SAE: Minimize timing differences in PWE derivation

The QR test result can provide information about the password to an
attacker, so try to minimize differences in how the
sae_test_pwd_seed_ecc() result is used. (CVE-2019-9494)

Use heap memory for the dummy password to allow the same password length
to be used even with long passwords.

Use constant time selection functions to track the real vs. dummy
variables so that the exact same operations can be performed for both QR
test results.

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

diff --git a/src/common/sae.c b/src/common/sae.c
index 8129a7c156da5cf221d2662613f5ae85e0741bc1..d55323bcdd7e48a05d367729fcc5aef17161d52d 100644 (file)
--- a/src/common/sae.c
+++ b/src/common/sae.c
@@ -9,6 +9,7 @@
 #include "includes.h"
 
 #include "common.h"
+#include "utils/const_time.h"
 #include "crypto/crypto.h"
 #include "crypto/sha256.h"
 #include "crypto/random.h"
@@ -292,15 +293,12 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
                                 const u8 *prime,
                                 const struct crypto_bignum *qr,
                                 const struct crypto_bignum *qnr,
-                                struct crypto_bignum **ret_x_cand)
+                                u8 *pwd_value)
 {
-       u8 pwd_value[SAE_MAX_ECC_PRIME_LEN];
        struct crypto_bignum *y_sqr, *x_cand;
        int res;
        size_t bits;
 
-       *ret_x_cand = NULL;
-
        wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-seed", pwd_seed, SHA256_MAC_LEN);
 
        /* pwd-value = KDF-z(pwd-seed, "SAE Hunting and Pecking", p) */
@@ -309,7 +307,7 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
                            prime, sae->tmp->prime_len, pwd_value, bits) < 0)
                return -1;
        if (bits % 8)
-               buf_shift_right(pwd_value, sizeof(pwd_value), 8 - bits % 8);
+               buf_shift_right(pwd_value, sae->tmp->prime_len, 8 - bits % 8);
        wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-value",
                        pwd_value, sae->tmp->prime_len);
 
@@ -320,20 +318,13 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
        if (!x_cand)
                return -1;
        y_sqr = crypto_ec_point_compute_y_sqr(sae->tmp->ec, x_cand);
-       if (!y_sqr) {
-               crypto_bignum_deinit(x_cand, 1);
+       crypto_bignum_deinit(x_cand, 1);
+       if (!y_sqr)
                return -1;
-       }
 
        res = is_quadratic_residue_blind(sae, prime, bits, qr, qnr, y_sqr);
        crypto_bignum_deinit(y_sqr, 1);
-       if (res <= 0) {
-               crypto_bignum_deinit(x_cand, 1);
-               return res;
-       }
-
-       *ret_x_cand = x_cand;
-       return 1;
+       return res;
 }
 
 
@@ -454,25 +445,30 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
        const u8 *addr[3];
        size_t len[3];
        size_t num_elem;
-       u8 dummy_password[32];
-       size_t dummy_password_len;
+       u8 *dummy_password, *tmp_password;
        int pwd_seed_odd = 0;
        u8 prime[SAE_MAX_ECC_PRIME_LEN];
        size_t prime_len;
-       struct crypto_bignum *x = NULL, *qr, *qnr;
+       struct crypto_bignum *x = NULL, *qr = NULL, *qnr = NULL;
+       u8 x_bin[SAE_MAX_ECC_PRIME_LEN];
+       u8 x_cand_bin[SAE_MAX_ECC_PRIME_LEN];
        size_t bits;
-       int res;
+       int res = -1;
+       u8 found = 0; /* 0 (false) or 0xff (true) to be used as const_time_*
+                      * mask */
 
-       dummy_password_len = password_len;
-       if (dummy_password_len > sizeof(dummy_password))
-               dummy_password_len = sizeof(dummy_password);
-       if (random_get_bytes(dummy_password, dummy_password_len) < 0)
-               return -1;
+       os_memset(x_bin, 0, sizeof(x_bin));
+
+       dummy_password = os_malloc(password_len);
+       tmp_password = os_malloc(password_len);
+       if (!dummy_password || !tmp_password ||
+           random_get_bytes(dummy_password, password_len) < 0)
+               goto fail;
 
        prime_len = sae->tmp->prime_len;
        if (crypto_bignum_to_bin(sae->tmp->prime, prime, sizeof(prime),
                                 prime_len) < 0)
-               return -1;
+               goto fail;
        bits = crypto_ec_prime_len_bits(sae->tmp->ec);
 
        /*
@@ -481,7 +477,7 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
         */
        if (get_random_qr_qnr(prime, prime_len, sae->tmp->prime, bits,
                              &qr, &qnr) < 0)
-               return -1;
+               goto fail;
 
        wpa_hexdump_ascii_key(MSG_DEBUG, "SAE: password",
                              password, password_len);
@@ -497,7 +493,7 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
         */
        sae_pwd_seed_key(addr1, addr2, addrs);
 
-       addr[0] = password;
+       addr[0] = tmp_password;
        len[0] = password_len;
        num_elem = 1;
        if (identifier) {
@@ -514,9 +510,8 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
         * attacks that attempt to determine the number of iterations required
         * in the loop.
         */
-       for (counter = 1; counter <= k || !x; counter++) {
+       for (counter = 1; counter <= k || !found; counter++) {
                u8 pwd_seed[SHA256_MAC_LEN];
-               struct crypto_bignum *x_cand;
 
                if (counter > 200) {
                        /* This should not happen in practice */
@@ -524,36 +519,45 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
                        break;
                }
 
-               wpa_printf(MSG_DEBUG, "SAE: counter = %u", counter);
+               wpa_printf(MSG_DEBUG, "SAE: counter = %03u", counter);
+               const_time_select_bin(found, dummy_password, password,
+                                     password_len, tmp_password);
                if (hmac_sha256_vector(addrs, sizeof(addrs), num_elem,
                                       addr, len, pwd_seed) < 0)
                        break;
 
                res = sae_test_pwd_seed_ecc(sae, pwd_seed,
-                                           prime, qr, qnr, &x_cand);
+                                           prime, qr, qnr, x_cand_bin);
+               const_time_select_bin(found, x_bin, x_cand_bin, prime_len,
+                                     x_bin);
+               pwd_seed_odd = const_time_select_u8(
+                       found, pwd_seed_odd,
+                       pwd_seed[SHA256_MAC_LEN - 1] & 0x01);
+               os_memset(pwd_seed, 0, sizeof(pwd_seed));
                if (res < 0)
                        goto fail;
-               if (res > 0 && !x) {
-                       wpa_printf(MSG_DEBUG,
-                                  "SAE: Selected pwd-seed with counter %u",
-                                  counter);
-                       x = x_cand;
-                       pwd_seed_odd = pwd_seed[SHA256_MAC_LEN - 1] & 0x01;
-                       os_memset(pwd_seed, 0, sizeof(pwd_seed));
+               /* Need to minimize differences in handling res == 0 and 1 here
+                * to avoid differences in timing and instruction cache access,
+                * so use const_time_select_*() to make local copies of the
+                * values based on whether this loop iteration was the one that
+                * found the pwd-seed/x. */
+
+               /* found is 0 or 0xff here and res is 0 or 1. Bitwise OR of them
+                * (with res converted to 0/0xff) handles this in constant time.
+                */
+               found |= res * 0xff;
+               wpa_printf(MSG_DEBUG, "SAE: pwd-seed result %d found=0x%02x",
+                          res, found);
+       }
 
-                       /*
-                        * Use a dummy password for the following rounds, if
-                        * any.
-                        */
-                       addr[0] = dummy_password;
-                       len[0] = dummy_password_len;
-               } else if (res > 0) {
-                       crypto_bignum_deinit(x_cand, 1);
-               }
+       if (!found) {
+               wpa_printf(MSG_DEBUG, "SAE: Could not generate PWE");
+               res = -1;
+               goto fail;
        }
 
+       x = crypto_bignum_init_set(x_bin, prime_len);
        if (!x) {
-               wpa_printf(MSG_DEBUG, "SAE: Could not generate PWE");
                res = -1;
                goto fail;
        }
@@ -566,7 +570,6 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
                res = crypto_ec_point_solve_y_coord(sae->tmp->ec,
                                                    sae->tmp->pwe_ecc, x,
                                                    pwd_seed_odd);
-       crypto_bignum_deinit(x, 1);
        if (res < 0) {
                /*
                 * This should not happen since we already checked that there
@@ -578,6 +581,11 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
 fail:
        crypto_bignum_deinit(qr, 0);
        crypto_bignum_deinit(qnr, 0);
+       os_free(dummy_password);
+       bin_clear_free(tmp_password, password_len);
+       crypto_bignum_deinit(x, 1);
+       os_memset(x_bin, 0, sizeof(x_bin));
+       os_memset(x_cand_bin, 0, sizeof(x_cand_bin));
 
        return res;
 }