mime/multipart: avoid excessive copy buffer allocations in ReadForm

When copying form data to disk with io.Copy,
allocate only one copy buffer and reuse it rather than
creating two buffers per file (one from io.multiReader.WriteTo,
and a second one from os.File.ReadFrom).

Thanks to Jakob Ackermann (@das7pad) for reporting this issue.

For CVE-2023-24536
For #59153

Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1802453
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Julie Qiu <julieqiu@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Change-Id: I732bd2e1e7467918cac8ab9d65d089272ba4656f
Reviewed-on: https://go-review.googlesource.com/c/go/+/482075
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/mime/multipart/formdata.go

@@ -88,6 +88,7 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 			maxMemoryBytes = math.MaxInt64
 		}
 	}
+	var copyBuf []byte
 	for {
 		p, err := r.nextPart(false, maxMemoryBytes)
 		if err == io.EOF {
@@ -151,14 +152,22 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 				}
 			}
 			numDiskFiles++
-			size, err := io.Copy(file, io.MultiReader(&b, p))
+			if _, err := file.Write(b.Bytes()); err != nil {
+				return nil, err
+			}
+			if copyBuf == nil {
+				copyBuf = make([]byte, 32*1024) // same buffer size as io.Copy uses
+			}
+			// os.File.ReadFrom will allocate its own copy buffer if we let io.Copy use it.
+			type writerOnly struct{ io.Writer }
+			remainingSize, err := io.CopyBuffer(writerOnly{file}, p, copyBuf)
 			if err != nil {
 				return nil, err
 			}
 			fh.tmpfile = file.Name()
-			fh.Size = size
+			fh.Size = int64(b.Len()) + remainingSize
 			fh.tmpoff = fileOff
-			fileOff += size
+			fileOff += fh.Size
 			if !combineFiles {
 				if err := file.Close(); err != nil {
 					return nil, err

src/mime/multipart/formdata_test.go

@@ -399,3 +399,52 @@ func testReadFormManyFiles(t *testing.T, distinct bool) {
 		t.Fatalf("temp dir contains %v files; want 0", len(names))
 	}
 }
+
+func BenchmarkReadForm(b *testing.B) {
+	for _, test := range []struct {
+		name string
+		form func(fw *Writer, count int)
+	}{{
+		name: "fields",
+		form: func(fw *Writer, count int) {
+			for i := 0; i < count; i++ {
+				w, _ := fw.CreateFormField(fmt.Sprintf("field%v", i))
+				fmt.Fprintf(w, "value %v", i)
+			}
+		},
+	}, {
+		name: "files",
+		form: func(fw *Writer, count int) {
+			for i := 0; i < count; i++ {
+				w, _ := fw.CreateFormFile(fmt.Sprintf("field%v", i), fmt.Sprintf("file%v", i))
+				fmt.Fprintf(w, "value %v", i)
+			}
+		},
+	}} {
+		b.Run(test.name, func(b *testing.B) {
+			for _, maxMemory := range []int64{
+				0,
+				1 << 20,
+			} {
+				var buf bytes.Buffer
+				fw := NewWriter(&buf)
+				test.form(fw, 10)
+				if err := fw.Close(); err != nil {
+					b.Fatal(err)
+				}
+				b.Run(fmt.Sprintf("maxMemory=%v", maxMemory), func(b *testing.B) {
+					b.ReportAllocs()
+					for i := 0; i < b.N; i++ {
+						fr := NewReader(bytes.NewReader(buf.Bytes()), fw.Boundary())
+						form, err := fr.ReadForm(maxMemory)
+						if err != nil {
+							b.Fatal(err)
+						}
+						form.RemoveAll()
+					}
+
+				})
+			}
+		})
+	}
+}