net/textproto: avoid overpredicting the number of MIME header keys

A parsed MIME header is a map[string][]string. In the common case,
a header contains many one-element []string slices. To avoid
allocating a separate slice for each key, ReadMIMEHeader looks
ahead in the input to predict the number of keys that will be
parsed, and allocates a single []string of that length.
The individual slices are then allocated out of the larger one.

The prediction of the number of header keys was done by counting
newlines in the input buffer, which does not take into account
header continuation lines (where a header key/value spans multiple
lines) or the end of the header block and the start of the body.
This could lead to a substantial amount of overallocation, for
example when the body consists of nothing but a large block of
newlines.

Fix header key count prediction to take into account the end of
the headers (indicated by a blank line) and continuation lines
(starting with whitespace).

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

For #58975
Fixes CVE-2023-24534

Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1802452
Run-TryBot: Damien Neil <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>
Change-Id: Iacc1c2b5ea6509529845a972414199f988ede1e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/481994
Reviewed-by: Matthew Dempsky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>

src/net/textproto/reader.go

@@ -489,8 +489,11 @@ func readMIMEHeader(r *Reader, lim int64) (MIMEHeader, error) {
 	// large one ahead of time which we'll cut up into smaller
 	// slices. If this isn't big enough later, we allocate small ones.
 	var strs []string
-	hint := r.upcomingHeaderNewlines()
+	hint := r.upcomingHeaderKeys()
 	if hint > 0 {
+		if hint > 1000 {
+			hint = 1000 // set a cap to avoid overallocation
+		}
 		strs = make([]string, hint)
 	}
 
@@ -581,17 +584,30 @@ func mustHaveFieldNameColon(line []byte) error {
 
 var nl = []byte("\n")
 
-// upcomingHeaderNewlines returns an approximation of the number of newlines
+// upcomingHeaderKeys returns an approximation of the number of keys
 // that will be in this header. If it gets confused, it returns 0.
-func (r *Reader) upcomingHeaderNewlines() (n int) {
+func (r *Reader) upcomingHeaderKeys() (n int) {
 	// Try to determine the 'hint' size.
 	r.R.Peek(1) // force a buffer load if empty
 	s := r.R.Buffered()
 	if s == 0 {
 		return
 	}
 	peek, _ := r.R.Peek(s)
-	return bytes.Count(peek, nl)
+	for len(peek) > 0 && n < 1000 {
+		var line []byte
+		line, peek, _ = bytes.Cut(peek, nl)
+		if len(line) == 0 || (len(line) == 1 && line[0] == '\r') {
+			// Blank line separating headers from the body.
+			break
+		}
+		if line[0] == ' ' || line[0] == '\t' {
+			// Folded continuation of the previous line.
+			continue
+		}
+		n++
+	}
+	return n
 }
 
 // CanonicalMIMEHeaderKey returns the canonical format of the

src/net/textproto/reader_test.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"net"
 	"reflect"
+	"runtime"
 	"strings"
 	"sync"
 	"testing"
@@ -129,6 +130,42 @@ func TestReadMIMEHeaderSingle(t *testing.T) {
 	}
 }
 
+// TestReaderUpcomingHeaderKeys is testing an internal function, but it's very
+// difficult to test well via the external API.
+func TestReaderUpcomingHeaderKeys(t *testing.T) {
+	for _, test := range []struct {
+		input string
+		want  int
+	}{{
+		input: "",
+		want:  0,
+	}, {
+		input: "A: v",
+		want:  1,
+	}, {
+		input: "A: v\r\nB: v\r\n",
+		want:  2,
+	}, {
+		input: "A: v\nB: v\n",
+		want:  2,
+	}, {
+		input: "A: v\r\n  continued\r\n  still continued\r\nB: v\r\n\r\n",
+		want:  2,
+	}, {
+		input: "A: v\r\n\r\nB: v\r\nC: v\r\n",
+		want:  1,
+	}, {
+		input: "A: v" + strings.Repeat("\n", 1000),
+		want:  1,
+	}} {
+		r := reader(test.input)
+		got := r.upcomingHeaderKeys()
+		if test.want != got {
+			t.Fatalf("upcomingHeaderKeys(%q): %v; want %v", test.input, got, test.want)
+		}
+	}
+}
+
 func TestReadMIMEHeaderNoKey(t *testing.T) {
 	r := reader(": bar\ntest-1: 1\n\n")
 	m, err := r.ReadMIMEHeader()
@@ -271,6 +308,28 @@ func TestReadMIMEHeaderTrimContinued(t *testing.T) {
 	}
 }
 
+// Test that reading a header doesn't overallocate. Issue 58975.
+func TestReadMIMEHeaderAllocations(t *testing.T) {
+	var totalAlloc uint64
+	const count = 200
+	for i := 0; i < count; i++ {
+		r := reader("A: b\r\n\r\n" + strings.Repeat("\n", 4096))
+		var m1, m2 runtime.MemStats
+		runtime.ReadMemStats(&m1)
+		_, err := r.ReadMIMEHeader()
+		if err != nil {
+			t.Fatalf("ReadMIMEHeader: %v", err)
+		}
+		runtime.ReadMemStats(&m2)
+		totalAlloc += m2.TotalAlloc - m1.TotalAlloc
+	}
+	// 32k is large and we actually allocate substantially less,
+	// but prior to the fix for #58975 we allocated ~400k in this case.
+	if got, want := totalAlloc/count, uint64(32768); got > want {
+		t.Fatalf("ReadMIMEHeader allocated %v bytes, want < %v", got, want)
+	}
+}
+
 type readResponseTest struct {
 	in       string
 	inCode   int