tiff: don't pre-allocate giant slices before reading

Use a copy of the standard libraries internal/saferio.ReadDataAt func to
create/read slices which have lengths supplied by the header. This
avoids allocating giant slices which we then learn there are not enough
bytes in the reader to fill. This makes DecodeConfig safe to use to
determine if the image is of a reasonable size to call Decode on.

This was found by the ngolo-fuzzing project running on OSS-Fuzz and
reported by Philippe Antoine (Catena cyber).

Fixes golang/go#58003
Fixes CVE-2022-41727

Change-Id: Iae53f78b840f3b8dbeab37fba8c0164054cbb4ed
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1680712
Reviewed-by: Damien Neil <[email protected]>
TryBot-Result: Security TryBots <[email protected]>
Run-TryBot: Roland Shoemaker <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/image/+/468195
Auto-Submit: Roland Shoemaker <[email protected]>
Run-TryBot: Roland Shoemaker <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

tiff/reader.go

@@ -38,6 +38,52 @@ func (e UnsupportedError) Error() string {
 
 var errNoPixels = FormatError("not enough pixel data")
 
+const maxChunkSize = 10 << 20 // 10M
+
+// safeReadtAt is a verbatim copy of internal/saferio.ReadDataAt from the
+// standard library, which is used to read data from a reader using a length
+// provided by untrusted data, without allocating the entire slice ahead of time
+// if it is large (>maxChunkSize). This allows us to avoid allocating giant
+// slices before learning that we can't actually read that much data from the
+// reader.
+func safeReadAt(r io.ReaderAt, n uint64, off int64) ([]byte, error) {
+	if int64(n) < 0 || n != uint64(int(n)) {
+		// n is too large to fit in int, so we can't allocate
+		// a buffer large enough. Treat this as a read failure.
+		return nil, io.ErrUnexpectedEOF
+	}
+
+	if n < maxChunkSize {
+		buf := make([]byte, n)
+		_, err := r.ReadAt(buf, off)
+		if err != nil {
+			// io.SectionReader can return EOF for n == 0,
+			// but for our purposes that is a success.
+			if err != io.EOF || n > 0 {
+				return nil, err
+			}
+		}
+		return buf, nil
+	}
+
+	var buf []byte
+	buf1 := make([]byte, maxChunkSize)
+	for n > 0 {
+		next := n
+		if next > maxChunkSize {
+			next = maxChunkSize
+		}
+		_, err := r.ReadAt(buf1[:next], off)
+		if err != nil {
+			return nil, err
+		}
+		buf = append(buf, buf1[:next]...)
+		n -= next
+		off += int64(next)
+	}
+	return buf, nil
+}
+
 type decoder struct {
 	r         io.ReaderAt
 	byteOrder binary.ByteOrder
@@ -82,8 +128,7 @@ func (d *decoder) ifdUint(p []byte) (u []uint, err error) {
 	}
 	if datalen := lengths[datatype] * count; datalen > 4 {
 		// The IFD contains a pointer to the real value.
-		raw = make([]byte, datalen)
-		_, err = d.r.ReadAt(raw, int64(d.byteOrder.Uint32(p[8:12])))
+		raw, err = safeReadAt(d.r, uint64(datalen), int64(d.byteOrder.Uint32(p[8:12])))
 	} else {
 		raw = p[8 : 8+datalen]
 	}
@@ -427,8 +472,9 @@ func newDecoder(r io.Reader) (*decoder, error) {
 	numItems := int(d.byteOrder.Uint16(p[0:2]))
 
 	// All IFD entries are read in one chunk.
-	p = make([]byte, ifdLen*numItems)
-	if _, err := d.r.ReadAt(p, ifdOffset+2); err != nil {
+	var err error
+	p, err = safeReadAt(d.r, uint64(ifdLen*numItems), ifdOffset+2)
+	if err != nil {
 		return nil, err
 	}
 
@@ -656,8 +702,7 @@ func Decode(r io.Reader) (img image.Image, err error) {
 				if b, ok := d.r.(*buffer); ok {
 					d.buf, err = b.Slice(int(offset), int(n))
 				} else {
-					d.buf = make([]byte, n)
-					_, err = d.r.ReadAt(d.buf, offset)
+					d.buf, err = safeReadAt(d.r, uint64(n), offset)
 				}
 			case cG3:
 				inv := d.firstVal(tPhotometricInterpretation) == pWhiteIsZero