mime/multipart: fix Reader.ReadForm(math.MaxInt64) overflow

Because "CopyN" will read one more byte, which will cause us
to overflow when calling "Reader.ReadForm(math.MaxInt64)".

So we should check if the parameter exceeds "math.MaxInt64"
to avoid returning no data.

Fixes #58384.

Change-Id: I30088ce6468176b21e4a9a0b8b6080f2986dda23
Reviewed-on: https://go-review.googlesource.com/c/go/+/467557
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Run-TryBot: hopehook <hopehook@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

2 files changed, 34 insertions, 0 deletions

diff --git a/src/mime/multipart/formdata.go b/src/mime/multipart/formdata.go
index 41bc886d16..e62727dbb1 100644
--- a/src/mime/multipart/formdata.go
+++ b/src/mime/multipart/formdata.go
@@ -77,6 +77,9 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 	// unconfigurable 10 MB added on to maxMemory, is unfortunate but difficult to change
 	// within the constraints of the API as documented.
 	maxFileMemoryBytes := maxMemory
+	if maxFileMemoryBytes == math.MaxInt64 {
+		maxFileMemoryBytes--
+	}
 	maxMemoryBytes := maxMemory + int64(10<<20)
 	if maxMemoryBytes <= 0 {
 		if maxMemory < 0 {
diff --git a/src/mime/multipart/formdata_test.go b/src/mime/multipart/formdata_test.go
index 8a862be717..9b3f9ec392 100644
--- a/src/mime/multipart/formdata_test.go
+++ b/src/mime/multipart/formdata_test.go
@@ -55,6 +55,23 @@ func TestReadFormWithNamelessFile(t *testing.T) {
 	}
 }
 
+// Issue 58384: Handle ReadForm(math.MaxInt64)
+func TestReadFormWitFileNameMaxMemoryOverflow(t *testing.T) {
+	b := strings.NewReader(strings.ReplaceAll(messageWithFileName, "\n", "\r\n"))
+	r := NewReader(b, boundary)
+	f, err := r.ReadForm(math.MaxInt64)
+	if err != nil {
+		t.Fatalf("ReadForm(MaxInt64): %v", err)
+	}
+	defer f.RemoveAll()
+
+	fd := testFile(t, f.File["filea"][0], "filea.txt", fileaContents)
+	if _, ok := fd.(*os.File); ok {
+		t.Error("file is *os.File, should not be")
+	}
+	fd.Close()
+}
+
 // Issue 40430: Handle ReadForm(math.MaxInt64)
 func TestReadFormMaxMemoryOverflow(t *testing.T) {
 	b := strings.NewReader(strings.ReplaceAll(messageWithTextContentType, "\n", "\r\n"))
@@ -66,6 +83,11 @@ func TestReadFormMaxMemoryOverflow(t *testing.T) {
 	if f == nil {
 		t.Fatal("ReadForm(MaxInt64): missing form")
 	}
+	defer f.RemoveAll()
+
+	if g, e := f.Value["texta"][0], textaValue; g != e {
+		t.Errorf("texta value = %q, want %q", g, e)
+	}
 }
 
 func TestReadFormWithTextContentType(t *testing.T) {
@@ -122,6 +144,15 @@ Content-Type: text/plain
 --MyBoundary--
 `
 
+const messageWithFileName = `
+--MyBoundary
+Content-Disposition: form-data; name="filea"; filename="filea.txt"
+Content-Type: text/plain
+
+` + fileaContents + `
+--MyBoundary--
+`
+
 const messageWithTextContentType = `
 --MyBoundary
 Content-Disposition: form-data; name="texta"