cifs: Fix validation of signed data in smb2

Fixes: c713c8770fa5 ("cifs: push rfc1002 generation down the stack")

We failed to validate signed data returned by the server because
__cifs_calc_signature() now expects to sign the actual data in iov but
we were also passing down the rfc1002 length.

Fix smb3_calc_signature() to calculate signature of rfc1002 length prior
to passing only the actual data iov[1-N] to __cifs_calc_signature(). In
addition, there are a few cases where no rfc1002 length is passed so we
make sure there's one (iov_len == 4).

Signed-off-by: Paulo Alcantara <[e-mail]>
Reviewed-by: Ronnie Sahlberg <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/smb2transport.c

···
 	struct kvec *iov = rqst->rq_iov;
 	struct smb2_sync_hdr *shdr = (struct smb2_sync_hdr *)iov[0].iov_base;
 	struct cifs_ses *ses;
+	struct shash_desc *shash = &server->secmech.sdeschmacsha256->shash;
+	struct smb_rqst drqst;
 
 	ses = smb2_find_smb_ses(server, shdr->SessionId);
 	if (!ses) {
···
 	}
 
 	rc = crypto_shash_setkey(server->secmech.hmacsha256,
-		ses->auth_key.response, SMB2_NTLMV2_SESSKEY_SIZE);
+				 ses->auth_key.response, SMB2_NTLMV2_SESSKEY_SIZE);
 	if (rc) {
 		cifs_dbg(VFS, "%s: Could not update with response\n", __func__);
 		return rc;
 	}
 
-	rc = crypto_shash_init(&server->secmech.sdeschmacsha256->shash);
+	rc = crypto_shash_init(shash);
 	if (rc) {
 		cifs_dbg(VFS, "%s: Could not init sha256", __func__);
 		return rc;
 	}
 
-	rc = __cifs_calc_signature(rqst, server, sigptr,
-		&server->secmech.sdeschmacsha256->shash);
+	/*
+	 * For SMB2+, __cifs_calc_signature() expects to sign only the actual
+	 * data, that is, iov[0] should not contain a rfc1002 length.
+	 *
+	 * Sign the rfc1002 length prior to passing the data (iov[1-N]) down to
+	 * __cifs_calc_signature().
+	 */
+	drqst = *rqst;
+	if (drqst.rq_nvec >= 2 && iov[0].iov_len == 4) {
+		rc = crypto_shash_update(shash, iov[0].iov_base,
+					 iov[0].iov_len);
+		if (rc) {
+			cifs_dbg(VFS, "%s: Could not update with payload\n",
+				 __func__);
+			return rc;
+		}
+		drqst.rq_iov++;
+		drqst.rq_nvec--;
+	}
 
+	rc = __cifs_calc_signature(&drqst, server, sigptr, shash);
 	if (!rc)
 		memcpy(shdr->Signature, sigptr, SMB2_SIGNATURE_SIZE);