1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
|
From cff138b0747fa39765cbc641b66cfa5d7f1735d1 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <jouni@codeaurora.org>
Date: Sat, 2 Mar 2019 16:05:56 +0200
Subject: [PATCH 09/14] SAE: Use constant time operations in
sae_test_pwd_seed_ffc()
Try to avoid showing externally visible timing or memory access
differences regardless of whether the derived pwd-value is smaller than
the group prime.
This is related to CVE-2019-9494.
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Upstream-Status: Backport
CVE: CVE-2019-9494
---
src/common/sae.c | 75 ++++++++++++++++++++++++++++++++++----------------------
1 file changed, 46 insertions(+), 29 deletions(-)
diff --git a/src/common/sae.c b/src/common/sae.c
index fa9a145..eaf825d 100644
--- a/src/common/sae.c
+++ b/src/common/sae.c
@@ -334,14 +334,17 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
}
+/* Returns -1 on fatal failure, 0 if PWE cannot be derived from the provided
+ * pwd-seed, or 1 if a valid PWE was derived from pwd-seed. */
static int sae_test_pwd_seed_ffc(struct sae_data *sae, const u8 *pwd_seed,
struct crypto_bignum *pwe)
{
u8 pwd_value[SAE_MAX_PRIME_LEN];
size_t bits = sae->tmp->prime_len * 8;
u8 exp[1];
- struct crypto_bignum *a, *b;
- int res;
+ struct crypto_bignum *a, *b = NULL;
+ int res, is_val;
+ u8 pwd_value_valid;
wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-seed", pwd_seed, SHA256_MAC_LEN);
@@ -353,16 +356,29 @@ static int sae_test_pwd_seed_ffc(struct sae_data *sae, const u8 *pwd_seed,
wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-value", pwd_value,
sae->tmp->prime_len);
- if (os_memcmp(pwd_value, sae->tmp->dh->prime, sae->tmp->prime_len) >= 0)
- {
- wpa_printf(MSG_DEBUG, "SAE: pwd-value >= p");
- return 0;
- }
+ /* Check whether pwd-value < p */
+ res = const_time_memcmp(pwd_value, sae->tmp->dh->prime,
+ sae->tmp->prime_len);
+ /* pwd-value >= p is invalid, so res is < 0 for the valid cases and
+ * the negative sign can be used to fill the mask for constant time
+ * selection */
+ pwd_value_valid = const_time_fill_msb(res);
+
+ /* If pwd-value >= p, force pwd-value to be < p and perform the
+ * calculations anyway to hide timing difference. The derived PWE will
+ * be ignored in that case. */
+ pwd_value[0] = const_time_select_u8(pwd_value_valid, pwd_value[0], 0);
/* PWE = pwd-value^((p-1)/r) modulo p */
+ res = -1;
a = crypto_bignum_init_set(pwd_value, sae->tmp->prime_len);
+ if (!a)
+ goto fail;
+ /* This is an optimization based on the used group that does not depend
+ * on the password in any way, so it is fine to use separate branches
+ * for this step without constant time operations. */
if (sae->tmp->dh->safe_prime) {
/*
* r = (p-1)/2 for the group used here, so this becomes:
@@ -376,33 +392,34 @@ static int sae_test_pwd_seed_ffc(struct sae_data *sae, const u8 *pwd_seed,
b = crypto_bignum_init_set(exp, sizeof(exp));
if (b == NULL ||
crypto_bignum_sub(sae->tmp->prime, b, b) < 0 ||
- crypto_bignum_div(b, sae->tmp->order, b) < 0) {
- crypto_bignum_deinit(b, 0);
- b = NULL;
- }
+ crypto_bignum_div(b, sae->tmp->order, b) < 0)
+ goto fail;
}
- if (a == NULL || b == NULL)
- res = -1;
- else
- res = crypto_bignum_exptmod(a, b, sae->tmp->prime, pwe);
-
- crypto_bignum_deinit(a, 0);
- crypto_bignum_deinit(b, 0);
+ if (!b)
+ goto fail;
- if (res < 0) {
- wpa_printf(MSG_DEBUG, "SAE: Failed to calculate PWE");
- return -1;
- }
+ res = crypto_bignum_exptmod(a, b, sae->tmp->prime, pwe);
+ if (res < 0)
+ goto fail;
- /* if (PWE > 1) --> found */
- if (crypto_bignum_is_zero(pwe) || crypto_bignum_is_one(pwe)) {
- wpa_printf(MSG_DEBUG, "SAE: PWE <= 1");
- return 0;
- }
+ /* There were no fatal errors in calculations, so determine the return
+ * value using constant time operations. We get here for number of
+ * invalid cases which are cleared here after having performed all the
+ * computation. PWE is valid if pwd-value was less than prime and
+ * PWE > 1. Start with pwd-value check first and then use constant time
+ * operations to clear res to 0 if PWE is 0 or 1.
+ */
+ res = const_time_select_u8(pwd_value_valid, 1, 0);
+ is_val = crypto_bignum_is_zero(pwe);
+ res = const_time_select_u8(const_time_is_zero(is_val), res, 0);
+ is_val = crypto_bignum_is_one(pwe);
+ res = const_time_select_u8(const_time_is_zero(is_val), res, 0);
- wpa_printf(MSG_DEBUG, "SAE: PWE found");
- return 1;
+fail:
+ crypto_bignum_deinit(a, 1);
+ crypto_bignum_deinit(b, 1);
+ return res;
}
--
2.7.4
|
