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centosplus patch [bug#15742]

commit 0beef634b86a1350c31da5fcc2992f0d7c8a622b                                                             
Author: Jan Beulich <JBeulich@suse.com>                                                           
Date:   Thu Jul 7 01:23:57 2016 -0600                                                             
                                                                                                  
    xenbus: don't BUG() on user mode induced condition                                            
                                                                                                  
    Inability to locate a user mode specified transaction ID should not                           
    lead to a kernel crash. For other than XS_TRANSACTION_START also                              
    don't issue anything to xenbus if the specified ID doesn't match that                         
    of any active transaction.                                                                    
                                                                                                  
    Signed-off-by: Jan Beulich <jbeulich@suse.com>                                                
    Cc: <stable@vger.kernel.org>                                                                  
    Signed-off-by: David Vrabel <david.vrabel@citrix.com>

    Applied-by: Akemi Yagi <toracat@centos.org>

--- a/drivers/xen/xenbus/xenbus_dev_frontend.c	2018-11-15 09:07:13.000000000 -0800
+++ b/drivers/xen/xenbus/xenbus_dev_frontend.c	2019-01-25 09:28:08.685086392 -0800
@@ -316,11 +316,18 @@ static int xenbus_write_transaction(unsi
 			rc = -ENOMEM;
 			goto out;
 		}
+	} else {
+		list_for_each_entry(trans, &u->transactions, list)
+			if (trans->handle.id == u->u.msg.tx_id)
+				break;
+		if (&trans->list == &u->transactions)
+			return -ESRCH;
 	}
 
 	reply = xenbus_dev_request_and_reply(&u->u.msg);
 	if (IS_ERR(reply)) {
-		kfree(trans);
+		if (msg_type == XS_TRANSACTION_START)
+			kfree(trans);
 		rc = PTR_ERR(reply);
 		goto out;
 	}
@@ -330,12 +337,7 @@ static int xenbus_write_transaction(unsi
 
 		list_add(&trans->list, &u->transactions);
 	} else if (msg_type == XS_TRANSACTION_END) {
-		list_for_each_entry(trans, &u->transactions, list)
-			if (trans->handle.id == u->u.msg.tx_id)
-				break;
-		BUG_ON(&trans->list == &u->transactions);
 		list_del(&trans->list);
-
 		kfree(trans);
 	}