Subject: Re: [RFC] netfilter: conntrack race between dump_table and destroy

From: Stephen Hemminger <[email-masked]>
Date: Wed, 24 Nov 2010 23:00:04 -0800

On Thu, 25 Nov 2010 07:34:33 +0100
Eric Dumazet <[email-masked]> wrote:

> Le mercredi 24 novembre 2010 à 22:27 -0800, Stephen Hemminger a écrit :
> > A customer reported a crash and the backtrace showed that
> > ctnetlink_dump_table was running while a conntrack entry was
> > being destroyed.  It looks like the code for walking the table
> > with hlist_nulls_for_each_entry_rcu is not correctly handling the
> > case where it finds a deleted entry.
> > 
> > According to RCU documentation, when using hlist_nulls the reader
> > must handle the case of seeing a deleted entry and not proceed
> > further down the linked list.  For lookup the correct behavior would
> > be to restart the scan, but that would generate duplicate entries.
> > 
> > This patch is the simplest one of three alternatives:
> >   1) if dead entry detected, skip the rest of the hash chain (see below)
> >   2) remember skb location at start of hash chain and rescan that chain
> >   3) switch to using a full lock when scanning rather than RCU.
> > It all depends on the amount of effort versus consistency of results.
> > 
> > Signed-off-by: Stephen Hemminger <[email-masked]>
> > 
> > 
> > --- a/net/netfilter/nf_conntrack_netlink.c	2010-11-24 14:11:27.661682148 -0800
> > +++ b/net/netfilter/nf_conntrack_netlink.c	2010-11-24 14:22:28.431980247 -0800
> > @@ -651,8 +651,12 @@ restart:
> >  			if (NF_CT_DIRECTION(h) != IP_CT_DIR_ORIGINAL)
> >  				continue;
> >  			ct = nf_ct_tuplehash_to_ctrack(h);
> > +
> > +			/* if entry is being deleted then can not proceed
> > +			 * past this point. */
> >  			if (!atomic_inc_not_zero(&ct->ct_general.use))
> > -				continue;
> > +				break;
> > +
> >  			/* Dump entries of a given L3 protocol number.
> >  			 * If it is not specified, ie. l3proto == 0,
> >  			 * then dump everything. */
> > --
> 
> Hmm...
> 
> How restarting the loop can be a problem ? 

At this point in the loop, some entries have been placed in the netlink
dump buffer. Restarting the loop will cause duplicate entries.

> There must be a bug somewhere else that your patch try to avoid, not to
> really fix.
> 
> Normally, destroyed ct is removed eventually from the chain, so this
> lookup should stop.

Because hlist_nulls it is possible to walk into a dead entry, in that
case the next pointer is no longer valid.

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