Subject: Re: [PATCH net-next 01/13] tipc: eliminate case of writing to freed memory

From: Jon Maloy <[email-masked]>
Date: Thu, 26 Jun 2014 22:33:04 -0500

On 06/26/2014 05:56 AM, Neil Horman wrote:
> On Wed, Jun 25, 2014 at 08:41:30PM -0500, Jon Maloy wrote:
>> In the function tipc_nodesub_notify() we call a function pointer
>> aggregated into the object to be notified, whereafter we set
>> the function pointer to NULL. However, in some cases the function
>> pointed to will free the struct containing the function pointer,
>> resulting in a write to already freed memory.
>>
>> This bug seems to always have been there, without causing any
>> notable harm.
>>
>> In this commit we fix the problem by inverting the order of the
>> zeroing and the function call.
>>
>> Signed-off-by: Jon Maloy <[email-masked]>
>> ---
>>  net/tipc/node_subscr.c |    6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/tipc/node_subscr.c b/net/tipc/node_subscr.c
>> index 7c59ab1..2d13eea 100644
>> --- a/net/tipc/node_subscr.c
>> +++ b/net/tipc/node_subscr.c
>> @@ -84,11 +84,13 @@ void tipc_nodesub_unsubscribe(struct tipc_node_subscr *node_sub)
>>  void tipc_nodesub_notify(struct list_head *nsub_list)
>>  {
>>  	struct tipc_node_subscr *ns, *safe;
>> +	net_ev_handler handle_node_down;
>>  
>>  	list_for_each_entry_safe(ns, safe, nsub_list, nodesub_list) {
>> -		if (ns->handle_node_down) {
>> -			ns->handle_node_down(ns->usr_handle);
>> +		handle_node_down = ns->handle_node_down;
>> +		if (handle_node_down) {
>>  			ns->handle_node_down = NULL;
>> +			handle_node_down(ns->usr_handle);
>>  		}
>>  	}
>>  }
> If its true that some functions pointed to by handle_node_down free the struct
> pointing to the function pointer, than this change isn't sufficient.  The only
> caller to tipc_nodesub_notify is node_lost_contact, which dereferences the same
> structure right after the call to tipc_nodesub_notify.

In think you misunderstand. The pointer (n_ptr*) passed as parameter in
node_sub_notify() points to a struct of type tipc_node. This one is *not*
freed during the call. But it aggregates a linked list of structs of type
nsub. Those are the ones containing the mentioned function pointer, and
the ones potentially being being released during the traversal of the list.

I cannot see that any of these struct are references after the call to
node_sub_notify().

///jon

>
> Neil
>
>> -- 
>> 1.7.9.5
>>
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