Subject: Re: [Bugme-new] [Bug 13760] New: 2.6.30 kernel locks up with pppoe in back trace (regression)

From: Eric Dumazet <[email-masked]>
Date: Tue, 28 Jul 2009 10:44:56 +0200

Igor M Podlesny a écrit :
> [...]
>> Could have been a problem in net core, perhaps.
>>
>> Below is a ppp fix from 2.6.31, but it seems unlikely to fix your problem.
>>
>> It would help if we could see that trace, please.  A digital photo
>> would suit.
> 
> 	Here it is:
> 
> 		http://bugzilla.kernel.org/attachment.cgi?id=22516
> 
> 	(It's 2.6.30.3)
> 	

Looking at this, I believe net_assign_generic() is not safe.

Two cpus could try to expand/update the array at same time, one update could be lost.

register_pernet_gen_device() has a mutex to guard against concurrent
calls, but net_assign_generic() has no locking at all.

I doubt this is the reason of the crash, still worth to mention it...

[PATCH] net: net_assign_generic() is not SMP safe

Two cpus could try to expand/update the array at same time, one update
could be lost during the copy of old array.

Re-using net_mutex is an easy way to fix this, it was used right
before to allocate the 'id'

Signed-off-by: Eric Dumazet <[email-masked]>
---

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index b7292a2..9c31ad1 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -467,15 +467,17 @@ int net_assign_generic(struct net *net, int id, void *data)
 	BUG_ON(!mutex_is_locked(&net_mutex));
 	BUG_ON(id == 0);
 
+	mutex_lock(&net_mutex);
 	ng = old_ng = net->gen;
 	if (old_ng->len >= id)
 		goto assign;
 
 	ng = kzalloc(sizeof(struct net_generic) +
 			id * sizeof(void *), GFP_KERNEL);
-	if (ng == NULL)
+	if (ng == NULL) {
+		mutex_unlock(&net_mutex);
 		return -ENOMEM;
-
+	}
 	/*
 	 * Some synchronisation notes:
 	 *
@@ -494,6 +496,7 @@ int net_assign_generic(struct net *net, int id, void *data)
 	call_rcu(&old_ng->rcu, net_generic_release);
 assign:
 	ng->ptr[id - 1] = data;
+	mutex_unlock(&net_mutex);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(net_assign_generic);