encoding/pem: be stricter about the ending line.

Previously the code didn't check the type and final five dashes of the
ending line of a PEM block.

Fixes #16335.

Change-Id: Ia544e8739ea738d767cfe56c8d46204214ec0b5a
Reviewed-on: https://go-review.googlesource.com/27391
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/pem/pem.go

@@ -119,19 +119,36 @@ func Decode(data []byte) (p *Block, rest []byte) {
 		rest = next
 	}
 
-	var endIndex int
+	var endIndex, endTrailerIndex int
+
 	// If there were no headers, the END line might occur
 	// immediately, without a leading newline.
 	if len(p.Headers) == 0 && bytes.HasPrefix(rest, pemEnd[1:]) {
 		endIndex = 0
+		endTrailerIndex = len(pemEnd) - 1
 	} else {
 		endIndex = bytes.Index(rest, pemEnd)
+		endTrailerIndex = endIndex + len(pemEnd)
 	}
 
 	if endIndex < 0 {
 		return decodeError(data, rest)
 	}
 
+	// After the "-----" of the ending line should be the same type and a
+	// final five dashes.
+	endTrailer := rest[endTrailerIndex:]
+	endTrailerLen := len(typeLine) + len(pemEndOfLine)
+	if len(endTrailer) < endTrailerLen {
+		return decodeError(data, rest)
+	}
+
+	endTrailer = endTrailer[:endTrailerLen]
+	if !bytes.HasPrefix(endTrailer, typeLine) ||
+		!bytes.HasSuffix(endTrailer, pemEndOfLine) {
+		return decodeError(data, rest)
+	}
+
 	base64Data := removeWhitespace(rest[:endIndex])
 	p.Bytes = make([]byte, base64.StdEncoding.DecodedLen(len(base64Data)))
 	n, err := base64.StdEncoding.Decode(p.Bytes, base64Data)

src/encoding/pem/pem_test.go

@@ -78,6 +78,48 @@ func TestDecode(t *testing.T) {
 	}
 }
 
+const pemTooFewEndingDashes = `
+-----BEGIN FOO-----
+dGVzdA==
+-----END FOO----`
+
+const pemWrongEndingType = `
+-----BEGIN FOO-----
+dGVzdA==
+-----END BAR-----`
+
+const pemMissingEndingSpace = `
+-----BEGIN FOO-----
+dGVzdA==
+-----ENDBAR-----`
+
+var badPEMTests = []struct {
+	name  string
+	input string
+}{
+	{
+		"too few trailing dashes",
+		pemTooFewEndingDashes,
+	},
+	{
+		"incorrect ending type",
+		pemWrongEndingType,
+	},
+	{
+		"missing ending space",
+		pemMissingEndingSpace,
+	},
+}
+
+func TestBadDecode(t *testing.T) {
+	for _, test := range badPEMTests {
+		result, _ := Decode([]byte(test.input))
+		if result != nil {
+			t.Errorf("unexpected success while parsing %q", test.name)
+		}
+	}
+}
+
 func TestEncode(t *testing.T) {
 	r := EncodeToMemory(privateKey2)
 	if string(r) != pemPrivateKey2 {