Fix DSA, preserve BN_FLG_CONSTTIME

Operations in the DSA signing algorithm should run in constant time in
order to avoid side channel attacks. A flaw in the OpenSSL DSA
implementation means that a non-constant time codepath is followed for
certain operations. This has been demonstrated through a cache-timing
attack to be sufficient for an attacker to recover the private DSA key.

CVE-2016-2178
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/dsa/dsa_ossl.c

@@ -204,10 +204,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
             goto err;
     } while (BN_is_zero(k));
 
-    if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) {
-        BN_set_flags(k, BN_FLG_CONSTTIME);
-    }
-
     if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
         if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
                                     dsa->lock, dsa->p, ctx))
@@ -238,6 +234,11 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
     } else {
         K = k;
     }
+
+    if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) {
+        BN_set_flags(K, BN_FLG_CONSTTIME);
+    }
+
     DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, K, dsa->p, ctx,
                    dsa->method_mont_p);
     if (!BN_mod(r, r, dsa->q, ctx))