Subject: Re: [PATCH net 1/3] sctp: fix the transport dead race check by using atomic_add_unless on refcnt

From: Vlad Yasevich <[email-masked]>
Date: Fri, 22 Jan 2016 13:54:09 -0500

On 01/22/2016 12:18 PM, Marcelo Ricardo Leitner wrote:
> On Fri, Jan 22, 2016 at 11:50:20AM -0500, Vlad Yasevich wrote:
>> On 01/21/2016 12:49 PM, Xin Long wrote:
>>> Now when __sctp_lookup_association is running in BH, it will try to
>>> check if t->dead is set, but meanwhile other CPUs may be freeing this
>>> transport and this assoc and if it happens that
>>> __sctp_lookup_association checked t->dead a bit too early, it may think
>>> that the association is still good while it was already freed.
>>>
>>> So we fix this race by using atomic_add_unless in sctp_transport_hold.
>>> After we get one transport from hashtable, we will hold it only when
>>> this transport's refcnt is not 0, so that we can make sure t->asoc
>>> cannot be freed before we hold the asoc again.
>>
>> atomic_add_unless() uses atomic_read() to check the value.  Since there
>> don't appear to be any barriers, what guarantees that the value
>> read will not have been modified in another thread under a proper lock?
>>
> 
> atomic_read() is used only as a starting point.  If it got changed in
> between, the new current value (return of atomic_cmpxchg) will be used
> then.
> 
>>>
>>> Note that sctp association is not freed using RCU so we can't use
>>> atomic_add_unless() with it as it may just be too late for that either.
>>>
>>> Fixes: 4f0087812648 ("sctp: apply rhashtable api to send/recv path")
>>> Reported-by: Vlad Yasevich <[email-masked]>
>>> Signed-off-by: Xin Long <[email-masked]>
>>> ---
>>>  include/net/sctp/structs.h |  2 +-
>>>  net/sctp/input.c           | 17 +++++++++++------
>>>  net/sctp/transport.c       |  4 ++--
>>>  3 files changed, 14 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
>>> index 20e7212..344da04 100644
>>> --- a/include/net/sctp/structs.h
>>> +++ b/include/net/sctp/structs.h
>>> @@ -955,7 +955,7 @@ void sctp_transport_route(struct sctp_transport *, union sctp_addr *,
>>>  void sctp_transport_pmtu(struct sctp_transport *, struct sock *sk);
>>>  void sctp_transport_free(struct sctp_transport *);
>>>  void sctp_transport_reset_timers(struct sctp_transport *);
>>> -void sctp_transport_hold(struct sctp_transport *);
>>> +int sctp_transport_hold(struct sctp_transport *);
>>>  void sctp_transport_put(struct sctp_transport *);
>>>  void sctp_transport_update_rto(struct sctp_transport *, __u32);
>>>  void sctp_transport_raise_cwnd(struct sctp_transport *, __u32, __u32);
>>> diff --git a/net/sctp/input.c b/net/sctp/input.c
>>> index bf61dfb..49d2cc7 100644
>>> --- a/net/sctp/input.c
>>> +++ b/net/sctp/input.c
>>> @@ -935,15 +935,22 @@ static struct sctp_association *__sctp_lookup_association(
>>>  					struct sctp_transport **pt)
>>>  {
>>>  	struct sctp_transport *t;
>>> +	struct sctp_association *asoc = NULL;
>>>  
>>> +	rcu_read_lock();
>>>  	t = sctp_addrs_lookup_transport(net, local, peer);
>>> -	if (!t || t->dead)
>>> -		return NULL;
>>> +	if (!t || !sctp_transport_hold(t))
>>> +		goto out;
>>>  
>>> -	sctp_association_hold(t->asoc);
>>> +	asoc = t->asoc;
>>> +	sctp_association_hold(asoc);
>>
>> I don't think you can modify the reference count on a transport, let alone
>> the association outside of a lock.
> 
> The transport memory is not freed, as it's protected by rcu_read_lock(),
> so we are safe to use it yet.
> atomic_ operations include an embedded lock instruction protecting the
> counter itself, there shouldn't be a need to use another lock around it.
> 
> And in the code above, as we could grab a hold on the transport, means
> the association was not freed yet because transports hold a ref on
> assoc. That's why the dance: hold(transport) hold(assoc) put(transport)
> 

OK,  I see how that holds together, but I think there might be hole wrt icmp
handling.  Some icmp processes assume transport can't disappear on them, but in
this case that last put(transport) may result in a call to sctp_transport_destroy()
and that might be bad.  I am looking at it now.

Thanks
-vlad

>   Marcelo
> 
>>
>> -vlad
>>
>>>  	*pt = t;
>>>  
>>> -	return t->asoc;
>>> +	sctp_transport_put(t);
>>> +
>>> +out:
>>> +	rcu_read_unlock();
>>> +	return asoc;
>>>  }
>>>  
>>>  /* Look up an association. protected by RCU read lock */
>>> @@ -955,9 +962,7 @@ struct sctp_association *sctp_lookup_association(struct net *net,
>>>  {
>>>  	struct sctp_association *asoc;
>>>  
>>> -	rcu_read_lock();
>>>  	asoc = __sctp_lookup_association(net, laddr, paddr, transportp);
>>> -	rcu_read_unlock();
>>>  
>>>  	return asoc;
>>>  }
>>> diff --git a/net/sctp/transport.c b/net/sctp/transport.c
>>> index aab9e3f..69f3799 100644
>>> --- a/net/sctp/transport.c
>>> +++ b/net/sctp/transport.c
>>> @@ -296,9 +296,9 @@ void sctp_transport_route(struct sctp_transport *transport,
>>>  }
>>>  
>>>  /* Hold a reference to a transport.  */
>>> -void sctp_transport_hold(struct sctp_transport *transport)
>>> +int sctp_transport_hold(struct sctp_transport *transport)
>>>  {
>>> -	atomic_inc(&transport->refcnt);
>>> +	return atomic_add_unless(&transport->refcnt, 1, 0);
>>>  }
>>>  
>>>  /* Release a reference to a transport and clean up
>>>
>>
>> --
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