintptr, err Errno)
 func Syscall18(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18 uintptr) (r1, r2 uintptr, err Errno)
 func loadlibrary(filename *uint16) (handle uintptr, err Errno)
-func loadsystemlibrary(filename *uint16) (handle uintptr, err Errno)
+func loadsystemlibrary(filename *uint16, absoluteFilepath *uint16) (handle uintptr, err Errno)
 func getprocaddress(handle uintptr, procname *uint8) (proc uintptr, err Errno)
 
 // A DLL implements access to a single DLL.
@@ -37,6 +37,26 @@ type DLL struct {
 	Handle Handle
 }
 
+// We use this for computing the absolute path for system DLLs on systems
+// where SEARCH_SYSTEM32 is not available.
+var systemDirectoryPrefix string
+
+func init() {
+	n := uint32(MAX_PATH)
+	for {
+		b := make([]uint16, n)
+		l, e := getSystemDirectory(&b[0], n)
+		if e != nil {
+			panic("Unable to determine system directory: " + e.Error())
+		}
+		if l <= n {
+			systemDirectoryPrefix = UTF16ToString(b[:l]) + "\\"
+			break
+		}
+		n = l
+	}
+}
+
 // LoadDLL loads the named DLL file into memory.
 //
 // If name is not an absolute path and is not a known system DLL used by
@@ -53,7 +73,11 @@ func LoadDLL(name string) (*DLL, error) {
 	var h uintptr
 	var e Errno
 	if sysdll.IsSystemDLL[name] {
-		h, e = loadsystemlibrary(namep)
+		absoluteFilepathp, err := UTF16PtrFromString(systemDirectoryPrefix + name)
+		if err != nil {
+			return nil, err
+		}
+		h, e = loadsystemlibrary(namep, absoluteFilepathp)
 	} else {
 		h, e = loadlibrary(namep)
 	}

src/syscall/security_windows.go

@@ -290,6 +290,7 @@ type Tokenprimarygroup struct {
 //sys	OpenProcessToken(h Handle, access uint32, token *Token) (err error) = advapi32.OpenProcessToken
 //sys	GetTokenInformation(t Token, infoClass uint32, info *byte, infoLen uint32, returnedLen *uint32) (err error) = advapi32.GetTokenInformation
 //sys	GetUserProfileDirectory(t Token, dir *uint16, dirLen *uint32) (err error) = userenv.GetUserProfileDirectoryW
+//sys	getSystemDirectory(dir *uint16, dirLen uint32) (len uint32, err error) = kernel32.GetSystemDirectoryW
 
 // An access token contains the security information for a logon session.
 // The system creates an access token when a user logs on, and every