archive/tar: add missing error checks to Reader.Next

A recursive call to Reader.Next did not check the error before
trying to use the result, leading to a nil pointer panic.
This specific CL addresses the immediate issue, which is the panic,
but does not solve the root issue, which is due to an integer
overflow in the base-256 parser.

Updates #12435

Change-Id: Ia908671f0f411a409a35e24f2ebf740d46734072
Reviewed-on: https://go-review.googlesource.com/15437
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

archive/tar/reader.go

@@ -820,40 +820,37 @@ func readGNUSparseMap1x0(r io.Reader) ([]sparseEntry, error) {
 	return sp, nil
 }
 
-// readGNUSparseMap0x1 reads the sparse map as stored in GNU's PAX sparse format version 0.1.
-// The sparse map is stored in the PAX headers.
-func readGNUSparseMap0x1(headers map[string]string) ([]sparseEntry, error) {
-	// Get number of entries
-	numEntriesStr, ok := headers[paxGNUSparseNumBlocks]
-	if !ok {
-		return nil, ErrHeader
-	}
-	numEntries, err := strconv.ParseInt(numEntriesStr, 10, 0)
-	if err != nil {
+// readGNUSparseMap0x1 reads the sparse map as stored in GNU's PAX sparse format
+// version 0.1. The sparse map is stored in the PAX headers.
+func readGNUSparseMap0x1(extHdrs map[string]string) ([]sparseEntry, error) {
+	// Get number of entries.
+	// Use integer overflow resistant math to check this.
+	numEntriesStr := extHdrs[paxGNUSparseNumBlocks]
+	numEntries, err := strconv.ParseInt(numEntriesStr, 10, 0) // Intentionally parse as native int
+	if err != nil || numEntries < 0 || int(2*numEntries) < int(numEntries) {
 		return nil, ErrHeader
 	}
 
-	sparseMap := strings.Split(headers[paxGNUSparseMap], ",")
-
-	// There should be two numbers in sparseMap for each entry
+	// There should be two numbers in sparseMap for each entry.
+	sparseMap := strings.Split(extHdrs[paxGNUSparseMap], ",")
 	if int64(len(sparseMap)) != 2*numEntries {
 		return nil, ErrHeader
 	}
 
-	// Loop through the entries in the sparse map
+	// Loop through the entries in the sparse map.
+	// numEntries is trusted now.
 	sp := make([]sparseEntry, 0, numEntries)
 	for i := int64(0); i < numEntries; i++ {
-		offset, err := strconv.ParseInt(sparseMap[2*i], 10, 0)
+		offset, err := strconv.ParseInt(sparseMap[2*i], 10, 64)
 		if err != nil {
 			return nil, ErrHeader
 		}
-		numBytes, err := strconv.ParseInt(sparseMap[2*i+1], 10, 0)
+		numBytes, err := strconv.ParseInt(sparseMap[2*i+1], 10, 64)
 		if err != nil {
 			return nil, ErrHeader
 		}
 		sp = append(sp, sparseEntry{offset: offset, numBytes: numBytes})
 	}
-
 	return sp, nil
 }
 

archive/tar/reader_test.go

@@ -672,23 +672,78 @@ func TestSparseFileReader(t *testing.T) {
 }
 
 func TestReadGNUSparseMap0x1(t *testing.T) {
-	headers := map[string]string{
-		paxGNUSparseNumBlocks: "4",
-		paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
-	}
-	expected := []sparseEntry{
-		{offset: 0, numBytes: 5},
-		{offset: 10, numBytes: 5},
-		{offset: 20, numBytes: 5},
-		{offset: 30, numBytes: 5},
-	}
+	const (
+		maxUint = ^uint(0)
+		maxInt  = int(maxUint >> 1)
+	)
+	var (
+		big1 = fmt.Sprintf("%d", int64(maxInt))
+		big2 = fmt.Sprintf("%d", (int64(maxInt)/2)+1)
+		big3 = fmt.Sprintf("%d", (int64(maxInt) / 3))
+	)
 
-	sp, err := readGNUSparseMap0x1(headers)
-	if err != nil {
-		t.Errorf("Unexpected error: %v", err)
-	}
-	if !reflect.DeepEqual(sp, expected) {
-		t.Errorf("Incorrect sparse map: got %v, wanted %v", sp, expected)
+	var vectors = []struct {
+		extHdrs   map[string]string // Input data
+		sparseMap []sparseEntry     // Expected sparse entries to be outputted
+		err       error             // Expected errors that may be raised
+	}{{
+		extHdrs: map[string]string{paxGNUSparseNumBlocks: "-4"},
+		err:     ErrHeader,
+	}, {
+		extHdrs: map[string]string{paxGNUSparseNumBlocks: "fee "},
+		err:     ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: big1,
+			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		},
+		err: ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: big2,
+			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		},
+		err: ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: big3,
+			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		},
+		err: ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0.5,5,10,5,20,5,30,5",
+		},
+		err: ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0,5.5,10,5,20,5,30,5",
+		},
+		err: ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0,fewafewa.5,fewafw,5,20,5,30,5",
+		},
+		err: ErrHeader,
+	}, {
+		extHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		},
+		sparseMap: []sparseEntry{{0, 5}, {10, 5}, {20, 5}, {30, 5}},
+	}}
+
+	for i, v := range vectors {
+		sp, err := readGNUSparseMap0x1(v.extHdrs)
+		if !reflect.DeepEqual(sp, v.sparseMap) && !(len(sp) == 0 && len(v.sparseMap) == 0) {
+			t.Errorf("test %d, readGNUSparseMap0x1(...): got %v, want %v", i, sp, v.sparseMap)
+		}
+		if err != v.err {
+			t.Errorf("test %d, unexpected error: got %v, want %v", i, err, v.err)
+		}
 	}
 }