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Timing vulnerability in DSA signature generation (CVE-2018-0734).
Avoid a timing attack that leaks information via a side channel that
triggers when a BN is resized. Increasing the size of the BNs
prior to doing anything with them suppresses the attack.
Thanks due to Samuel Weiser for finding and locating this.
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from #7486)
(cherry picked from commit a9cfb8c)
https://github.com/openssl/openssl/commit/8abfe72e8c1de1b95f50aa0d9134803b4d00070f
--- crypto/dsa/dsa_ossl.c.orig 2018-09-11 12:48:21 UTC
+++ crypto/dsa/dsa_ossl.c
@@ -9,6 +9,7 @@
#include <stdio.h>
#include "internal/cryptlib.h"
+#include "internal/bn_int.h"
#include <openssl/bn.h>
#include <openssl/sha.h>
#include "dsa_locl.h"
@@ -178,9 +179,9 @@ static int dsa_sign_setup(DSA *dsa, BN_C
{
BN_CTX *ctx = NULL;
BIGNUM *k, *kinv = NULL, *r = *rp;
- BIGNUM *l, *m;
+ BIGNUM *l;
int ret = 0;
- int q_bits;
+ int q_bits, q_words;
if (!dsa->p || !dsa->q || !dsa->g) {
DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
@@ -189,8 +190,7 @@ static int dsa_sign_setup(DSA *dsa, BN_C
k = BN_new();
l = BN_new();
- m = BN_new();
- if (k == NULL || l == NULL || m == NULL)
+ if (k == NULL || l == NULL)
goto err;
if (ctx_in == NULL) {
@@ -201,9 +201,9 @@ static int dsa_sign_setup(DSA *dsa, BN_C
/* Preallocate space */
q_bits = BN_num_bits(dsa->q);
- if (!BN_set_bit(k, q_bits)
- || !BN_set_bit(l, q_bits)
- || !BN_set_bit(m, q_bits))
+ q_words = bn_get_top(dsa->q);
+ if (!bn_wexpand(k, q_words + 2)
+ || !bn_wexpand(l, q_words + 2))
goto err;
/* Get random k */
@@ -238,14 +238,17 @@ static int dsa_sign_setup(DSA *dsa, BN_C
* small timing information leakage. We then choose the sum that is
* one bit longer than the modulus.
*
- * TODO: revisit the BN_copy aiming for a memory access agnostic
- * conditional copy.
+ * There are some concerns about the efficacy of doing this. More
+ * specificly refer to the discussion starting with:
+ * https://github.com/openssl/openssl/pull/7486#discussion_r228323705
+ * The fix is to rework BN so these gymnastics aren't required.
*/
if (!BN_add(l, k, dsa->q)
- || !BN_add(m, l, dsa->q)
- || !BN_copy(k, BN_num_bits(l) > q_bits ? l : m))
+ || !BN_add(k, l, dsa->q))
goto err;
+ BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
+
if ((dsa)->meth->bn_mod_exp != NULL) {
if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,
dsa->method_mont_p))
@@ -258,7 +261,7 @@ static int dsa_sign_setup(DSA *dsa, BN_C
if (!BN_mod(r, r, dsa->q, ctx))
goto err;
- /* Compute part of 's = inv(k) (m + xr) mod q' */
+ /* Compute part of 's = inv(k) (m + xr) mod q' */
if ((kinv = BN_mod_inverse(NULL, k, dsa->q, ctx)) == NULL)
goto err;
@@ -273,7 +276,6 @@ static int dsa_sign_setup(DSA *dsa, BN_C
BN_CTX_free(ctx);
BN_clear_free(k);
BN_clear_free(l);
- BN_clear_free(m);
return ret;
}
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