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encoding/json: incorrect usage of sync.Pool #27735

dsnet opened this Issue Sep 18, 2018 · 1 comment


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commented Sep 18, 2018 introduces a denial-of-service attack on json.Marshal via a live-lock situation with sync.Pool.

Consider this snippet:

type CustomMarshaler int

func (c CustomMarshaler) MarshalJSON() ([]byte, error) {
	time.Sleep(500 * time.Millisecond) // simulate processing time
	b := make([]byte, int(c))
	b[0] = '"'
	for i := 1; i < len(b)-1; i++ {
		b[i] = 'a'
	b[len(b)-1] = '"'
	return b, nil

func main() {
	processRequest := func(size int) {
		time.Sleep(1 * time.Millisecond) // simulate idle time

	// Simulate a steady stream of infrequent large requests.
	go func() {
		for {
			processRequest(1 << 28) // 256MiB

	// Simulate a storm of small requests.
	for i := 0; i < 1000; i++ {
		go func() {
			for {
				processRequest(1 << 10) // 1KiB

	// Continually run a GC and track the allocated bytes.
	var stats runtime.MemStats
	for i := 0; ; i++ {
		fmt.Printf("Cycle %d: %dB\n", i, stats.Alloc)

This is a variation of #23199 (comment) of a situation suggested by @bcmills.

Essentially, we have a 1-to-1000 ratio of a routines that either use 1KiB or 256MiB, respectively. The occasional insertion of a 256MiB buffer into the sync.Pool gets continually held by the 1KiB routines. On my machine, after 300 GC cycles, the above program occupies 6GiB of my heap, when I expect it to be 256MiB in the worst-case.

\cc @jnjackins @bradfitz


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commented Sep 18, 2018

Change mentions this issue: encoding/json: fix usage of sync.Pool

@bcmills bcmills added this to the Go1.12 milestone Sep 19, 2018

@ianlancetaylor ianlancetaylor modified the milestones: Go1.12, Go1.13 Dec 12, 2018

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