Fix CVE-2016-2178. Source: From 621eaf49a289bfac26d4cbcdb7396e796784c534 Mon Sep 17 00:00:00 2001 From: Cesar Pereida Date: Mon, 23 May 2016 12:45:25 +0300 Subject: [PATCH 1/2] Fix DSA, preserve BN_FLG_CONSTTIME Operations in the DSA signing algorithm should run in constant time in order to avoid side channel attacks. A flaw in the OpenSSL DSA implementation means that a non-constant time codepath is followed for certain operations. This has been demonstrated through a cache-timing attack to be sufficient for an attacker to recover the private DSA key. CVE-2016-2178 Reviewed-by: Richard Levitte Reviewed-by: Matt Caswell --- crypto/dsa/dsa_ossl.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index efc4f1b..b29eb4b 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -248,9 +248,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, if (!BN_rand_range(&k, dsa->q)) goto err; while (BN_is_zero(&k)) ; - if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) { - BN_set_flags(&k, BN_FLG_CONSTTIME); - } if (dsa->flags & DSA_FLAG_CACHE_MONT_P) { if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p, @@ -279,9 +276,12 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, } K = &kq; + + BN_set_flags(K, BN_FLG_CONSTTIME); } else { K = &k; } + DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, K, dsa->p, ctx, dsa->method_mont_p); if (!BN_mod(r, r, dsa->q, ctx)) -- 2.8.4 From b7d0f2834e139a20560d64c73e2565e93715ce2b Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Tue, 7 Jun 2016 09:12:51 +0100 Subject: [PATCH 2/2] More fix DSA, preserve BN_FLG_CONSTTIME The previous "fix" still left "k" exposed to constant time problems in the later BN_mod_inverse() call. Ensure both k and kq have the BN_FLG_CONSTTIME flag set at the earliest opportunity after creation. CVE-2016-2178 Reviewed-by: Rich Salz --- crypto/dsa/dsa_ossl.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index b29eb4b..58013a4 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -247,7 +247,12 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, do if (!BN_rand_range(&k, dsa->q)) goto err; - while (BN_is_zero(&k)) ; + while (BN_is_zero(&k)); + + if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) { + BN_set_flags(&k, BN_FLG_CONSTTIME); + } + if (dsa->flags & DSA_FLAG_CACHE_MONT_P) { if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p, @@ -261,6 +266,8 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, if (!BN_copy(&kq, &k)) goto err; + BN_set_flags(&kq, BN_FLG_CONSTTIME); + /* * We do not want timing information to leak the length of k, so we * compute g^k using an equivalent exponent of fixed length. (This @@ -276,8 +283,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, } K = &kq; - - BN_set_flags(K, BN_FLG_CONSTTIME); } else { K = &k; } -- 2.8.4