Remove broken DSA private key workarounds.

Remove old code that handled various invalid DSA formats in ancient
software.

This also fixes a double free bug when parsing malformed DSA private keys.

Thanks to Adam Langley (Google/BoringSSL) for discovering this bug using
libFuzzer.

CVE-2016-0705
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

crypto/dsa/dsa_ameth.c

@@ -183,7 +183,7 @@ static int dsa_pub_encode(X509_PUBKEY *pk, const EVP_PKEY *pkey)
 
 static int dsa_priv_decode(EVP_PKEY *pkey, PKCS8_PRIV_KEY_INFO *p8)
 {
-    const unsigned char *p, *pm;
+    const unsigned char *p, *q, *pm;
     int pklen, pmlen;
     int ptype;
     void *pval;
@@ -192,53 +192,26 @@ static int dsa_priv_decode(EVP_PKEY *pkey, PKCS8_PRIV_KEY_INFO *p8)
     ASN1_INTEGER *privkey = NULL;
     BN_CTX *ctx = NULL;
 
-    STACK_OF(ASN1_TYPE) *ndsa = NULL;
     DSA *dsa = NULL;
 
+    int ret = 0;
+
     if (!PKCS8_pkey_get0(NULL, &p, &pklen, &palg, p8))
         return 0;
     X509_ALGOR_get0(NULL, &ptype, &pval, palg);
 
-    /* Check for broken DSA PKCS#8, UGH! */
-    if (*p == (V_ASN1_SEQUENCE | V_ASN1_CONSTRUCTED)) {
-        ASN1_TYPE *t1, *t2;
-        if ((ndsa = d2i_ASN1_SEQUENCE_ANY(NULL, &p, pklen)) == NULL)
-            goto decerr;
-        if (sk_ASN1_TYPE_num(ndsa) != 2)
-            goto decerr;
-        /*-
-         * Handle Two broken types:
-         * SEQUENCE {parameters, priv_key}
-         * SEQUENCE {pub_key, priv_key}
-         */
-
-        t1 = sk_ASN1_TYPE_value(ndsa, 0);
-        t2 = sk_ASN1_TYPE_value(ndsa, 1);
-        if (t1->type == V_ASN1_SEQUENCE) {
-            p8->broken = PKCS8_EMBEDDED_PARAM;
-            pval = t1->value.ptr;
-        } else if (ptype == V_ASN1_SEQUENCE)
-            p8->broken = PKCS8_NS_DB;
-        else
-            goto decerr;
-
-        if (t2->type != V_ASN1_INTEGER)
-            goto decerr;
+    q = p;
 
-        privkey = t2->value.integer;
-    } else {
-        const unsigned char *q = p;
-        if ((privkey = d2i_ASN1_INTEGER(NULL, &p, pklen)) == NULL)
-            goto decerr;
-        if (privkey->type == V_ASN1_NEG_INTEGER) {
-            p8->broken = PKCS8_NEG_PRIVKEY;
-            ASN1_STRING_clear_free(privkey);
-            if ((privkey = d2i_ASN1_UINTEGER(NULL, &q, pklen)) == NULL)
-                goto decerr;
-        }
-        if (ptype != V_ASN1_SEQUENCE)
+    if ((privkey = d2i_ASN1_INTEGER(NULL, &p, pklen)) == NULL)
+        goto decerr;
+    if (privkey->type == V_ASN1_NEG_INTEGER) {
+        p8->broken = PKCS8_NEG_PRIVKEY;
+        ASN1_STRING_clear_free(privkey);
+        if ((privkey = d2i_ASN1_UINTEGER(NULL, &q, pklen)) == NULL)
             goto decerr;
     }
+    if (ptype != V_ASN1_SEQUENCE)
+        goto decerr;
 
     pstr = pval;
     pm = pstr->data;
@@ -267,22 +240,18 @@ static int dsa_priv_decode(EVP_PKEY *pkey, PKCS8_PRIV_KEY_INFO *p8)
     }
 
     EVP_PKEY_assign_DSA(pkey, dsa);
-    BN_CTX_free(ctx);
-    if (ndsa)
-        sk_ASN1_TYPE_pop_free(ndsa, ASN1_TYPE_free);
-    else
-        ASN1_STRING_clear_free(privkey);
 
-    return 1;
+    ret = 1;
+    goto done;
 
  decerr:
     DSAerr(DSA_F_DSA_PRIV_DECODE, DSA_R_DECODE_ERROR);
  dsaerr:
+    DSA_free(dsa);
+ done:
     BN_CTX_free(ctx);
     ASN1_STRING_clear_free(privkey);
-    sk_ASN1_TYPE_pop_free(ndsa, ASN1_TYPE_free);
-    DSA_free(dsa);
-    return 0;
+    return ret;
 }
 
 static int dsa_priv_encode(PKCS8_PRIV_KEY_INFO *p8, const EVP_PKEY *pkey)