fix: bound debian ParseControlFile to a single control stanza (#38044)

**Packages-index stanza injection via Debian control file**

A `.deb` whose `control` file appends extra paragraphs after a blank
line was still accepted, and `ParseControlFile` stored the whole
multi-stanza blob in `p.Control`. That blob is re-emitted verbatim into
the generated `Packages` index, so the embedded blank line splits it
into separate stanzas and an uploader can smuggle a package entry with
an attacker-chosen `Filename` into the shared index. A binary control
file only holds one stanza, so parsing now stops at the blank line that
terminates it; well-formed packages are unaffected and the new subtest
covers the trailing-stanza case.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/packages/debian/metadata.go

@@ -146,15 +146,26 @@ func ParseControlFile(r io.Reader) (*Package, error) {
 	var depends strings.Builder
 	var control strings.Builder
 
-	s := bufio.NewScanner(io.TeeReader(r, &control))
+	// https://www.debian.org/doc/debian-policy/ch-controlfields.html#syntax-of-control-files
+	s := bufio.NewScanner(r)
 	for s.Scan() {
 		line := s.Text()
 
 		trimmed := strings.TrimSpace(line)
 		if trimmed == "" {
-			continue
+			// A binary package control file holds exactly one stanza. Stop at the
+			// blank line that terminates it, otherwise a crafted control file could
+			// smuggle additional stanzas (with attacker-chosen Filename/Package
+			// fields) into the generated repository "Packages" index.
+			if control.Len() == 0 {
+				continue
+			}
+			break
 		}
 
+		control.WriteString(line)
+		control.WriteByte('\n')
+
 		if line[0] == ' ' || line[0] == '\t' {
 			switch key {
 			case "Description":

modules/packages/debian/metadata_test.go

@@ -184,4 +184,19 @@ func TestParseControlFile(t *testing.T) {
 			assert.NotNil(t, p)
 		}
 	})
+
+	t.Run("SingleStanzaOnly", func(t *testing.T) {
+		// A control file with a trailing stanza must not leak the extra fields into
+		// p.Control, otherwise buildPackagesIndices would emit a second package entry
+		// with an attacker-chosen Filename into the repository "Packages" index.
+		content := bytes.NewBufferString("Package: realpkg\nVersion: 1.0.0\nArchitecture: amd64\nMaintainer: a <a@b.c>\nDescription: real\n\nPackage: openssl\nVersion: 99.0\nArchitecture: amd64\nFilename: pool/main/o/openssl/evil.deb\nDescription: spoofed\n")
+
+		p, err := ParseControlFile(content)
+		assert.NoError(t, err)
+		assert.NotNil(t, p)
+		assert.Equal(t, "realpkg", p.Name)
+		assert.Equal(t, "1.0.0", p.Version)
+		assert.NotContains(t, p.Control, "openssl")
+		assert.NotContains(t, p.Control, "evil.deb")
+	})
 }