110 lines
3.6 KiB
Diff
110 lines
3.6 KiB
Diff
From 8abfe72e8c1de1b95f50aa0d9134803b4d00070f Mon Sep 17 00:00:00 2001
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From: Pauli <paul.dale@oracle.com>
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Date: Wed, 24 Oct 2018 07:42:46 +1000
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Subject: [PATCH 2/3] Timing vulnerability in DSA signature generation
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(CVE-2018-0734).
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Avoid a timing attack that leaks information via a side channel that
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triggers when a BN is resized. Increasing the size of the BNs
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prior to doing anything with them suppresses the attack.
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Thanks due to Samuel Weiser for finding and locating this.
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Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
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(Merged from https://github.com/openssl/openssl/pull/7486)
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(cherry picked from commit a9cfb8c2aa7254a4aa6a1716909e3f8cb78049b6)
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---
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crypto/dsa/dsa_ossl.c | 28 +++++++++++++++-------------
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1 file changed, 15 insertions(+), 13 deletions(-)
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diff --git x/crypto/dsa/dsa_ossl.c y/crypto/dsa/dsa_ossl.c
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index ca20811200..2dd2d7489a 100644
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--- x/crypto/dsa/dsa_ossl.c
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+++ y/crypto/dsa/dsa_ossl.c
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@@ -9,6 +9,7 @@
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#include <stdio.h>
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#include "internal/cryptlib.h"
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+#include "internal/bn_int.h"
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#include <openssl/bn.h>
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#include <openssl/sha.h>
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#include "dsa_locl.h"
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@@ -180,9 +181,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
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{
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BN_CTX *ctx = NULL;
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BIGNUM *k, *kinv = NULL, *r = *rp;
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- BIGNUM *l, *m;
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+ BIGNUM *l;
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int ret = 0;
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- int q_bits;
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+ int q_bits, q_words;
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if (!dsa->p || !dsa->q || !dsa->g) {
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DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
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@@ -191,8 +192,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
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k = BN_new();
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l = BN_new();
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- m = BN_new();
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- if (k == NULL || l == NULL || m == NULL)
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+ if (k == NULL || l == NULL)
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goto err;
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if (ctx_in == NULL) {
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@@ -203,9 +203,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
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/* Preallocate space */
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q_bits = BN_num_bits(dsa->q);
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- if (!BN_set_bit(k, q_bits)
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- || !BN_set_bit(l, q_bits)
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- || !BN_set_bit(m, q_bits))
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+ q_words = bn_get_top(dsa->q);
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+ if (!bn_wexpand(k, q_words + 2)
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+ || !bn_wexpand(l, q_words + 2))
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goto err;
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/* Get random k */
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@@ -240,14 +240,17 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
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* small timing information leakage. We then choose the sum that is
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* one bit longer than the modulus.
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*
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- * TODO: revisit the BN_copy aiming for a memory access agnostic
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- * conditional copy.
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+ * There are some concerns about the efficacy of doing this. More
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+ * specificly refer to the discussion starting with:
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+ * https://github.com/openssl/openssl/pull/7486#discussion_r228323705
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+ * The fix is to rework BN so these gymnastics aren't required.
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*/
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if (!BN_add(l, k, dsa->q)
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- || !BN_add(m, l, dsa->q)
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- || !BN_copy(k, BN_num_bits(l) > q_bits ? l : m))
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+ || !BN_add(k, l, dsa->q))
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goto err;
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+ BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
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+
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if ((dsa)->meth->bn_mod_exp != NULL) {
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if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,
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dsa->method_mont_p))
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@@ -260,7 +263,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
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if (!BN_mod(r, r, dsa->q, ctx))
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goto err;
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- /* Compute part of 's = inv(k) (m + xr) mod q' */
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+ /* Compute part of 's = inv(k) (m + xr) mod q' */
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if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)
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goto err;
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@@ -275,7 +278,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
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BN_CTX_free(ctx);
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BN_clear_free(k);
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BN_clear_free(l);
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- BN_clear_free(m);
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return ret;
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}
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--
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2.19.1
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