netdev - Re: [PATCH net-next] rds: tcp: per-netns flag to stop new connection creation when rds-tcp is being dismantled

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Date:   Thu, 25 Jan 2018 11:38:01 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     santosh.shilimkar@...cle.com
Cc:     sowmini.varadhan@...cle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] rds: tcp: per-netns flag to stop new
 connection creation when rds-tcp is being dismantled

From: Santosh Shilimkar <santosh.shilimkar@...cle.com>
Date: Wed, 24 Jan 2018 13:32:20 -0800

> On 1/24/2018 1:03 PM, Sowmini Varadhan wrote:
>> An rds_connection can get added during netns deletion between lines
>> 528
>> and 529 of
>>    506 static void rds_tcp_kill_sock(struct net *net)
>>    :
>>    /* code to pull out all the rds_connections that should be destroyed
>>    */
>>    :
>>    528         spin_unlock_irq(&rds_tcp_conn_lock);
>>    529         list_for_each_entry_safe(tc, _tc, &tmp_list, t_tcp_node)
>>    530                 rds_conn_destroy(tc->t_cpath->cp_conn);
>> Such an rds_connection would miss out the rds_conn_destroy()
>> loop (that cancels all pending work) and (if it was scheduled
>> after netns deletion) could trigger the use-after-free.
>> A similar race-window exists for the module unload path
>> in rds_tcp_exit -> rds_tcp_destroy_conns
>> To avoid the addition of new rds_connections during kill_sock
>> or netns_delete, this patch introduces a per-netns flag,
>> RTN_DELETE_PENDING, that will cause RDS connection creation to fail.
>> RCU is used to make sure that we wait for the critical
>> section of __rds_conn_create threads (that may have started before
>> the setting of RTN_DELETE_PENDING) to complete before starting
>> the connection destruction.
>> Reported-by: syzbot+bbd8e9a06452cc48059b@...kaller.appspotmail.com
>> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
>> ---
>>   net/rds/connection.c |    3 ++
>>   net/rds/tcp.c | 82 ++++++++++++++++++++++++++++++++-----------------
>>   net/rds/tcp.h        |    1 +
>>   3 files changed, 57 insertions(+), 29 deletions(-)
>> 
> FWIW,
> Acked-by: Santosh Shilimkar <santosh.shilimkar@...cle.com>
> 
> Just for archives, just summarizing off-list discussion. Netns
> destroy making use of conn_destroy now which in past was used for only
> module unload is racy.
> 
> Its not possible to make it race free with just flags alone and needs
> rcu sync kind of mechanism. RDS being sensitive to brownouts on
> reconnects, rcu usage was has been minimised. Netns delete
> is expected to be non-frequent operation and hence usage of rcu as
> done in this patch is probably ok. If needed it will be revisited in
> future for optimization.

Please have a look at:

commit 4ee806d51176ba7b8ff1efd81f271d7252e03a1d
Author: Dan Streetman <ddstreet@...e.org>
Date:   Thu Jan 18 16:14:26 2018 -0500

    net: tcp: close sock if net namespace is exiting

in the 'net' tree, it adds a helper 'check_net()' that you can probably
use to guard RDS connection creation in order to avoid these races,
and therefore without the RDS specific RTN_PENDING_DELETE flag.

Thank you.