encoding/gob: use saferio to read large buffer

Avoid allocating large amounts of memory for corrupt input.

No test case because the problem can only happen for invalid data.
Let the fuzzer find cases like this.

Fixes #53369

Change-Id: I67c5e75bf181ad84988d6d6da12507df0e6df8e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/413979
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/encoding/gob/decode.go

@@ -57,17 +57,6 @@ func (d *decBuffer) Drop(n int) {
 	d.offset += n
 }
 
-// Size grows the buffer to exactly n bytes, so d.Bytes() will
-// return a slice of length n. Existing data is first discarded.
-func (d *decBuffer) Size(n int) {
-	d.Reset()
-	if cap(d.data) < n {
-		d.data = make([]byte, n)
-	} else {
-		d.data = d.data[0:n]
-	}
-}
-
 func (d *decBuffer) ReadByte() (byte, error) {
 	if d.offset >= len(d.data) {
 		return 0, io.EOF
@@ -85,6 +74,12 @@ func (d *decBuffer) Bytes() []byte {
 	return d.data[d.offset:]
 }
 
+// SetBytes sets the buffer to the bytes, discarding any existing data.
+func (d *decBuffer) SetBytes(data []byte) {
+	d.data = data
+	d.offset = 0
+}
+
 func (d *decBuffer) Reset() {
 	d.data = d.data[0:0]
 	d.offset = 0

src/encoding/gob/decoder.go

@@ -7,6 +7,7 @@ package gob
 import (
 	"bufio"
 	"errors"
+	"internal/saferio"
 	"io"
 	"reflect"
 	"sync"
@@ -98,8 +99,9 @@ func (dec *Decoder) readMessage(nbytes int) {
 		panic("non-empty decoder buffer")
 	}
 	// Read the data
-	dec.buf.Size(nbytes)
+	var buf []byte
-	_, dec.err = io.ReadFull(dec.r, dec.buf.Bytes())
+	buf, dec.err = saferio.ReadData(dec.r, uint64(nbytes))
+	dec.buf.SetBytes(buf)
 	if dec.err == io.EOF {
 		dec.err = io.ErrUnexpectedEOF
 	}

src/go/build/deps_test.go

@@ -187,6 +187,9 @@ var depsRules = `
 	  text/scanner,
 	  text/tabwriter;
 
+	io, reflect
+	< internal/saferio;
+
 	# encodings
 	# core ones do not use fmt.
 	io, strconv
@@ -200,7 +203,7 @@ var depsRules = `
 
 	fmt !< encoding/base32, encoding/base64;
 
-	FMT, encoding/base32, encoding/base64
+	FMT, encoding/base32, encoding/base64, internal/saferio
 	< encoding/ascii85, encoding/csv, encoding/gob, encoding/hex,
 	  encoding/json, encoding/pem, encoding/xml, mime;
 
@@ -239,9 +242,6 @@ var depsRules = `
 	encoding/binary, regexp
 	< index/suffixarray;
 
-	io, reflect
-	< internal/saferio;
-
 	# executable parsing
 	FMT, encoding/binary, compress/zlib, internal/saferio
 	< runtime/debug