ksmbd: fix null pointer dereference in destroy_previous_session

commit 7ac5b66 upstream.

If client set ->PreviousSessionId on kerberos session setup stage,
NULL pointer dereference error will happen. Since sess->user is not
set yet, It can pass the user argument as NULL to destroy_previous_session.
sess->user will be set in ksmbd_krb5_authenticate(). So this patch move
calling destroy_previous_session() after ksmbd_krb5_authenticate().

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-27391
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: namjaejeon

fs/smb/server/smb2pdu.c

@@ -1605,17 +1605,18 @@ static int krb5_authenticate(struct ksmbd_work *work,
 	out_len = work->response_sz -
 		(le16_to_cpu(rsp->SecurityBufferOffset) + 4);
 
-	/* Check previous session */
-	prev_sess_id = le64_to_cpu(req->PreviousSessionId);
-	if (prev_sess_id && prev_sess_id != sess->id)
-		destroy_previous_session(conn, sess->user, prev_sess_id);
-
 	retval = ksmbd_krb5_authenticate(sess, in_blob, in_len,
 					 out_blob, &out_len);
 	if (retval) {
 		ksmbd_debug(SMB, "krb5 authentication failed\n");
 		return -EINVAL;
 	}
+
+	/* Check previous session */
+	prev_sess_id = le64_to_cpu(req->PreviousSessionId);
+	if (prev_sess_id && prev_sess_id != sess->id)
+		destroy_previous_session(conn, sess->user, prev_sess_id);
+
 	rsp->SecurityBufferLength = cpu_to_le16(out_len);
 
 	if ((conn->sign || server_conf.enforced_signing) ||