SAE: Mask timing of MODP groups 22, 23, 24

These groups have significant probability of coming up with pwd-value
that is equal or greater than the prime and as such, need for going
through the PWE derivation loop multiple times. This can result in
sufficient timing different to allow an external observer to determine
how many rounds are needed and that can leak information about the used
password.

Force at least 40 loop rounds for these MODP groups similarly to the ECC
group design to mask timing. This behavior is not described in IEEE Std
802.11-2016 for SAE, but it does not result in different values (i.e.,
only different timing), so such implementation specific countermeasures
can be done without breaking interoperability with other implementation.

Note: These MODP groups 22, 23, and 24 are not considered sufficiently
strong to be used with SAE (or more or less anything else). As such,
they should never be enabled in runtime configuration for any production
use cases. These changes to introduce additional protection to mask
timing is only for completeness of implementation and not an indication
that these groups should be used.

This is related to CVE-2019-9494.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>

diff --git a/src/common/sae.c b/src/common/sae.c
index 5df9b95aae705f9381162cd3a511268414316509..75b1b4a83777d0a5ad7dc3f10fb2f11b0a5f44ba 100644 (file)
--- a/src/common/sae.c
+++ b/src/common/sae.c
@@ -601,22 +601,27 @@ fail:
 }
 
 
+static int sae_modp_group_require_masking(int group)
+{
+       /* Groups for which pwd-value is likely to be >= p frequently */
+       return group == 22 || group == 23 || group == 24;
+}
+
+
 static int sae_derive_pwe_ffc(struct sae_data *sae, const u8 *addr1,
                              const u8 *addr2, const u8 *password,
                              size_t password_len, const char *identifier)
 {
-       u8 counter;
+       u8 counter, k;
        u8 addrs[2 * ETH_ALEN];
        const u8 *addr[3];
        size_t len[3];
        size_t num_elem;
        int found = 0;
+       struct crypto_bignum *pwe = NULL;
 
-       if (sae->tmp->pwe_ffc == NULL) {
-               sae->tmp->pwe_ffc = crypto_bignum_init();
-               if (sae->tmp->pwe_ffc == NULL)
-                       return -1;
-       }
+       crypto_bignum_deinit(sae->tmp->pwe_ffc, 1);
+       sae->tmp->pwe_ffc = NULL;
 
        wpa_hexdump_ascii_key(MSG_DEBUG, "SAE: password",
                              password, password_len);
@@ -640,7 +645,9 @@ static int sae_derive_pwe_ffc(struct sae_data *sae, const u8 *addr1,
        len[num_elem] = sizeof(counter);
        num_elem++;
 
-       for (counter = 1; !found; counter++) {
+       k = sae_modp_group_require_masking(sae->group) ? 40 : 1;
+
+       for (counter = 1; counter <= k || !found; counter++) {
                u8 pwd_seed[SHA256_MAC_LEN];
                int res;
 
@@ -650,19 +657,30 @@ static int sae_derive_pwe_ffc(struct sae_data *sae, const u8 *addr1,
                        break;
                }
 
-               wpa_printf(MSG_DEBUG, "SAE: counter = %u", counter);
+               wpa_printf(MSG_DEBUG, "SAE: counter = %02u", counter);
                if (hmac_sha256_vector(addrs, sizeof(addrs), num_elem,
                                       addr, len, pwd_seed) < 0)
                        break;
-               res = sae_test_pwd_seed_ffc(sae, pwd_seed, sae->tmp->pwe_ffc);
+               if (!pwe) {
+                       pwe = crypto_bignum_init();
+                       if (!pwe)
+                               break;
+               }
+               res = sae_test_pwd_seed_ffc(sae, pwd_seed, pwe);
                if (res < 0)
                        break;
                if (res > 0) {
-                       wpa_printf(MSG_DEBUG, "SAE: Use this PWE");
                        found = 1;
+                       if (!sae->tmp->pwe_ffc) {
+                               wpa_printf(MSG_DEBUG, "SAE: Use this PWE");
+                               sae->tmp->pwe_ffc = pwe;
+                               pwe = NULL;
+                       }
                }
        }
 
+       crypto_bignum_deinit(pwe, 1);
+
        return found ? 0 : -1;
 }