fix: bound debian ParseControlFile to a single control stanza (#38044) (#38055)

Backport #38044 by @metsw24-max

**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: metsw24-max <metsw24@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>

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")
+	})
 }