netdev - Re: [PATCH] SCTP: fix race between sctp_bind_addr_free() and sctp_bind_addr

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Date:	Wed, 18 May 2011 10:06:01 +0200
From:	Jacek Luczak <difrost.kernel@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Vlad Yasevich <vladislav.yasevich@...com>
Subject: Re: [PATCH] SCTP: fix race between sctp_bind_addr_free() and sctp_bind_addr_conflict()

2011/5/18 Eric Dumazet <eric.dumazet@...il.com>:
> Le mercredi 18 mai 2011 à 09:01 +0200, Jacek Luczak a écrit :
>> During the sctp_close() call, we do not use rcu primitives to
>> destroy the address list attached to the endpoint.  At the same
>> time, we do the removal of addresses from this list before
>> attempting to remove the socket from the port hash
>>
>> As a result, it is possible for another process to find the socket
>> in the port hash that is in the process of being closed.  It then
>> proceeds to traverse the address list to find the conflict, only
>> to have that address list suddenly disappear without rcu() critical
>> section.
>>
>> This can result in a kernel crash with general protection fault or
>> kernel NULL pointer dereference.
>>
>> Fix issue by closing address list removal inside RCU critical
>> section.
>>
>> Signed-off-by: Jacek Luczak <luczak.jacek@...il.com>
>> Acked-by: Vlad Yasevich <vladislav.yasevich@...com>
>>
>> ---
>>  bind_addr.c |   12 ++++++++++--
>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/sctp/bind_addr.c b/net/sctp/bind_addr.c
>> index faf71d1..19d1329 100644
>> --- a/net/sctp/bind_addr.c
>> +++ b/net/sctp/bind_addr.c
>> @@ -155,8 +155,16 @@ static void sctp_bind_addr_clean(struct sctp_bind_addr *bp)
>>  /* Dispose of an SCTP_bind_addr structure  */
>>  void sctp_bind_addr_free(struct sctp_bind_addr *bp)
>>  {
>> -       /* Empty the bind address list. */
>> -       sctp_bind_addr_clean(bp);
>> +       struct sctp_sockaddr_entry *addr;
>> +
>> +       /* Empty the bind address list inside RCU section. */
>> +       rcu_read_lock();
>> +       list_for_each_entry_rcu(addr, &bp->address_list, list) {
>> +               list_del_rcu(&addr->list);
>> +               call_rcu(&addr->rcu, sctp_local_addr_free);
>> +               SCTP_DBG_OBJCNT_DEC(addr);
>> +       }
>> +       rcu_read_unlock();
>>
>
> Sorry this looks odd.
>
> If you're removing items from this list, you must be a writer here, with
> exclusive access. So rcu_read_lock()/rcu_read_unlock() is not necessary.

I could agree to some extend ... but strict RCU section IMO is needed here.
I can check this if the issue exists.

> Therefore, I guess following code is better :
>
> list_for_each_entry(addr, &bp->address_list, list) {
>        list_del_rcu(&addr->list);
>        call_rcu(&addr->rcu, sctp_local_addr_free);
>        SCTP_DBG_OBJCNT_DEC(addr);
> }
>
> Then, why dont you fix sctp_bind_addr_clean() instead ?
>
> if 'struct sctp_sockaddr_entry' is recu protected, then all frees should
> be protected as well.

The _clean() as claimed by Vlad is called many times from various places
in code and this could give a overhead. I guess Vlad would need to comment.

-Jacek
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