Re: [PATCH] nf_conntrack_core.c: fix for dead connection after flushing conntrack cache

[Joerg Marx <joerg.marx@secunet.com>]

On 05/10/2010 05:23 PM, Patrick McHardy wrote:

...
> 
> I think this should be fine since the race you describe only affects
> unconfirmed conntracks, but it took me a while to realize that all
> the other spots where the DYING bit is set are fine without holding
> the conntrack lock.
> 
> Could you please add a comment to the check in __nf_conntrack_confirm()
> stating that the dying check is supposed to prevent races against
> nf_ct_get_next_corpse()? The semantic of the DYING bit is unfortunately
> a bit overloaded.
> 
> Also, since the condition unconfirmed + dying in nf_conntrack_confirm()
> is highly unlikely, I'd suggest to remove the dying check there and only
> perform it in __nf_conntrack_confirm().
> 

I hope the comment is clearly pointing to the (solved) problem now. I
also removed the obsolete check in nf_conntrack_confirm.

diff --git a/include/net/netfilter/nf_conntrack_core.h
b/include/net/netfilter/nf_conntrack_core.h
index dffde8e..3d7524f 100644
--- a/include/net/netfilter/nf_conntrack_core.h
+++ b/include/net/netfilter/nf_conntrack_core.h
@@ -61,7 +61,7 @@ static inline int nf_conntrack_confirm(struct sk_buff
*skb)
 	int ret = NF_ACCEPT;

 	if (ct && ct != &nf_conntrack_untracked) {
-		if (!nf_ct_is_confirmed(ct) && !nf_ct_is_dying(ct))
+		if (!nf_ct_is_confirmed(ct))
 			ret = __nf_conntrack_confirm(skb);
 		if (likely(ret == NF_ACCEPT))
 			nf_ct_deliver_cached_events(ct);
diff --git a/net/netfilter/nf_conntrack_core.c
b/net/netfilter/nf_conntrack_core.c
index 0c9bbe9..7ff9a40 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -422,6 +422,16 @@ __nf_conntrack_confirm(struct sk_buff *skb)

 	spin_lock_bh(&nf_conntrack_lock);

+	/* We have to check the DYING flag inside the lock to prevent
+	   a race against nf_ct_get_next_corpse() possibly called from
+	   user context, else we insert an already 'dead' hash, blocking
+	   further use of that particular connection -JM */
+
+	if (unlikely(nf_ct_is_dying(ct))) {
+		spin_unlock_bh(&nf_conntrack_lock);
+		return NF_ACCEPT;
+	}
+
 	/* See if there's one in the list already, including reverse:
 	   NAT could have grabbed it without realizing, since we're
 	   not in the hash.  If there is, we lost race. */



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