encoding/pem: refuse extra data on ending line

Previously the code didn't check for extra data after the final five
dashes of the ending line of a PEM block.

Fixes #19147
Fixes #7042

Change-Id: Idaab2390914a2bed8c2c12b14dfb6d68233fdfec
Reviewed-on: https://go-review.googlesource.com/37147Reviewed-by: Adam Langley <agl@golang.org>

src/encoding/pem/pem.go

@@ -135,20 +135,26 @@ func Decode(data []byte) (p *Block, rest []byte) {
 		return decodeError(data, rest)
 	}
 
-	// After the "-----" of the ending line should be the same type and a
-	// final five dashes.
+	// After the "-----" of the ending line, there should be the same type
+	// and then a final five dashes.
 	endTrailer := rest[endTrailerIndex:]
 	endTrailerLen := len(typeLine) + len(pemEndOfLine)
 	if len(endTrailer) < endTrailerLen {
 		return decodeError(data, rest)
 	}
 
+	restOfEndLine := endTrailer[endTrailerLen:]
 	endTrailer = endTrailer[:endTrailerLen]
 	if !bytes.HasPrefix(endTrailer, typeLine) ||
 		!bytes.HasSuffix(endTrailer, pemEndOfLine) {
 		return decodeError(data, rest)
 	}
 
+	// The line must end with only whitespace.
+	if s, _ := getLine(restOfEndLine); len(s) != 0 {
+		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

@@ -83,6 +83,16 @@ const pemTooFewEndingDashes = `
 dGVzdA==
 -----END FOO----`
 
+const pemTooManyEndingDashes = `
+-----BEGIN FOO-----
+dGVzdA==
+-----END FOO------`
+
+const pemTrailingNonWhitespace = `
+-----BEGIN FOO-----
+dGVzdA==
+-----END FOO----- .`
+
 const pemWrongEndingType = `
 -----BEGIN FOO-----
 dGVzdA==
@@ -101,6 +111,14 @@ var badPEMTests = []struct {
 		"too few trailing dashes",
 		pemTooFewEndingDashes,
 	},
+	{
+		"too many trailing dashes",
+		pemTooManyEndingDashes,
+	},
+	{
+		"trailing non-whitespace",
+		pemTrailingNonWhitespace,
+	},
 	{
 		"incorrect ending type",
 		pemWrongEndingType,