Re: [PATCH] target: Fix a double put in transport_free_session

[michael.christie@oracle.com]

On 3/22/21 9:58 PM, Lv Yunlong wrote:
> In transport_free_session, se_nacl is got from se_sess
> with the initial reference. If se_nacl->acl_sess_list is
> empty, se_nacl->dynamic_stop is set to true. Then the first
> target_put_nacl(se_nacl) will drop the initial reference
> and free se_nacl. Later there is a second target_put_nacl()
> to put se_nacl. It may cause error in race.
>> My patch sets se_nacl->dynamic_stop to false to avoid the
> double put.
> 
> Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
> ---
>  drivers/target/target_core_transport.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
> index 5ecb9f18a53d..c266defe694f 100644
> --- a/drivers/target/target_core_transport.c
> +++ b/drivers/target/target_core_transport.c
> @@ -584,8 +584,10 @@ void transport_free_session(struct se_session *se_sess)
>  		}
>  		mutex_unlock(&se_tpg->acl_node_mutex);
>  
> -		if (se_nacl->dynamic_stop)
> +		if (se_nacl->dynamic_stop) {
>  			target_put_nacl(se_nacl);
> +			se_nacl->dynamic_stop = false;
> +		}
>  
>  		target_put_nacl(se_nacl);
Could you describe the race a little more?

Is the race:

1. thread1 called core_tpg_check_initiator_node_acl and found the acl.
sess->se_node_acl is set to the found acl.
2. thread2 is running transport_free_session. It now grabs the acl_node_mutex
and sees se_nacl->acl_sess_list is empty.
3. thread2 does the dynamic_stop=true operations in transport_free_session.
4. thread1 now calls transport_register_session now adds the sess to acl's
acl_sess_list.

Later when the session that thread 1 created is deleted dynamic_stop is still
set, so we do an extra target_put_nacl?

I'm not sure your patch will handle this race. When we delete the session thread1
created dynamic_node_acl is still set, so this:

                mutex_lock(&se_tpg->acl_node_mutex);
                if (se_nacl->dynamic_node_acl &&
                    !se_tfo->tpg_check_demo_mode_cache(se_tpg)) {
                        spin_lock_irqsave(&se_nacl->nacl_sess_lock, flags);
                        if (list_empty(&se_nacl->acl_sess_list))
                                se_nacl->dynamic_stop = true;

can set dynamic_stop to true again and we can end up doing the extra put still.

On top of the extra put we also do

list_del(&se_nacl->acl_list);

twice so we have to handle that as well.

Is there also another bug in this code. If someone adds an acl while there is a
dynamic acl in place core_tpg_add_initiator_node_acl will clear dynamic_node_acl
but we leave the extra reference, so later when transport_free_session is called
we will not do the extra put.