compress/bzip2: fix panics on malformed input.

Fixes 8363.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/114810043

src/pkg/compress/bzip2/bzip2.go

@@ -261,6 +261,11 @@ func (bz2 *reader) readBlock() (err error) {
 		}
 	}
 
+	if numSymbols == 0 {
+		// There must be an EOF symbol.
+		return StructuralError("no symbols in input")
+	}
+
 	// A block uses between two and six different Huffman trees.
 	numHuffmanTrees := br.ReadBits(3)
 	if numHuffmanTrees < 2 || numHuffmanTrees > 6 {
@@ -307,10 +312,10 @@ func (bz2 *reader) readBlock() (err error) {
 
 	// Now we decode the arrays of code-lengths for each tree.
 	lengths := make([]uint8, numSymbols)
-	for i := 0; i < numHuffmanTrees; i++ {
+	for i := range huffmanTrees {
 		// The code lengths are delta encoded from a 5-bit base value.
 		length := br.ReadBits(5)
-		for j := 0; j < numSymbols; j++ {
+		for j := range lengths {
 			for {
 				if !br.ReadBit() {
 					break
@@ -333,6 +338,12 @@ func (bz2 *reader) readBlock() (err error) {
 	}
 
 	selectorIndex := 1 // the next tree index to use
+	if len(treeIndexes) == 0 {
+		return StructuralError("no tree selectors given")
+	}
+	if int(treeIndexes[0]) >= len(huffmanTrees) {
+		return StructuralError("tree selector out of range")
+	}
 	currentHuffmanTree := huffmanTrees[treeIndexes[0]]
 	bufIndex := 0 // indexes bz2.buf, the output buffer.
 	// The output of the move-to-front transform is run-length encoded and
@@ -350,6 +361,12 @@ func (bz2 *reader) readBlock() (err error) {
 	decoded := 0 // counts the number of symbols decoded by the current tree.
 	for {
 		if decoded == 50 {
+			if selectorIndex >= numSelectors {
+				return StructuralError("insufficient selector indices for number of symbols")
+			}
+			if int(treeIndexes[selectorIndex]) >= len(huffmanTrees) {
+				return StructuralError("tree selector out of range")
+			}
 			currentHuffmanTree = huffmanTrees[treeIndexes[selectorIndex]]
 			selectorIndex++
 			decoded = 0

src/pkg/compress/bzip2/bzip2_test.go

@@ -208,6 +208,14 @@ func TestBufferOverrun(t *testing.T) {
 	ioutil.ReadAll(decompressor)
 }
 
+func TestOutOfRangeSelector(t *testing.T) {
+	// Tests https://code.google.com/p/go/issues/detail?id=8363.
+	buffer := bytes.NewReader(outOfRangeSelector)
+	decompressor := NewReader(buffer)
+	// This shouldn't panic.
+	ioutil.ReadAll(decompressor)
+}
+
 var bufferOverrunBase64 string = `
 QlpoNTFBWSZTWTzyiGcACMP/////////////////////////////////3/7f3///
 ////4N/fCZODak2Xo44GIHZgkGzDRbFAuwAAKoFV7T6AO6qwA6APb6s2rOoAkAAD
@@ -361,3 +369,13 @@ O0A8s/iua5oFdNZTWvbVI4FUH9sKcLiB3/fIAF+sB4n8q6L+UCfmbPcAo/crQ6b3
 HqhDBMY9J0q/jdz9GNYZ/1fbXdkUqAQKFePhtzJDRBZba27+LPQNMCcrHMq06F1T
 4QmLmkHt7LxB2pAczUO+T2O9bHEw/HWw+dYf2MoRDUw=
 `
+
+var outOfRangeSelector = []byte{
+	0x42, 0x5a, 0x68, 0x39, 0x31, 0x41, 0x59, 0x26,
+	0x53, 0x59, 0x4e, 0xec, 0xe8, 0x36, 0x00, 0x00,
+	0x02, 0x51, 0x80, 0x00, 0x10, 0x40, 0x00, 0x06,
+	0x44, 0x90, 0x80, 0x20, 0x00, 0x31, 0x06, 0x4c,
+	0x41, 0x01, 0xa7, 0xa9, 0xa5, 0x80, 0xbb, 0x94,
+	0x31, 0x17, 0x72, 0x45, 0x38, 0x50, 0x90, 0x00,
+	0x00, 0x00, 0x00,
+}