File 0008-rsa-rsa_oaep.c-remove-memcpy-calls-from-RSA_padding_.patch of Package openssl1

From b29b91bc7ea8dfe47d873d7953175c57556a4488 Mon Sep 17 00:00:00 2001
From: Andy Polyakov <appro@openssl.org>
Date: Thu, 6 Sep 2018 21:54:23 +0200
Subject: [PATCH 4/5] rsa/rsa_oaep.c: remove memcpy calls from
 RSA_padding_check_PKCS1_OAEP.

And make RSAErr call unconditional.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 75f5e944be97f28867e7c489823c889d89d0bd06)

Resolved conflicts:
	crypto/rsa/rsa_oaep.c

(Merged from https://github.com/openssl/openssl/pull/7737)
---
 crypto/rsa/rsa_oaep.c | 96 +++++++++++++++++++++++++------------------
 1 file changed, 56 insertions(+), 40 deletions(-)

Index: openssl-1.0.1i/crypto/rsa/rsa_oaep.c
===================================================================
--- openssl-1.0.1i.orig/crypto/rsa/rsa_oaep.c
+++ openssl-1.0.1i/crypto/rsa/rsa_oaep.c
@@ -121,7 +121,7 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(un
 	const EVP_MD *md, const EVP_MD *mgf1md)
 	{
 	int i, dblen, mlen = -1, one_index = 0, msg_index;
-	unsigned int good, found_one_byte;
+	unsigned int good = 0, found_one_byte, mask;
 	const unsigned char *maskedseed, *maskeddb;
 	/* |em| is the encoded message, zero-padded to exactly |num| bytes:
 	 * em = Y || maskedSeed || maskedDB */
@@ -146,8 +146,11 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(un
 	 * the ciphertext, see PKCS #1 v2.2, section 7.1.2.
 	 * This does not leak any side-channel information.
 	 */
-	if (num < flen || num < 2 * mdlen + 2)
-		goto decoding_err;
+	if (num < flen || num < 2 * mdlen + 2) {
+		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1,
+		       RSA_R_OAEP_DECODING_ERROR);
+		return -1;
+	}
 
 	dblen = num - mdlen - 1;
 	db = OPENSSL_malloc(dblen);
@@ -156,26 +159,25 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(un
 	    goto cleanup;
 	}
 
-	if (flen != num) {
-	    em = OPENSSL_malloc(num);
-	    if (em == NULL) {
+	em = OPENSSL_malloc(num);
+	if (em == NULL) {
 		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1,
 		       ERR_R_MALLOC_FAILURE);
 		goto cleanup;
-	    }
-
-	    /*
-	     * Caller is encouraged to pass zero-padded message created with
-	     * BN_bn2binpad, but if it doesn't, we do this zero-padding copy
-	     * to avoid leaking that information. The copy still leaks some
-	     * side-channel information, but it's impossible to have a fixed
-	     * memory access pattern since we can't read out of the bounds of
-	     * |from|.
-	     */
-	    memset(em, 0, num);
-	    memcpy(em + num - flen, from, flen);
-	    from = em;
 	}
+	/*
+	 * Caller is encouraged to pass zero-padded message created with
+	 * BN_bn2binpad. Trouble is that since we can't read out of |from|'s
+	 * bounds, it's impossible to have an invariant memory access pattern
+	 * in case |from| was not zero-padded in advance.
+	 */
+	for (from += flen, em += num, i = 0; i < num; i++) {
+		mask = ~constant_time_is_zero(flen);
+		flen -= 1 & mask;
+		from -= 1 & mask;
+		*--em = *from & mask;
+	}
+	from = em;
 
 	/*
 	 * The first byte must be zero, however we must not leak if this is
@@ -221,31 +223,46 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(un
 	 * so plaintext-awareness ensures timing side-channels are no longer a
 	 * concern.
 	 */
-	if (!good)
-		goto decoding_err;
 
 	msg_index = one_index + 1;
 	mlen = dblen - msg_index;
 
-	if (tlen < mlen)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1, RSA_R_DATA_TOO_LARGE);
-		mlen = -1;
-		}
-	else
-		{
-		memcpy(to, db + msg_index, mlen);
-		goto cleanup;
-		}
+	/*
+	 * For good measure, do this check in constant tine as well.
+	 */
+	good &= constant_time_ge(tlen, mlen);
+	/*
+	 * Even though we can't fake result's length, we can pretend copying
+	 * |tlen| bytes where |mlen| bytes would be real. Last |tlen| of |dblen|
+	 * bytes are viewed as circular buffer with start at |tlen|-|mlen'|,
+	 * where |mlen'| is "saturated" |mlen| value. Deducing information
+	 * about failure or |mlen| would take attacker's ability to observe
+	 * memory access pattern with byte granularity *as it occurs*. It
+	 * should be noted that failure is indistinguishable from normal
+	 * operation if |tlen| is fixed by protocol.
+	 */
+	tlen = constant_time_select_int(constant_time_lt(dblen, tlen), dblen, tlen);
+	msg_index = constant_time_select_int(good, msg_index, dblen - tlen);
+	mlen = dblen - msg_index;
+	for (from = db + msg_index, mask = good, i = 0; i < tlen; i++) {
+		unsigned int equals = constant_time_eq(i, mlen);
+		from -= dblen & equals; /* if (i == dblen) rewind   */
+		mask &= mask ^ equals;  /* if (i == dblen) mask = 0 */
+		to[i] = constant_time_select_8(mask, from[i], to[i]);
+	}
 
-decoding_err:
 	/* To avoid chosen ciphertext attacks, the error message should not reveal
 	 * which kind of decoding error happened. */
 	RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP_MGF1, RSA_R_OAEP_DECODING_ERROR);
+	err_clear_last_constant_time(1 & good);
 cleanup:
-	if (db != NULL) OPENSSL_free(db);
-	if (em != NULL) OPENSSL_free(em);
-	return mlen;
+	OPENSSL_cleanse(seed, sizeof(seed));
+	OPENSSL_cleanse(db, dblen);
+	OPENSSL_free(db);
+	OPENSSL_cleanse(em, num);
+	OPENSSL_free(em);
+
+	return constant_time_select_int(good, mlen, -1);
 	}
 
 int PKCS1_MGF1(unsigned char *mask, long len,