From: Florian Westphal <email@example.com> To: Pablo Neira Ayuso <firstname.lastname@example.org> Cc: Florian Westphal <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH nf 0/4] netfilter: conntrack: allow insertion of clashing entries Date: Mon, 17 Feb 2020 21:12:40 +0100 Message-ID: <20200217201240.GH19559@breakpoint.cc> (raw) In-Reply-To: <20200217192546.pa26vfni4kmhlpng@salvia> Pablo Neira Ayuso <email@example.com> wrote: > On Mon, Feb 03, 2020 at 05:37:03PM +0100, Florian Westphal wrote: > > This series allows conntrack to insert a duplicate conntrack entry > > if the reply direction doesn't result in a clash with a different > > original connection. > > Applied, thanks for your patience. > > I introduced the late clash resolution approach to deal with nfqueue, > now this is extended to cover more cases, let's give it a try. Yes, nfqueue is one way this can happen, changes to resolver libraries to issue parallel requests have exposed this race for non-nfqueue case too. > >Alternatives considered were: > >1. Confirm ct entries at allocation time, not in postrouting. > > a. will cause uneccesarry work when the skb that creates the > > conntrack is dropped by ruleset. > > b. in case nat is applied, ct entry would need to be moved in > > the table, which requires another spinlock pair to be taken. > > c. breaks the 'unconfirmed entry is private to cpu' assumption: > > we would need to guard all nfct->ext allocation requests with > > ct->lock spinlock. > > > >2. Make the unconfirmed list a hash table instead of a pcpu list. > > Shares drawback c) of the first alternative. > > The spinlock would need to be grabbed rarely, right? My mean, most > extension allocations happen before insertion to the unconfirmed list. > Only _ext_add() invocations coming after init_conntrack() might > require this. Right, we could add __nf_ct_ext_add() which is unlocked and convert the additions happening before unconfirmed list insertion there. But there are additional problems that I forgot: a) need for one additional lookup after negative result from main table (this time in unconfirmed list). b) Need to asynchronously re-insert the skb at a later time, once the racing entry is confirmed. We can't use the unconfirmed ct as-is, because it may be incomplete. For instance, the racing skb might not yet have hit the nat table, so the ct contains wrong NAT info. I think b) is a non-starter for all of the alternatives, unfortunately.
prev parent reply index Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-03 16:37 Florian Westphal 2020-02-03 16:37 ` [PATCH nf 1/4] netfilter: conntrack: remove two args from resolve_clash Florian Westphal 2020-02-03 16:37 ` [PATCH nf 2/4] netfilter: conntrack: place confirm-bit setting in a helper Florian Westphal 2020-02-03 16:37 ` [PATCH nf 3/4] netfilter: conntrack: split resolve_clash function Florian Westphal 2020-02-03 16:37 ` [PATCH nf 4/4] netfilter: conntrack: allow insertion of clashing entries Florian Westphal 2020-02-17 19:25 ` [PATCH nf 0/4] " Pablo Neira Ayuso 2020-02-17 20:12 ` Florian Westphal [this message]
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