http2/hpack: fix nil pointer dereference crash in huffman decoder

Return an error instead.

Fixes bradfitz/http2#56

Change-Id: I3d1e80a214a8635932479943f0ef9610ee02b233
Reviewed-on: https://go-review.googlesource.com/15738
Reviewed-by: Andrew Gerrand <adg@golang.org>

http2/hpack/hpack.go

@@ -473,12 +473,10 @@ func readString(p []byte, wantStr bool) (s string, remain []byte, err error) {
 	}
 
 	if wantStr {
-		// TODO: optimize this garbage:
-		var buf bytes.Buffer
-		if _, err := HuffmanDecode(&buf, p[:strLen]); err != nil {
+		s, err = HuffmanDecodeToString(p[:strLen])
+		if err != nil {
 			return "", nil, err
 		}
-		s = buf.String()
 	}
 	return s, p[strLen:], nil
 }

http2/hpack/hpack_test.go

@@ -10,11 +10,13 @@ import (
 	"bytes"
 	"encoding/hex"
 	"fmt"
+	"math/rand"
 	"reflect"
 	"regexp"
 	"strconv"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestStaticTable(t *testing.T) {
@@ -582,6 +584,77 @@ func TestAppendHuffmanString(t *testing.T) {
 	}
 }
 
+func TestHuffmanRoundtripStress(t *testing.T) {
+	const Len = 50 // of uncompressed string
+	input := make([]byte, Len)
+	var output bytes.Buffer
+	var huff []byte
+
+	n := 5000
+	if testing.Short() {
+		n = 100
+	}
+	seed := time.Now().UnixNano()
+	t.Logf("Seed = %v", seed)
+	src := rand.New(rand.NewSource(seed))
+	var encSize int64
+	for i := 0; i < n; i++ {
+		for l := range input {
+			input[l] = byte(src.Intn(256))
+		}
+		huff = AppendHuffmanString(huff[:0], string(input))
+		encSize += int64(len(huff))
+		output.Reset()
+		if err := huffmanDecode(&output, huff); err != nil {
+			t.Errorf("Failed to decode %q -> %q -> error %v", input, huff, err)
+			continue
+		}
+		if !bytes.Equal(output.Bytes(), input) {
+			t.Errorf("Roundtrip failure on %q -> %q -> %q", input, huff, output.Bytes())
+		}
+	}
+	t.Logf("Compressed size of original: %0.02f%% (%v -> %v)", 100*(float64(encSize)/(Len*float64(n))), Len*n, encSize)
+}
+
+func TestHuffmanDecodeFuzz(t *testing.T) {
+	const Len = 50 // of compressed
+	var buf, zbuf bytes.Buffer
+
+	n := 5000
+	if testing.Short() {
+		n = 100
+	}
+	seed := time.Now().UnixNano()
+	t.Logf("Seed = %v", seed)
+	src := rand.New(rand.NewSource(seed))
+	numFail := 0
+	for i := 0; i < n; i++ {
+		zbuf.Reset()
+		if i == 0 {
+			// Start with at least one invalid one.
+			zbuf.WriteString("00\x91\xff\xff\xff\xff\xc8")
+		} else {
+			for l := 0; l < Len; l++ {
+				zbuf.WriteByte(byte(src.Intn(256)))
+			}
+		}
+
+		buf.Reset()
+		if err := huffmanDecode(&buf, zbuf.Bytes()); err != nil {
+			if err == ErrInvalidHuffman {
+				numFail++
+				continue
+			}
+			t.Errorf("Failed to decode %q: %v", zbuf.Bytes(), err)
+			continue
+		}
+	}
+	t.Logf("%0.02f%% are invalid (%d / %d)", 100*float64(numFail)/float64(n), numFail, n)
+	if numFail < 1 {
+		t.Error("expected at least one invalid huffman encoding (test starts with one)")
+	}
+}
+
 func TestReadVarInt(t *testing.T) {
 	type res struct {
 		i        uint64
@@ -637,6 +710,17 @@ func TestReadVarInt(t *testing.T) {
 	}
 }
 
+// Fuzz crash, originally reported at https://github.com/bradfitz/http2/issues/56
+func TestHuffmanFuzzCrash(t *testing.T) {
+	got, err := HuffmanDecodeToString([]byte("00\x91\xff\xff\xff\xff\xc8"))
+	if got != "" {
+		t.Errorf("Got %q; want empty string", got)
+	}
+	if err != ErrInvalidHuffman {
+		t.Errorf("Err = %v; want ErrInvalidHuffman", err)
+	}
+}
+
 func dehex(s string) []byte {
 	s = strings.Replace(s, " ", "", -1)
 	s = strings.Replace(s, "\n", "", -1)

http2/hpack/huffman.go

@@ -7,6 +7,7 @@ package hpack
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"sync"
 )
@@ -22,14 +23,39 @@ func HuffmanDecode(w io.Writer, v []byte) (int, error) {
 	buf := bufPool.Get().(*bytes.Buffer)
 	buf.Reset()
 	defer bufPool.Put(buf)
+	if err := huffmanDecode(buf, v); err != nil {
+		return 0, err
+	}
+	return w.Write(buf.Bytes())
+}
 
+// HuffmanDecodeToString decodes the string in v.
+func HuffmanDecodeToString(v []byte) (string, error) {
+	buf := bufPool.Get().(*bytes.Buffer)
+	buf.Reset()
+	defer bufPool.Put(buf)
+	if err := huffmanDecode(buf, v); err != nil {
+		return "", err
+	}
+	return buf.String(), nil
+}
+
+// ErrInvalidHuffman is returned for errors found decoding
+// Huffman-encoded strings.
+var ErrInvalidHuffman = errors.New("hpack: invalid Huffman-encoded data")
+
+func huffmanDecode(buf *bytes.Buffer, v []byte) error {
 	n := rootHuffmanNode
 	cur, nbits := uint(0), uint8(0)
 	for _, b := range v {
 		cur = cur<<8 | uint(b)
 		nbits += 8
 		for nbits >= 8 {
-			n = n.children[byte(cur>>(nbits-8))]
+			idx := byte(cur >> (nbits - 8))
+			n = n.children[idx]
+			if n == nil {
+				return ErrInvalidHuffman
+			}
 			if n.children == nil {
 				buf.WriteByte(n.sym)
 				nbits -= n.codeLen
@@ -48,7 +74,7 @@ func HuffmanDecode(w io.Writer, v []byte) (int, error) {
 		nbits -= n.codeLen
 		n = rootHuffmanNode
 	}
-	return w.Write(buf.Bytes())
+	return nil
 }
 
 type node struct {
@@ -67,10 +93,10 @@ func newInternalNode() *node {
 var rootHuffmanNode = newInternalNode()
 
 func init() {
+	if len(huffmanCodes) != 256 {
+		panic("unexpected size")
+	}
 	for i, code := range huffmanCodes {
-		if i > 255 {
-			panic("too many huffman codes")
-		}
 		addDecoderNode(byte(i), code, huffmanCodeLen[i])
 	}
 }

http2/hpack/tables.go

@@ -74,7 +74,7 @@ var staticTable = [...]HeaderField{
 	pair("www-authenticate", ""),
 }
 
-var huffmanCodes = []uint32{
+var huffmanCodes = [256]uint32{
 	0x1ff8,
 	0x7fffd8,
 	0xfffffe2,
@@ -333,7 +333,7 @@ var huffmanCodes = []uint32{
 	0x3ffffee,
 }
 
-var huffmanCodeLen = []uint8{
+var huffmanCodeLen = [256]uint8{
 	13, 23, 28, 28, 28, 28, 28, 28, 28, 24, 30, 28, 28, 30, 28, 28,
 	28, 28, 28, 28, 28, 28, 30, 28, 28, 28, 28, 28, 28, 28, 28, 28,
 	6, 10, 10, 12, 13, 6, 8, 11, 10, 10, 8, 11, 8, 6, 6, 6,