Re: [PATCH 2/2] target: iscsi: fix a race condition when aborting a task

[Maurizio Lombardi <mlombard@redhat.com>]

Dne 27. 10. 20 v 21:03 Michael Christie napsal(a):
> 
> 
>> On Oct 27, 2020, at 12:54 PM, Mike Christie <michael.christie@oracle.com> wrote:
>>
>> On 10/27/20 8:49 AM, Maurizio Lombardi wrote:
>>> Hello Mike,
>>>
>>> Dne 22. 10. 20 v 4:42 Mike Christie napsal(a):
>>>> If we free the cmd from the abort path, then for your conn stop plus abort race case, could we do:
>>>>
>>>> 1. thread1 runs iscsit_release_commands_from_conn and sets CMD_T_FABRIC_STOP.
>>>> 2. thread2 runs iscsit_aborted_task and then does __iscsit_free_cmd. It then returns from the aborted_task callout and we finish target_handle_abort and do:
>>>>
>>>> target_handle_abort -> transport_cmd_check_stop_to_fabric -> lio_check_stop_free -> target_put_sess_cmd
>>>>
>>>> The cmd is now freed.
>>>> 3. thread1 now finishes iscsit_release_commands_from_conn and runs iscsit_free_cmd while accessing a command we just released.
>>>>
>>>>
>>>
>>> Thanks for the review!
>>>
>>> There are definitely some problems with task aborts and commands' refcounting *
>>> but this is a different bug than the one this patch is trying to solve (a race to list_del_init());
>>> unless you are saying that abort tasks should never be executed when the connection 
>>> is going down and we have to prevent such cases from happening at all.
>>
>> Yeah, I think if we prevent the race then we fix the refcount issue and your issue.
>> Here is a patch that is only compile tested:
>>
>> From 209709bcedd9a6ce6003e6bb86f3ebf547dca6af Mon Sep 17 00:00:00 2001
>> From: Mike Christie <michael.christie@oracle.com>
>> Date: Tue, 27 Oct 2020 12:30:53 -0500
>> Subject: [PATCH] iscsi target: fix cmd abort vs fabric stop race
>>
>> The abort and cmd stop paths can race where:
>>
>> 1. thread1 runs iscsit_release_commands_from_conn and sets
>> CMD_T_FABRIC_STOP.
>> 2. thread2 runs iscsit_aborted_task and then does __iscsit_free_cmd. It
>> then returns from the aborted_task callout and we finish
>> target_handle_abort and do:
>>
>> target_handle_abort -> transport_cmd_check_stop_to_fabric ->
>> lio_check_stop_free -> target_put_sess_cmd
>>
>> The cmd is now freed.
>> 3. thread1 now finishes iscsit_release_commands_from_conn and runs
>> iscsit_free_cmd while accessing a command we just released.
>>
>> In __target_check_io_state we check for CMD_T_FABRIC_STOP and set the
>> CMD_T_ABORTED if the driver is not cleaning up the cmd because of
>> a session shutdown. However, iscsit_release_commands_from_conn only
>> sets the CMD_T_FABRIC_STOP and does not check to see if the abort path
>> has claimed completion ownership of the command.
>>
>> This adds a check in iscsit_release_commands_from_conn so only the
>> abort or fabric stop path cleanup the command.
>> ---
>> drivers/target/iscsi/iscsi_target.c | 13 +++++++++++--
>> 1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
>> index f77e5ee..85027d3 100644
>> --- a/drivers/target/iscsi/iscsi_target.c
>> +++ b/drivers/target/iscsi/iscsi_target.c
>> @@ -483,8 +483,7 @@ int iscsit_queue_rsp(struct iscsi_conn *conn, struct iscsi_cmd *cmd)
>> void iscsit_aborted_task(struct iscsi_conn *conn, struct iscsi_cmd *cmd)
>> {
>> 	spin_lock_bh(&conn->cmd_lock);
>> -	if (!list_empty(&cmd->i_conn_node) &&
>> -	    !(cmd->se_cmd.transport_state & CMD_T_FABRIC_STOP))
>> +	if (!list_empty(&cmd->i_conn_node))
>> 		list_del_init(&cmd->i_conn_node);
>> 	spin_unlock_bh(&conn->cmd_lock);
>>
>> @@ -4088,6 +4087,16 @@ static void iscsit_release_commands_from_conn(struct iscsi_conn *conn)
>>
>> 		if (se_cmd->se_tfo != NULL) {
>> 			spin_lock_irq(&se_cmd->t_state_lock);
>> +			if (se_cmd->transport_state & CMD_T_ABORTED) {
>> +				/*
>> +				 * LIO's abort path owns the cleanup for this,
>> +				 * so put it back on the list and let
>> +				 * aborted_task handle it.
>> +				 */
>> +				list_add_tail(&cmd->i_conn_node,
>> +					      &conn->conn_cmd_list);
> 
> 
> That should have been a move from the tmp list back to the conn_cmd_list.

Nice, it looks simple and I will test it.
I am a bit worried there could be other possible race conditions.

Example: 

thread1: connection is going to be closed,
iscsit_release_commands_from_conn() finds a command that is about
to be aborted, re-adds it to conn_cmd_list and proceeds.
iscsit_close_connection() decreases the conn usage count and finally calls iscsit_free_conn(conn)
that destroys the conn structure.

thread2: iscsit_aborted_task() gets called and tries to lock the conn->cmd_lock spinlock, dereferencing
an invalid pointer.

Possible solutions that I can think of:

- Make iscsit_release_commands_from_conn() wait for the abort task to finish
or
- abort handler could hold a reference to the conn structure so that iscsit_close_connection()
will sleep when calling iscsit_check_conn_usage_count(conn) until abort finishes.


Maurizio