Fix a padding oracle in PKCS7_dataDecode and CMS_decrypt_set1_pkey

An attack is simple, if the first CMS_recipientInfo is valid but the
second CMS_recipientInfo is chosen ciphertext. If the second
recipientInfo decodes to PKCS #1 v1.5 form plaintext, the correct
encryption key will be replaced by garbage, and the message cannot be
decoded, but if the RSA decryption fails, the correct encryption key is
used and the recipient will not notice the attack.

As a work around for this potential attack the length of the decrypted
key must be equal to the cipher default key length, in case the
certifiate is not given and all recipientInfo are tried out.

The old behaviour can be re-enabled in the CMS code by setting the
CMS_DEBUG_DECRYPT flag.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #9777)

(cherry picked from commit 5840ed0)

CHANGES

@@ -39,6 +39,20 @@
      (CVE-2019-1547)
      [Billy Bob Brumley]
 
+  *) Fixed a padding oracle in PKCS7_dataDecode and CMS_decrypt_set1_pkey.
+     An attack is simple, if the first CMS_recipientInfo is valid but the
+     second CMS_recipientInfo is chosen ciphertext. If the second
+     recipientInfo decodes to PKCS #1 v1.5 form plaintext, the correct
+     encryption key will be replaced by garbage, and the message cannot be
+     decoded, but if the RSA decryption fails, the correct encryption key is
+     used and the recipient will not notice the attack.
+     As a work around for this potential attack the length of the decrypted
+     key must be equal to the cipher default key length, in case the
+     certifiate is not given and all recipientInfo are tried out.
+     The old behaviour can be re-enabled in the CMS code by setting the
+     CMS_DEBUG_DECRYPT flag.
+     [Bernd Edlinger]
+
   *) Document issue with installation paths in diverse Windows builds
 
      '/usr/local/ssl' is an unsafe prefix for location to install OpenSSL

crypto/cms/cms_env.c

@@ -422,6 +422,7 @@ static int cms_RecipientInfo_ktri_decrypt(CMS_ContentInfo *cms,
     unsigned char *ek = NULL;
     size_t eklen;
     int ret = 0;
+    size_t fixlen = 0;
     CMS_EncryptedContentInfo *ec;
     ec = cms->d.envelopedData->encryptedContentInfo;
 
@@ -430,6 +431,19 @@ static int cms_RecipientInfo_ktri_decrypt(CMS_ContentInfo *cms,
         return 0;
     }
 
+    if (cms->d.envelopedData->encryptedContentInfo->havenocert
+            && !cms->d.envelopedData->encryptedContentInfo->debug) {
+        X509_ALGOR *calg = ec->contentEncryptionAlgorithm;
+        const EVP_CIPHER *ciph = EVP_get_cipherbyobj(calg->algorithm);
+
+        if (ciph == NULL) {
+            CMSerr(CMS_F_CMS_RECIPIENTINFO_KTRI_DECRYPT, CMS_R_UNKNOWN_CIPHER);
+            return 0;
+        }
+
+        fixlen = EVP_CIPHER_key_length(ciph);
+    }
+
     ktri->pctx = EVP_PKEY_CTX_new(pkey, NULL);
     if (!ktri->pctx)
         return 0;
@@ -460,7 +474,9 @@ static int cms_RecipientInfo_ktri_decrypt(CMS_ContentInfo *cms,
 
     if (EVP_PKEY_decrypt(ktri->pctx, ek, &eklen,
                          ktri->encryptedKey->data,
-                         ktri->encryptedKey->length) <= 0) {
+                         ktri->encryptedKey->length) <= 0
+            || eklen == 0
+            || (fixlen != 0 && eklen != fixlen)) {
         CMSerr(CMS_F_CMS_RECIPIENTINFO_KTRI_DECRYPT, CMS_R_CMS_LIB);
         goto err;
     }

crypto/cms/cms_lcl.h

@@ -172,6 +172,8 @@ struct CMS_EncryptedContentInfo_st {
     size_t keylen;
     /* Set to 1 if we are debugging decrypt and don't fake keys for MMA */
     int debug;
+    /* Set to 1 if we have no cert and need extra safety measures for MMA */
+    int havenocert;
 };
 
 struct CMS_RecipientInfo_st {

crypto/cms/cms_smime.c

@@ -737,6 +737,10 @@ int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert,
         cms->d.envelopedData->encryptedContentInfo->debug = 1;
     else
         cms->d.envelopedData->encryptedContentInfo->debug = 0;
+    if (!cert)
+        cms->d.envelopedData->encryptedContentInfo->havenocert = 1;
+    else
+        cms->d.envelopedData->encryptedContentInfo->havenocert = 0;
     if (!pk && !cert && !dcont && !out)
         return 1;
     if (pk && !CMS_decrypt_set1_pkey(cms, pk, cert))

crypto/pkcs7/pk7_doit.c

@@ -191,7 +191,8 @@ static int pkcs7_encode_rinfo(PKCS7_RECIP_INFO *ri,
 }
 
 static int pkcs7_decrypt_rinfo(unsigned char **pek, int *peklen,
-                               PKCS7_RECIP_INFO *ri, EVP_PKEY *pkey)
+                               PKCS7_RECIP_INFO *ri, EVP_PKEY *pkey,
+                               size_t fixlen)
 {
     EVP_PKEY_CTX *pctx = NULL;
     unsigned char *ek = NULL;
@@ -224,7 +225,9 @@ 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) {
+                         ri->enc_key->data, ri->enc_key->length) <= 0
+            || eklen == 0
+            || (fixlen != 0 && eklen != fixlen)) {
         ret = 0;
         PKCS7err(PKCS7_F_PKCS7_DECRYPT_RINFO, ERR_R_EVP_LIB);
         goto err;
@@ -571,13 +574,14 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
             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)
+                if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey,
+                        EVP_CIPHER_key_length(evp_cipher)) < 0)
                     goto err;
                 ERR_clear_error();
             }
         } else {
             /* Only exit on fatal errors, not decrypt failure */
-            if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey) < 0)
+            if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey, 0) < 0)
                 goto err;
             ERR_clear_error();
         }