reflect: make Elem panic on bad notinheap pointers

This CL fixes the subtle issue that Elem can promote a
not-in-heap pointer, which could be any bit pattern, into an
unsafe.Pointer, which the garbage collector can see. If that
resulting value is bad, it can crash the GC.

Make sure that we don't introduce bad pointers that way. We can
make Elem() panic, because any such bad pointers are in the Go heap,
and not-in-heap pointers are not allowed to point into the Go heap.

Update #48399

Change-Id: Ieaf35a611b16b4dfb5e907e229ed4a2aed30e18c
Reviewed-on: https://go-review.googlesource.com/c/go/+/350153
Trust: Keith Randall <khr@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/all_test.go

@@ -7697,3 +7697,23 @@ func TestSetIter(t *testing.T) {
 		t.Errorf("pointer incorrect: got %d want %d", got, b)
 	}
 }
+
+//go:notinheap
+type nih struct{ x int }
+
+var global_nih = nih{x: 7}
+
+func TestNotInHeapDeref(t *testing.T) {
+	// See issue 48399.
+	v := ValueOf((*nih)(nil))
+	v.Elem()
+	shouldPanic("reflect: call of reflect.Value.Field on zero Value", func() { v.Elem().Field(0) })
+
+	v = ValueOf(&global_nih)
+	if got := v.Elem().Field(0).Int(); got != 7 {
+		t.Fatalf("got %d, want 7", got)
+	}
+
+	v = ValueOf((*nih)(unsafe.Pointer(new(int))))
+	shouldPanic("reflect: reflect.Value.Elem on an invalid notinheap pointer", func() { v.Elem() })
+}

src/reflect/value.go

@@ -1169,6 +1169,21 @@ func (v Value) Elem() Value {
 	case Ptr:
 		ptr := v.ptr
 		if v.flag&flagIndir != 0 {
+			if ifaceIndir(v.typ) {
+				// This is a pointer to a not-in-heap object. ptr points to a uintptr
+				// in the heap. That uintptr is the address of a not-in-heap object.
+				// In general, pointers to not-in-heap objects can be total junk.
+				// But Elem() is asking to dereference it, so the user has asserted
+				// that at least it is a valid pointer (not just an integer stored in
+				// a pointer slot). So let's check, to make sure that it isn't a pointer
+				// that the runtime will crash on if it sees it during GC or write barriers.
+				// Since it is a not-in-heap pointer, all pointers to the heap are
+				// forbidden! That makes the test pretty easy.
+				// See issue 48399.
+				if !verifyNotInHeapPtr(*(*uintptr)(ptr)) {
+					panic("reflect: reflect.Value.Elem on an invalid notinheap pointer")
+				}
+			}
 			ptr = *(*unsafe.Pointer)(ptr)
 		}
 		// The returned value's address is v's value.
@@ -3406,6 +3421,8 @@ func typedslicecopy(elemType *rtype, dst, src unsafeheader.Slice) int
 //go:noescape
 func typehash(t *rtype, p unsafe.Pointer, h uintptr) uintptr
 
+func verifyNotInHeapPtr(p uintptr) bool
+
 // Dummy annotation marking that the value x escapes,
 // for use in cases where the reflect code is so clever that
 // the compiler cannot follow.

src/runtime/mbitmap.go

@@ -417,6 +417,15 @@ func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex ui
 	return
 }
 
+// verifyNotInHeapPtr reports whether converting the not-in-heap pointer into a unsafe.Pointer is ok.
+//go:linkname reflect_verifyNotInHeapPtr reflect.verifyNotInHeapPtr
+func reflect_verifyNotInHeapPtr(p uintptr) bool {
+	// Conversion to a pointer is ok as long as findObject above does not call badPointer.
+	// Since we're already promised that p doesn't point into the heap, just disallow heap
+	// pointers and the special clobbered pointer.
+	return spanOf(p) == nil && p != clobberdeadPtr
+}
+
 // next returns the heapBits describing the next pointer-sized word in memory.
 // That is, if h describes address p, h.next() describes p+ptrSize.
 // Note that next does not modify h. The caller must record the result.