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Message-ID: <f8247247-4109-18bc-c422-a69619b50258@nvidia.com>
Date: Tue, 17 May 2022 13:04:53 +0300
From: Oz Shlomo <ozsh@...dia.com>
To: Pablo Neira Ayuso <pablo@...filter.org>,
netfilter-devel@...r.kernel.org
Cc: nbd@....name, fw@...len.de, paulb@...dia.com,
netdev@...r.kernel.org, sven.auhagen@...eatech.de
Subject: Re: [PATCH nf,v2] netfilter: flowtable: fix TCP flow teardown
Signed-off-by: Oz Shlomo <ozsh@...dia.com>
On 5/17/2022 12:42 PM, Pablo Neira Ayuso wrote:
> This patch addresses three possible problems:
>
> 1. ct gc may race to undo the timeout adjustment of the packet path, leaving
> the conntrack entry in place with the internal offload timeout (one day).
>
> 2. ct gc removes the ct because the IPS_OFFLOAD_BIT is not set and the CLOSE
> timeout is reached before the flow offload del.
>
> 3. tcp ct is always set to ESTABLISHED with a very long timeout
> in flow offload teardown/delete even though the state might be already
> CLOSED. Also as a remark we cannot assume that the FIN or RST packet
> is hitting flow table teardown as the packet might get bumped to the
> slow path in nftables.
>
> This patch resets IPS_OFFLOAD_BIT from flow_offload_teardown(), so
> conntrack handles the tcp rst/fin packet which triggers the CLOSE/FIN
> state transition.
>
> Moreover, teturn the connection's ownership to conntrack upon teardown
> by clearing the offload flag and fixing the established timeout value.
> The flow table GC thread will asynchonrnously free the flow table and
> hardware offload entries.
>
> Before this patch, the IPS_OFFLOAD_BIT remained set for expired flows on
> which is also misleading since the flow is back to classic conntrack
> path.
>
> If nf_ct_delete() removes the entry from the conntrack table, then it
> calls nf_ct_put() which decrements the refcnt. This is not a problem
> because the flowtable holds a reference to the conntrack object from
> flow_offload_alloc() path which is released via flow_offload_free().
>
> This patch also updates nft_flow_offload to skip packets in SYN_RECV
> state. Since we might miss or bump packets to slow path, we do not know
> what will happen there while we are still in SYN_RECV, this patch
> postpones offload up to the next packet which also aligns to the
> existing behaviour in tc-ct.
>
> flow_offload_teardown() does not reset the existing tcp state from
> flow_offload_fixup_tcp() to ESTABLISHED anymore, packets bump to slow
> path might have already update the state to CLOSE/FIN.
>
> Joint work with Oz and Sven.
>
> Fixes: 1e5b2471bcc4 ("netfilter: nf_flow_table: teardown flow timeout race")
> Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
> ---
> v2: fix nf_conntrack_tcp_established() call, reported by Oz
>
> net/netfilter/nf_flow_table_core.c | 33 +++++++-----------------------
> net/netfilter/nft_flow_offload.c | 3 ++-
> 2 files changed, 9 insertions(+), 27 deletions(-)
>
> diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
> index 20b4a14e5d4e..ebdf5332e838 100644
> --- a/net/netfilter/nf_flow_table_core.c
> +++ b/net/netfilter/nf_flow_table_core.c
> @@ -179,12 +179,11 @@ EXPORT_SYMBOL_GPL(flow_offload_route_init);
>
> static void flow_offload_fixup_tcp(struct ip_ct_tcp *tcp)
> {
> - tcp->state = TCP_CONNTRACK_ESTABLISHED;
> tcp->seen[0].td_maxwin = 0;
> tcp->seen[1].td_maxwin = 0;
> }
>
> -static void flow_offload_fixup_ct_timeout(struct nf_conn *ct)
> +static void flow_offload_fixup_ct(struct nf_conn *ct)
> {
> struct net *net = nf_ct_net(ct);
> int l4num = nf_ct_protonum(ct);
> @@ -193,7 +192,9 @@ static void flow_offload_fixup_ct_timeout(struct nf_conn *ct)
> if (l4num == IPPROTO_TCP) {
> struct nf_tcp_net *tn = nf_tcp_pernet(net);
>
> - timeout = tn->timeouts[TCP_CONNTRACK_ESTABLISHED];
> + flow_offload_fixup_tcp(&ct->proto.tcp);
> +
> + timeout = tn->timeouts[ct->proto.tcp.state];
> timeout -= tn->offload_timeout;
> } else if (l4num == IPPROTO_UDP) {
> struct nf_udp_net *tn = nf_udp_pernet(net);
> @@ -211,18 +212,6 @@ static void flow_offload_fixup_ct_timeout(struct nf_conn *ct)
> WRITE_ONCE(ct->timeout, nfct_time_stamp + timeout);
> }
>
> -static void flow_offload_fixup_ct_state(struct nf_conn *ct)
> -{
> - if (nf_ct_protonum(ct) == IPPROTO_TCP)
> - flow_offload_fixup_tcp(&ct->proto.tcp);
> -}
> -
> -static void flow_offload_fixup_ct(struct nf_conn *ct)
> -{
> - flow_offload_fixup_ct_state(ct);
> - flow_offload_fixup_ct_timeout(ct);
> -}
> -
> static void flow_offload_route_release(struct flow_offload *flow)
> {
> nft_flow_dst_release(flow, FLOW_OFFLOAD_DIR_ORIGINAL);
> @@ -361,22 +350,14 @@ static void flow_offload_del(struct nf_flowtable *flow_table,
> rhashtable_remove_fast(&flow_table->rhashtable,
> &flow->tuplehash[FLOW_OFFLOAD_DIR_REPLY].node,
> nf_flow_offload_rhash_params);
> -
> - clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
> -
> - if (nf_flow_has_expired(flow))
> - flow_offload_fixup_ct(flow->ct);
> - else
> - flow_offload_fixup_ct_timeout(flow->ct);
> -
> flow_offload_free(flow);
> }
>
> void flow_offload_teardown(struct flow_offload *flow)
> {
> + clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
> set_bit(NF_FLOW_TEARDOWN, &flow->flags);
> -
> - flow_offload_fixup_ct_state(flow->ct);
> + flow_offload_fixup_ct(flow->ct);
> }
> EXPORT_SYMBOL_GPL(flow_offload_teardown);
>
> @@ -466,7 +447,7 @@ static void nf_flow_offload_gc_step(struct nf_flowtable *flow_table,
> if (nf_flow_has_expired(flow) ||
> nf_ct_is_dying(flow->ct) ||
> nf_flow_has_stale_dst(flow))
> - set_bit(NF_FLOW_TEARDOWN, &flow->flags);
> + flow_offload_teardown(flow);
>
> if (test_bit(NF_FLOW_TEARDOWN, &flow->flags)) {
> if (test_bit(NF_FLOW_HW, &flow->flags)) {
> diff --git a/net/netfilter/nft_flow_offload.c b/net/netfilter/nft_flow_offload.c
> index 187b8cb9a510..6f0b07fe648d 100644
> --- a/net/netfilter/nft_flow_offload.c
> +++ b/net/netfilter/nft_flow_offload.c
> @@ -298,7 +298,8 @@ static void nft_flow_offload_eval(const struct nft_expr *expr,
> case IPPROTO_TCP:
> tcph = skb_header_pointer(pkt->skb, nft_thoff(pkt),
> sizeof(_tcph), &_tcph);
> - if (unlikely(!tcph || tcph->fin || tcph->rst))
> + if (unlikely(!tcph || tcph->fin || tcph->rst ||
> + !nf_conntrack_tcp_established(ct)))
> goto out;
> break;
> case IPPROTO_UDP:
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