rsa/rsa_oaep.c: remove memcpy calls from RSA_padding_check_PKCS1_OAEP.

And make RSAErr call unconditional.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(cherry picked from commit 75f5e944be97f28867e7c489823c889d89d0bd06)

crypto/rsa/rsa_oaep.c

@@ -120,7 +120,7 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                                       const EVP_MD *mgf1md)
 {
     int i, dblen = 0, mlen = -1, one_index = 0, msg_index;
-    unsigned int good, found_one_byte;
+    unsigned int good = 0, found_one_byte, mask;
     const unsigned char *maskedseed, *maskeddb;
     /*
      * |em| is the encoded message, zero-padded to exactly |num| bytes: em =
@@ -147,8 +147,11 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
      * the ciphertext, see PKCS #1 v2.2, section 7.1.2.
      * This does not leak any side-channel information.
      */
-    if (num < flen || num < 2 * mdlen + 2)
-        goto decoding_err;
+    if (num < flen || num < 2 * mdlen + 2) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1,
+               RSA_R_OAEP_DECODING_ERROR);
+        return -1;
+    }
 
     dblen = num - mdlen - 1;
     db = OPENSSL_malloc(dblen);
@@ -157,25 +160,26 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
         goto cleanup;
     }
 
-    if (flen != num) {
-        em = OPENSSL_zalloc(num);
-        if (em == NULL) {
-            RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1,
-                   ERR_R_MALLOC_FAILURE);
-            goto cleanup;
-        }
+    em = OPENSSL_malloc(num);
+    if (em == NULL) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1,
+               ERR_R_MALLOC_FAILURE);
+        goto cleanup;
+    }
 
-        /*
-         * Caller is encouraged to pass zero-padded message created with
-         * BN_bn2binpad, but if it doesn't, we do this zero-padding copy
-         * to avoid leaking that information. The copy still leaks some
-         * side-channel information, but it's impossible to have a fixed
-         * memory access pattern since we can't read out of the bounds of
-         * |from|.
-         */
-        memcpy(em + num - flen, from, flen);
-        from = em;
+    /*
+     * Caller is encouraged to pass zero-padded message created with
+     * BN_bn2binpad. Trouble is that since we can't read out of |from|'s
+     * bounds, it's impossible to have an invariant memory access pattern
+     * in case |from| was not zero-padded in advance.
+     */
+    for (from += flen, em += num, i = 0; i < num; i++) {
+        mask = ~constant_time_is_zero(flen);
+        flen -= 1 & mask;
+        from -= 1 & mask;
+        *--em = *from & mask;
     }
+    from = em;
 
     /*
      * The first byte must be zero, however we must not leak if this is
@@ -222,32 +226,48 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
      * so plaintext-awareness ensures timing side-channels are no longer a
      * concern.
      */
-    if (!good)
-        goto decoding_err;
-
     msg_index = one_index + 1;
     mlen = dblen - msg_index;
 
-    if (tlen < mlen) {
-        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1, RSA_R_DATA_TOO_LARGE);
-        mlen = -1;
-    } else {
-        memcpy(to, db + msg_index, mlen);
-        goto cleanup;
+    /*
+     * For good measure, do this check in constant tine as well.
+     */
+    good &= constant_time_ge(tlen, mlen);
+
+    /*
+     * Even though we can't fake result's length, we can pretend copying
+     * |tlen| bytes where |mlen| bytes would be real. Last |tlen| of |dblen|
+     * bytes are viewed as circular buffer with start at |tlen|-|mlen'|,
+     * where |mlen'| is "saturated" |mlen| value. Deducing information
+     * about failure or |mlen| would take attacker's ability to observe
+     * memory access pattern with byte granularity *as it occurs*. It
+     * should be noted that failure is indistinguishable from normal
+     * operation if |tlen| is fixed by protocol.
+     */
+    tlen = constant_time_select_int(constant_time_lt(dblen, tlen), dblen, tlen);
+    msg_index = constant_time_select_int(good, msg_index, dblen - tlen);
+    mlen = dblen - msg_index;
+    for (from = db + msg_index, mask = good, i = 0; i < tlen; i++) {
+        unsigned int equals = constant_time_eq(i, mlen);
+
+        from -= dblen & equals; /* if (i == dblen) rewind   */
+        mask &= mask ^ equals;  /* if (i == dblen) mask = 0 */
+        to[i] = constant_time_select_8(mask, from[i], to[i]);
     }
 
- decoding_err:
     /*
      * To avoid chosen ciphertext attacks, the error message should not
      * reveal which kind of decoding error happened.
      */
     RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1,
            RSA_R_OAEP_DECODING_ERROR);
+    err_clear_last_constant_time(1 & good);
  cleanup:
     OPENSSL_cleanse(seed, sizeof(seed));
     OPENSSL_clear_free(db, dblen);
     OPENSSL_clear_free(em, num);
-    return mlen;
+
+    return constant_time_select_int(good, mlen, -1);
 }
 
 int PKCS1_MGF1(unsigned char *mask, long len,