reflect: zero stack slots before writing to them with write barriers

reflect.assignTo writes to the target using write barriers. Make sure
that the memory it is writing to is zeroed, so the write barrier does
not read pointers from uninitialized memory.

Fixes #39541

Change-Id: Ia64b2cacc193bffd0c1396bbce1dfb8182d4905b
Reviewed-on: https://go-review.googlesource.com/c/go/+/238760
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/type.go

@@ -3068,6 +3068,7 @@ func ifaceIndir(t *rtype) bool {
 	return t.kind&kindDirectIface == 0
 }
 
+// Note: this type must agree with runtime.bitvector.
 type bitVector struct {
 	n    uint32 // number of bits
 	data []byte

src/reflect/value.go

@@ -589,6 +589,13 @@ func callReflect(ctxt *makeFuncImpl, frame unsafe.Pointer, retValid *bool) {
 			// Convert v to type typ if v is assignable to a variable
 			// of type t in the language spec.
 			// See issue 28761.
+			if typ.Kind() == Interface {
+				// We must clear the destination before calling assignTo,
+				// in case assignTo writes (with memory barriers) to the
+				// target location used as scratch space. See issue 39541.
+				*(*uintptr)(addr) = 0
+				*(*uintptr)(add(addr, ptrSize, "typ.size == 2*ptrSize")) = 0
+			}
 			v = v.assignTo("reflect.MakeFunc", typ, addr)
 
 			// We are writing to stack. No write barrier.
@@ -2381,6 +2388,7 @@ func NewAt(typ Type, p unsafe.Pointer) Value {
 // assignTo returns a value v that can be assigned directly to typ.
 // It panics if v is not assignable to typ.
 // For a conversion to an interface type, target is a suggested scratch space to use.
+// target must be initialized memory (or nil).
 func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
 	if v.flag&flagMethod != 0 {
 		v = makeMethodValue(context, v)

src/runtime/stack.go

@@ -556,6 +556,7 @@ func adjustpointer(adjinfo *adjustinfo, vpp unsafe.Pointer) {
 }
 
 // Information from the compiler about the layout of stack frames.
+// Note: this type must agree with reflect.bitVector.
 type bitvector struct {
 	n        int32 // # of bits
 	bytedata *uint8

test/fixedbugs/issue39541.go

@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2020 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 main
+
+import "reflect"
+
+func sub(args []reflect.Value) []reflect.Value {
+	type A struct {
+		s int
+		t int
+	}
+	return []reflect.Value{reflect.ValueOf(A{1, 2})}
+}
+
+func main() {
+	f := reflect.MakeFunc(reflect.TypeOf((func() interface{})(nil)), sub).Interface().(func() interface{})
+	c := make(chan bool, 100)
+	for i := 0; i < 100; i++ {
+		go func() {
+			for j := 0; j < 10000; j++ {
+				f()
+			}
+			c <- true
+		}()
+	}
+	for i := 0; i < 100; i++ {
+		<-c
+	}
+}