crypto/rand,runtime: switch RtlGenRandom for ProcessPrng

RtlGenRandom is a semi-undocumented API, also known as
SystemFunction036, which we use to generate random data on Windows.
It's definition, in cryptbase.dll, is an opaque wrapper for the
documented API ProcessPrng. Instead of using RtlGenRandom, switch to
using ProcessPrng, since the former is simply a wrapper for the latter,
there should be no practical change on the user side, other than a minor
change in the DLLs we load.

Change-Id: Ie6891bf97b1d47f5368cccbe92f374dba2c2672a
Reviewed-on: https://go-review.googlesource.com/c/go/+/536235
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Quim Muntal <[email protected]>
Auto-Submit: Roland Shoemaker <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

src/crypto/rand/rand.go

@@ -15,7 +15,7 @@ import "io"
 // available, /dev/urandom otherwise.
 // On OpenBSD and macOS, Reader uses getentropy(2).
 // On other Unix-like systems, Reader reads from /dev/urandom.
-// On Windows systems, Reader uses the RtlGenRandom API.
+// On Windows systems, Reader uses the ProcessPrng API.
 // On JS/Wasm, Reader uses the Web Crypto API.
 // On WASIP1/Wasm, Reader uses random_get from wasi_snapshot_preview1.
 var Reader io.Reader

src/crypto/rand/rand_windows.go

@@ -15,11 +15,8 @@ func init() { Reader = &rngReader{} }
 
 type rngReader struct{}
 
-func (r *rngReader) Read(b []byte) (n int, err error) {
-	// RtlGenRandom only returns 1<<32-1 bytes at a time. We only read at
-	// most 1<<31-1 bytes at a time so that  this works the same on 32-bit
-	// and 64-bit systems.
-	if err := batched(windows.RtlGenRandom, 1<<31-1)(b); err != nil {
+func (r *rngReader) Read(b []byte) (int, error) {
+	if err := windows.ProcessPrng(b); err != nil {
 		return 0, err
 	}
 	return len(b), nil

src/internal/syscall/windows/syscall_windows.go

@@ -373,7 +373,7 @@ func ErrorLoadingGetTempPath2() error {
 //sys	DestroyEnvironmentBlock(block *uint16) (err error) = userenv.DestroyEnvironmentBlock
 //sys	CreateEvent(eventAttrs *SecurityAttributes, manualReset uint32, initialState uint32, name *uint16) (handle syscall.Handle, err error) = kernel32.CreateEventW
 
-//sys	RtlGenRandom(buf []byte) (err error) = advapi32.SystemFunction036
+//sys	ProcessPrng(buf []byte) (err error) = bcryptprimitives.ProcessPrng
 
 type FILE_ID_BOTH_DIR_INFO struct {
 	NextEntryOffset uint32

src/runtime/os_windows.go

@@ -127,15 +127,8 @@ var (
 	_WriteFile,
 	_ stdFunction
 
-	// Use RtlGenRandom to generate cryptographically random data.
-	// This approach has been recommended by Microsoft (see issue
-	// 15589 for details).
-	// The RtlGenRandom is not listed in advapi32.dll, instead
-	// RtlGenRandom function can be found by searching for SystemFunction036.
-	// Also some versions of Mingw cannot link to SystemFunction036
-	// when building executable as Cgo. So load SystemFunction036
-	// manually during runtime startup.
-	_RtlGenRandom stdFunction
+	// Use ProcessPrng to generate cryptographically random data.
+	_ProcessPrng stdFunction
 
 	// Load ntdll.dll manually during startup, otherwise Mingw
 	// links wrong printf function to cgo executable (see issue
@@ -151,11 +144,11 @@ var (
 )
 
 var (
-	advapi32dll = [...]uint16{'a', 'd', 'v', 'a', 'p', 'i', '3', '2', '.', 'd', 'l', 'l', 0}
-	ntdlldll    = [...]uint16{'n', 't', 'd', 'l', 'l', '.', 'd', 'l', 'l', 0}
-	powrprofdll = [...]uint16{'p', 'o', 'w', 'r', 'p', 'r', 'o', 'f', '.', 'd', 'l', 'l', 0}
-	winmmdll    = [...]uint16{'w', 'i', 'n', 'm', 'm', '.', 'd', 'l', 'l', 0}
-	ws2_32dll   = [...]uint16{'w', 's', '2', '_', '3', '2', '.', 'd', 'l', 'l', 0}
+	bcryptprimitivesdll = [...]uint16{'b', 'c', 'r', 'y', 'p', 't', 'p', 'r', 'i', 'm', 'i', 't', 'i', 'v', 'e', 's', '.', 'd', 'l', 'l', 0}
+	ntdlldll            = [...]uint16{'n', 't', 'd', 'l', 'l', '.', 'd', 'l', 'l', 0}
+	powrprofdll         = [...]uint16{'p', 'o', 'w', 'r', 'p', 'r', 'o', 'f', '.', 'd', 'l', 'l', 0}
+	winmmdll            = [...]uint16{'w', 'i', 'n', 'm', 'm', '.', 'd', 'l', 'l', 0}
+	ws2_32dll           = [...]uint16{'w', 's', '2', '_', '3', '2', '.', 'd', 'l', 'l', 0}
 )
 
 // Function to be called by windows CreateThread
@@ -251,11 +244,11 @@ func windowsLoadSystemLib(name []uint16) uintptr {
 }
 
 func loadOptionalSyscalls() {
-	a32 := windowsLoadSystemLib(advapi32dll[:])
-	if a32 == 0 {
-		throw("advapi32.dll not found")
+	bcryptPrimitives := windowsLoadSystemLib(bcryptprimitivesdll[:])
+	if bcryptPrimitives == 0 {
+		throw("bcryptprimitives.dll not found")
 	}
-	_RtlGenRandom = windowsFindfunc(a32, []byte("SystemFunction036\000"))
+	_ProcessPrng = windowsFindfunc(bcryptPrimitives, []byte("ProcessPrng\000"))
 
 	n32 := windowsLoadSystemLib(ntdlldll[:])
 	if n32 == 0 {
@@ -528,7 +521,7 @@ func osinit() {
 //go:nosplit
 func getRandomData(r []byte) {
 	n := 0
-	if stdcall2(_RtlGenRandom, uintptr(unsafe.Pointer(&r[0])), uintptr(len(r)))&0xff != 0 {
+	if stdcall2(_ProcessPrng, uintptr(unsafe.Pointer(&r[0])), uintptr(len(r)))&0xff != 0 {
 		n = len(r)
 	}
 	extendRandom(r, n)