2 files changed, 62 insertions, 2 deletions
diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index b81e505866..39cdaebde7 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -153,7 +153,7 @@ import (
 //
 // JSON cannot represent cyclic data structures and Marshal does not
 // handle them. Passing cyclic structures to Marshal will result in
-// an infinite recursion.
+// an error.
 //
 func Marshal(v interface{}) ([]byte, error) {
 	e := newEncodeState()
@@ -285,17 +285,31 @@ var hex = "0123456789abcdef"
 type encodeState struct {
 	bytes.Buffer // accumulated output
 	scratch      [64]byte
+
+	// Keep track of what pointers we've seen in the current recursive call
+	// path, to avoid cycles that could lead to a stack overflow. Only do
+	// the relatively expensive map operations if ptrLevel is larger than
+	// startDetectingCyclesAfter, so that we skip the work if we're within a
+	// reasonable amount of nested pointers deep.
+	ptrLevel uint
+	ptrSeen  map[interface{}]struct{}
 }
 
+const startDetectingCyclesAfter = 1000
+
 var encodeStatePool sync.Pool
 
 func newEncodeState() *encodeState {
 	if v := encodeStatePool.Get(); v != nil {
 		e := v.(*encodeState)
 		e.Reset()
+		if len(e.ptrSeen) > 0 {
+			panic("ptrEncoder.encode should have emptied ptrSeen via defers")
+		}
+		e.ptrLevel = 0
 		return e
 	}
-	return new(encodeState)
+	return &encodeState{ptrSeen: make(map[interface{}]struct{})}
 }
 
 // jsonError is an error wrapper type for internal use only.
@@ -887,7 +901,18 @@ func (pe ptrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
+	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
+		// We're a large number of nested ptrEncoder.encode calls deep;
+		// start checking if we've run into a pointer cycle.
+		ptr := v.Interface()
+		if _, ok := e.ptrSeen[ptr]; ok {
+			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
+		}
+		e.ptrSeen[ptr] = struct{}{}
+		defer delete(e.ptrSeen, ptr)
+	}
 	pe.elemEnc(e, v.Elem(), opts)
+	e.ptrLevel--
 }
 
 func newPtrEncoder(t reflect.Type) encoderFunc {
diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index 40f16d86ff..5110c7de9b 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -138,10 +138,45 @@ func TestEncodeRenamedByteSlice(t *testing.T) {
 	}
 }
 
+type SamePointerNoCycle struct {
+	Ptr1, Ptr2 *SamePointerNoCycle
+}
+
+var samePointerNoCycle = &SamePointerNoCycle{}
+
+type PointerCycle struct {
+	Ptr *PointerCycle
+}
+
+var pointerCycle = &PointerCycle{}
+
+type PointerCycleIndirect struct {
+	Ptrs []interface{}
+}
+
+var pointerCycleIndirect = &PointerCycleIndirect{}
+
+func init() {
+	ptr := &SamePointerNoCycle{}
+	samePointerNoCycle.Ptr1 = ptr
+	samePointerNoCycle.Ptr2 = ptr
+
+	pointerCycle.Ptr = pointerCycle
+	pointerCycleIndirect.Ptrs = []interface{}{pointerCycleIndirect}
+}
+
+func TestSamePointerNoCycle(t *testing.T) {
+	if _, err := Marshal(samePointerNoCycle); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 var unsupportedValues = []interface{}{
 	math.NaN(),
 	math.Inf(-1),
 	math.Inf(1),
+	pointerCycle,
+	pointerCycleIndirect,
 }
 
 func TestUnsupportedValues(t *testing.T) {