Check for leading zeros in CBS_get_asn1_uint64.

The encoding of an INTEGER should not have leading zeros, except to pad for the
sign bit.

Change-Id: I80d22818cf1d2ca9d27e215620392e1725372aa5
Reviewed-on: https://boringssl-review.googlesource.com/4218
Reviewed-by: Adam Langley <agl@google.com>

crypto/bytestring/bytestring_test.cc

@@ -596,8 +596,10 @@ static const ASN1InvalidUint64Test kASN1InvalidUint64Tests[] = {
     {"\x02\x00", 2},
     // Negative number.
     {"\x02\x01\x80", 3},
-    // Overflow
+    // Overflow.
     {"\x02\x09\x01\x00\x00\x00\x00\x00\x00\x00\x00", 11},
+    // Leading zeros.
+    {"\x02\x02\x00\x01", 4},
 };
 
 static bool TestASN1Uint64() {

crypto/bytestring/cbs.c

@@ -291,7 +291,12 @@ int CBS_get_asn1_uint64(CBS *cbs, uint64_t *out) {
   }
 
   if ((data[0] & 0x80) != 0) {
-    /* negative number */
+    /* Negative number. */
+    return 0;
+  }
+
+  if (data[0] == 0 && len > 1 && (data[1] & 0x80) == 0) {
+    /* Extra leading zeros. */
     return 0;
   }