Fix for CMS/PKCS7 MMA. If RSA decryption fails use a random key and

continue with symmetric decryption process to avoid leaking timing
information to an attacker.

Thanks to Ivan Nestlerode <inestlerode@us.ibm.com> for discovering
this issue. (CVE-2012-0884)

1 files changed, 47 insertions, 18 deletions

diff --git a/crypto/pkcs7/pk7_doit.c b/crypto/pkcs7/pk7_doit.c
index cb8434b322..fae8eda46c 100644
--- a/crypto/pkcs7/pk7_doit.c
+++ b/crypto/pkcs7/pk7_doit.c
@@ -204,11 +204,11 @@ static int pkcs7_decrypt_rinfo(unsigned char **pek, int *peklen,
 	unsigned char *ek = NULL;
 	size_t eklen;
 
-	int ret = 0;
+	int ret = -1;
 
 	pctx = EVP_PKEY_CTX_new(pkey, NULL);
 	if (!pctx)
-		return 0;
+		return -1;
 
 	if (EVP_PKEY_decrypt_init(pctx) <= 0)
 		goto err;
@@ -235,12 +235,19 @@ static int pkcs7_decrypt_rinfo(unsigned char **pek, int *peklen,
 	if (EVP_PKEY_decrypt(pctx, ek, &eklen,
 				ri->enc_key->data, ri->enc_key->length) <= 0)
 		{
+		ret = 0;
 		PKCS7err(PKCS7_F_PKCS7_DECRYPT_RINFO, ERR_R_EVP_LIB);
 		goto err;
 		}
 
 	ret = 1;
 
+	if (*pek)
+		{
+		OPENSSL_cleanse(*pek, *peklen);
+		OPENSSL_free(*pek);
+		}
+
 	*pek = ek;
 	*peklen = eklen;
 
@@ -500,8 +507,8 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
 		int max;
 		X509_OBJECT ret;
 #endif
-		unsigned char *ek = NULL;
-		int eklen;
+		unsigned char *ek = NULL, *tkey = NULL;
+		int eklen, tkeylen;
 
 		if ((etmp=BIO_new(BIO_f_cipher())) == NULL)
 			{
@@ -534,29 +541,28 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
 			}
 
 		/* If we haven't got a certificate try each ri in turn */
-
 		if (pcert == NULL)
 			{
+			/* Always attempt to decrypt all rinfo even
+			 * after sucess as a defence against MMA timing
+			 * attacks.
+			 */
 			for (i=0; i<sk_PKCS7_RECIP_INFO_num(rsk); i++)
 				{
 				ri=sk_PKCS7_RECIP_INFO_value(rsk,i);
+				
 				if (pkcs7_decrypt_rinfo(&ek, &eklen,
-							ri, pkey) > 0)
-					break;
+							ri, pkey) < 0)
+					goto err;
 				ERR_clear_error();
-				ri = NULL;
-				}
-			if (ri == NULL)
-				{
-				PKCS7err(PKCS7_F_PKCS7_DATADECODE,
-				      PKCS7_R_NO_RECIPIENT_MATCHES_KEY);
-				goto err;
 				}
 			}
 		else
 			{
-			if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey) <= 0)
+			/* Only exit on fatal errors, not decrypt failure */
+			if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey) < 0)
 				goto err;
+			ERR_clear_error();
 			}
 
 		evp_ctx=NULL;
@@ -565,6 +571,19 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
 			goto err;
 		if (EVP_CIPHER_asn1_to_param(evp_ctx,enc_alg->parameter) < 0)
 			goto err;
+		/* Generate random key as MMA defence */
+		tkeylen = EVP_CIPHER_CTX_key_length(evp_ctx);
+		tkey = OPENSSL_malloc(tkeylen);
+		if (!tkey)
+			goto err;
+		if (EVP_CIPHER_CTX_rand_key(evp_ctx, tkey) <= 0)
+			goto err;
+		if (ek == NULL)
+			{
+			ek = tkey;
+			eklen = tkeylen;
+			tkey = NULL;
+			}
 
 		if (eklen != EVP_CIPHER_CTX_key_length(evp_ctx)) {
 			/* Some S/MIME clients don't use the same key
@@ -573,11 +592,16 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
 			 */
 			if(!EVP_CIPHER_CTX_set_key_length(evp_ctx, eklen))
 				{
-				PKCS7err(PKCS7_F_PKCS7_DATADECODE,
-					PKCS7_R_DECRYPTED_KEY_IS_WRONG_LENGTH);
-				goto err;
+				/* Use random key as MMA defence */
+				OPENSSL_cleanse(ek, eklen);
+				OPENSSL_free(ek);
+				ek = tkey;
+				eklen = tkeylen;
+				tkey = NULL;
 				}
 		} 
+		/* Clear errors so we don't leak information useful in MMA */
+		ERR_clear_error();
 		if (EVP_CipherInit_ex(evp_ctx,NULL,NULL,ek,NULL,0) <= 0)
 			goto err;
 
@@ -586,6 +610,11 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
 			OPENSSL_cleanse(ek,eklen);
 			OPENSSL_free(ek);
 			}
+		if (tkey)
+			{
+			OPENSSL_cleanse(tkey,tkeylen);
+			OPENSSL_free(tkey);
+			}
 
 		if (out == NULL)
 			out=etmp;