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From 362704dda04507e7ebb8035122e83d9f0ae7c320 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <jouni@codeaurora.org>
Date: Tue, 26 Feb 2019 19:34:38 +0200
Subject: [PATCH] SAE: Avoid branches in is_quadratic_residue_blind()

Make the non-failure path in the function proceed without branches based
on r_odd and in constant time to minimize risk of observable differences
in timing or cache use. (CVE-2019-9494)

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
---
 src/common/sae.c | 64 ++++++++++++++++++++++++++++--------------------
 1 file changed, 37 insertions(+), 27 deletions(-)

diff --git a/src/common/sae.c b/src/common/sae.c
index d55323bcd..5df9b95aa 100644
--- a/src/common/sae.c
+++ b/src/common/sae.c
@@ -232,12 +232,14 @@ get_rand_1_to_p_1(const u8 *prime, size_t prime_len, size_t prime_bits,
 
 static int is_quadratic_residue_blind(struct sae_data *sae,
 				      const u8 *prime, size_t bits,
-				      const struct crypto_bignum *qr,
-				      const struct crypto_bignum *qnr,
+				      const u8 *qr, const u8 *qnr,
 				      const struct crypto_bignum *y_sqr)
 {
-	struct crypto_bignum *r, *num;
+	struct crypto_bignum *r, *num, *qr_or_qnr = NULL;
 	int r_odd, check, res = -1;
+	u8 qr_or_qnr_bin[SAE_MAX_ECC_PRIME_LEN];
+	size_t prime_len = sae->tmp->prime_len;
+	unsigned int mask;
 
 	/*
 	 * Use the blinding technique to mask y_sqr while determining
@@ -248,7 +250,7 @@ static int is_quadratic_residue_blind(struct sae_data *sae,
 	 * r = a random number between 1 and p-1, inclusive
 	 * num = (v * r * r) modulo p
 	 */
-	r = get_rand_1_to_p_1(prime, sae->tmp->prime_len, bits, &r_odd);
+	r = get_rand_1_to_p_1(prime, prime_len, bits, &r_odd);
 	if (!r)
 		return -1;
 
@@ -258,41 +260,45 @@ static int is_quadratic_residue_blind(struct sae_data *sae,
 	    crypto_bignum_mulmod(num, r, sae->tmp->prime, num) < 0)
 		goto fail;
 
-	if (r_odd) {
-		/*
-		 * num = (num * qr) module p
-		 * LGR(num, p) = 1 ==> quadratic residue
-		 */
-		if (crypto_bignum_mulmod(num, qr, sae->tmp->prime, num) < 0)
-			goto fail;
-		check = 1;
-	} else {
-		/*
-		 * num = (num * qnr) module p
-		 * LGR(num, p) = -1 ==> quadratic residue
-		 */
-		if (crypto_bignum_mulmod(num, qnr, sae->tmp->prime, num) < 0)
-			goto fail;
-		check = -1;
-	}
+	/*
+	 * Need to minimize differences in handling different cases, so try to
+	 * avoid branches and timing differences.
+	 *
+	 * If r_odd:
+	 * num = (num * qr) module p
+	 * LGR(num, p) = 1 ==> quadratic residue
+	 * else:
+	 * num = (num * qnr) module p
+	 * LGR(num, p) = -1 ==> quadratic residue
+	 */
+	mask = const_time_is_zero(r_odd);
+	const_time_select_bin(mask, qnr, qr, prime_len, qr_or_qnr_bin);
+	qr_or_qnr = crypto_bignum_init_set(qr_or_qnr_bin, prime_len);
+	if (!qr_or_qnr ||
+	    crypto_bignum_mulmod(num, qr_or_qnr, sae->tmp->prime, num) < 0)
+		goto fail;
+	/* r_odd is 0 or 1; branchless version of check = r_odd ? 1 : -1, */
+	check = const_time_select_int(mask, -1, 1);
 
 	res = crypto_bignum_legendre(num, sae->tmp->prime);
 	if (res == -2) {
 		res = -1;
 		goto fail;
 	}
-	res = res == check;
+	/* branchless version of res = res == check
+	 * (res is -1, 0, or 1; check is -1 or 1) */
+	mask = const_time_eq(res, check);
+	res = const_time_select_int(mask, 1, 0);
 fail:
 	crypto_bignum_deinit(num, 1);
 	crypto_bignum_deinit(r, 1);
+	crypto_bignum_deinit(qr_or_qnr, 1);
 	return res;
 }
 
 
 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,
+				 const u8 *prime, const u8 *qr, const u8 *qnr,
 				 u8 *pwd_value)
 {
 	struct crypto_bignum *y_sqr, *x_cand;
@@ -452,6 +458,8 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
 	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];
+	u8 qr_bin[SAE_MAX_ECC_PRIME_LEN];
+	u8 qnr_bin[SAE_MAX_ECC_PRIME_LEN];
 	size_t bits;
 	int res = -1;
 	u8 found = 0; /* 0 (false) or 0xff (true) to be used as const_time_*
@@ -476,7 +484,9 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
 	 * (qnr) modulo p for blinding purposes during the loop.
 	 */
 	if (get_random_qr_qnr(prime, prime_len, sae->tmp->prime, bits,
-			      &qr, &qnr) < 0)
+			      &qr, &qnr) < 0 ||
+	    crypto_bignum_to_bin(qr, qr_bin, sizeof(qr_bin), prime_len) < 0 ||
+	    crypto_bignum_to_bin(qnr, qnr_bin, sizeof(qnr_bin), prime_len) < 0)
 		goto fail;
 
 	wpa_hexdump_ascii_key(MSG_DEBUG, "SAE: password",
@@ -527,7 +537,7 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1,
 			break;
 
 		res = sae_test_pwd_seed_ecc(sae, pwd_seed,
-					    prime, qr, qnr, x_cand_bin);
+					    prime, qr_bin, qnr_bin, x_cand_bin);
 		const_time_select_bin(found, x_bin, x_cand_bin, prime_len,
 				      x_bin);
 		pwd_seed_odd = const_time_select_u8(
-- 
2.21.0