File openssl-CVE-2019-1547.patch of Package openssl.12525

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File openssl-CVE-2019-1547.patch of Package openssl.12525

From 21c856b75d81eff61aa63b4f036bb64a85bf6d46 Mon Sep 17 00:00:00 2001
From: Billy Brumley <bbrumley@gmail.com>
Date: Sat, 7 Sep 2019 10:50:58 +0300
Subject: [PATCH] [crypto/ec] for ECC parameters with NULL or zero cofactor,
 compute it

The cofactor argument to EC_GROUP_set_generator is optional, and SCA
mitigations for ECC currently use it. So the library currently falls
back to very old SCA-vulnerable code if the cofactor is not present.

This PR allows EC_GROUP_set_generator to compute the cofactor for all
curves of cryptographic interest. Steering scalar multiplication to more
SCA-robust code.

This issue affects persisted private keys in explicit parameter form,
where the (optional) cofactor field is zero or absent.

It also affects curves not built-in to the library, but constructed
programatically with explicit parameters, then calling
EC_GROUP_set_generator with a nonsensical value (NULL, zero).

The very old scalar multiplication code is known to be vulnerable to
local uarch attacks, outside of the OpenSSL threat model. New results
suggest the code path is also vulnerable to traditional wall clock
timing attacks.

CVE-2019-1547

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9799)
---
 CHANGES            |   7 ++++
 crypto/ec/ec.h     |   6 ++-
 crypto/ec/ec_err.c |   3 +-
 crypto/ec/ec_lib.c | 102 +++++++++++++++++++++++++++++++++++++++++----
 4 files changed, 108 insertions(+), 10 deletions(-)

Index: openssl-1.0.1i/crypto/ec/ec.h
===================================================================
--- openssl-1.0.1i.orig/crypto/ec/ec.h
+++ openssl-1.0.1i/crypto/ec/ec.h
@@ -1155,6 +1155,7 @@ void ERR_load_EC_strings(void);
 #define EC_R_SLOT_FULL					 108
 #define EC_R_UNDEFINED_GENERATOR			 113
 #define EC_R_UNDEFINED_ORDER				 128
+#define EC_R_UNKNOWN_COFACTOR				 152
 #define EC_R_UNKNOWN_GROUP				 129
 #define EC_R_UNKNOWN_ORDER				 114
 #define EC_R_UNSUPPORTED_FIELD				 131
Index: openssl-1.0.1i/crypto/ec/ec_err.c
===================================================================
--- openssl-1.0.1i.orig/crypto/ec/ec_err.c
+++ openssl-1.0.1i/crypto/ec/ec_err.c
@@ -253,6 +253,7 @@ static ERR_STRING_DATA EC_str_reasons[]=
 {ERR_REASON(EC_R_SLOT_FULL)              ,"slot full"},
 {ERR_REASON(EC_R_UNDEFINED_GENERATOR)    ,"undefined generator"},
 {ERR_REASON(EC_R_UNDEFINED_ORDER)        ,"undefined order"},
+{ERR_REASON(EC_R_UNKNOWN_COFACTOR)       ,"unknown cofactor"},
 {ERR_REASON(EC_R_UNKNOWN_GROUP)          ,"unknown group"},
 {ERR_REASON(EC_R_UNKNOWN_ORDER)          ,"unknown order"},
 {ERR_REASON(EC_R_UNSUPPORTED_FIELD)      ,"unsupported field"},
Index: openssl-1.0.1i/crypto/ec/ec_lib.c
===================================================================
--- openssl-1.0.1i.orig/crypto/ec/ec_lib.c
+++ openssl-1.0.1i/crypto/ec/ec_lib.c
@@ -280,6 +280,66 @@ int EC_METHOD_get_field_type(const EC_ME
         return meth->field_type;
         }
 
+/*-
+ * Try computing cofactor from the generator order (n) and field cardinality (q).
+ * This works for all curves of cryptographic interest.
+ *
+ * Hasse thm: q + 1 - 2*sqrt(q) <= n*h <= q + 1 + 2*sqrt(q)
+ * h_min = (q + 1 - 2*sqrt(q))/n
+ * h_max = (q + 1 + 2*sqrt(q))/n
+ * h_max - h_min = 4*sqrt(q)/n
+ * So if n > 4*sqrt(q) holds, there is only one possible value for h:
+ * h = \lfloor (h_min + h_max)/2 \rceil = \lfloor (q + 1)/n \rceil
+ *
+ * Otherwise, zero cofactor and return success.
+ */
+static int ec_guess_cofactor(EC_GROUP *group) {
+	int ret = 0;
+	BN_CTX *ctx = NULL;
+	BIGNUM *q = NULL;
+
+	/*-
+	 * If the cofactor is too large, we cannot guess it.
+	 * The RHS of below is a strict overestimate of lg(4 * sqrt(q))
+	 */
+	if (BN_num_bits(&group->order) <= (BN_num_bits(&group->field) + 1) / 2 + 3) {
+		/* default to 0 */
+		BN_zero(&group->cofactor);
+		/* return success */
+		return 1;
+	}
+
+	if ((ctx = BN_CTX_new()) == NULL)
+        return 0;
+
+	BN_CTX_start(ctx);
+	if ((q = BN_CTX_get(ctx)) == NULL)
+		goto err;
+
+	/* set q = 2**m for binary fields; q = p otherwise */
+	if (group->meth->field_type == NID_X9_62_characteristic_two_field) {
+		BN_zero(q);
+		if (!BN_set_bit(q, BN_num_bits(&group->field) - 1))
+			goto err;
+	} else {
+		if (!BN_copy(q, &group->field))
+			goto err;
+	}
+
+	/* compute h = \lfloor (q + 1)/n \rceil = \lfloor (q + 1 + n/2)/n \rfloor */
+	if (!BN_rshift1(&group->cofactor, &group->order) /* n/2 */
+	    || !BN_add(&group->cofactor, &group->cofactor, q) /* q + n/2 */
+	    /* q + 1 + n/2 */
+	    || !BN_add(&group->cofactor, &group->cofactor, BN_value_one())
+	    /* (q + 1 + n/2)/n */
+	    || !BN_div(&group->cofactor, NULL, &group->cofactor, &group->order, ctx))
+		goto err;
+	ret = 1;
+ err:
+	BN_CTX_end(ctx);
+	BN_CTX_free(ctx);
+	return ret;
+}
 
 int EC_GROUP_set_generator(EC_GROUP *group, const EC_POINT *generator, const BIGNUM *order, const BIGNUM *cofactor)
 	{
@@ -289,6 +349,33 @@ int EC_GROUP_set_generator(EC_GROUP *gro
 		return 0   ;
 		}
 
+	/* require group->field >= 1 */
+	if (BN_is_zero(&group->field) || BN_is_negative(&group->field)) {
+		ECerr(EC_F_EC_GROUP_SET_GENERATOR, EC_R_INVALID_FIELD);
+		return 0;
+	}
+
+	/*-
+	 * - require order >= 1
+	 * - enforce upper bound due to Hasse thm: order can be no more than one bit
+	 *   longer than field cardinality
+	 */
+	if (order == NULL || BN_is_zero(order) || BN_is_negative(order)
+	    || BN_num_bits(order) > BN_num_bits(&group->field) + 1) {
+		ECerr(EC_F_EC_GROUP_SET_GENERATOR, EC_R_INVALID_GROUP_ORDER);
+		return 0;
+	}
+
+	/*-
+	 * Unfortunately the cofactor is an optional field in many standards.
+	 * Internally, the lib uses 0 cofactor as a marker for "unknown cofactor".
+	 * So accept cofactor == NULL or cofactor >= 0.
+	 */
+	if (cofactor != NULL && BN_is_negative(cofactor)) {
+		ECerr(EC_F_EC_GROUP_SET_GENERATOR, EC_R_UNKNOWN_COFACTOR);
+		return 0;
+	}
+
 	if (group->generator == NULL)
 		{
 		group->generator = EC_POINT_new(group);
@@ -296,15 +383,17 @@ int EC_GROUP_set_generator(EC_GROUP *gro
 		}
 	if (!EC_POINT_copy(group->generator, generator)) return 0;
 
-	if (order != NULL)
-		{ if (!BN_copy(&group->order, order)) return 0; }	
-	else
-		BN_zero(&group->order);
-
-	if (cofactor != NULL)
-		{ if (!BN_copy(&group->cofactor, cofactor)) return 0; }	
-	else
+	if (!BN_copy(&group->order, order))
+		return 0;
+
+	/* Either take the provided positive cofactor, or try to compute it */
+	if (cofactor != NULL && !BN_is_zero(cofactor)) {
+		if (!BN_copy(&group->cofactor, cofactor))
+			return 0;
+	} else if (!ec_guess_cofactor(group)) {
 		BN_zero(&group->cofactor);
+		return 0;
+	}
 
 	return 1;
 	}

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