netdev - Re: [PATCH] sctp: Fix sk_ack_backlog wrap-around problem

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Date:	Wed, 11 Jun 2014 12:55:31 -0400
From:	Vlad Yasevich <vyasevich@...il.com>
To:	Xufeng Zhang <xufeng.zhang@...driver.com>, nhorman@...driver.com,
	davem@...emloft.net
CC:	linux-sctp@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sctp: Fix sk_ack_backlog wrap-around problem

On 06/11/2014 08:55 AM, Vlad Yasevich wrote:
> On 06/10/2014 10:37 PM, Xufeng Zhang wrote:
>> Consider the scenario:
>> For a TCP-style socket, while processing the COOKIE_ECHO chunk in
>> sctp_sf_do_5_1D_ce(), after it has passed a series of sanity check,
>> a new association would be created in sctp_unpack_cookie(), but afterwards,
>> some processing maybe failed, and sctp_association_free() will be called to
>> free the previously allocated association, in sctp_association_free(),
>> sk_ack_backlog value is decremented for this socket, since the initial
>> value for sk_ack_backlog is 0, after the decrement, it will be 65535,
>> a wrap-around problem happens, and if we want to establish new associations
>> afterward in the same socket, ABORT would be triggered since sctp deem the
>> accept queue as full.
>> Fix this issue by only decrementing sk_ack_backlog for associations in
>> the endpoint's list.
>>
>> Fix-suggested-by: Neil Horman <nhorman@...driver.com>
>> Signed-off-by: Xufeng Zhang <xufeng.zhang@...driver.com>
>> ---
>>  net/sctp/associola.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
>> index 39579c3..60564f2 100644
>> --- a/net/sctp/associola.c
>> +++ b/net/sctp/associola.c
>> @@ -330,7 +330,7 @@ void sctp_association_free(struct sctp_association *asoc)
>>  	/* Only real associations count against the endpoint, so
>>  	 * don't bother for if this is a temporary association.
>>  	 */
>> -	if (!asoc->temp) {
>> +	if (!asoc->temp && !list_empty(&asoc->asocs)) {
>>  		list_del(&asoc->asocs);
>>  
>>  		/* Decrement the backlog value for a TCP-style listening
>>
> 
> I am not crazy about this patch.   It's been suggested before that may
> be duplicate cookie processing should really be creating a temporary
> association since that's is how that association is being used.

I had another look at the description for triggering this issue and
realized that I was thinking about something else when looking at
this solution.

There is however no need to test both the list and temp value.
We can simply always test the that list is not empty before doing
list_del().

-vlad

>  It
> might be nice at that approach.  It actually benefits us as the
> association destruction would happen immediately instead of being delayed.

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