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netfilter: conntrack: improve RST handling when tuple is re-used

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If we receive a SYN packet in original direction on an existing
connection tracking entry, we let this SYN through because conntrack
might be out-of-sync.

Conntrack gets back in sync when server responds with SYN/ACK and state
gets updated accordingly.

However, if server replies with RST, this packet might be marked as
INVALID because td_maxack value reflects the *old* conntrack state
and not the state of the originator of the RST.

Avoid td_maxack-based checks if previous packet was a SYN.

Unfortunately that is not be enough: an out of order ACK in original
direction updates last_index, so we still end up marking valid RST.

Thus disable the sequence check when we are not in established state and
the received RST has a sequence of 0.

Because marking RSTs as invalid usually leads to unwanted timeouts,
also skip RST sequence checks if a conntrack entry is already closing.

Such entries can already be evicted via GC in case the table is full.

Co-developed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Ali Abdallah <aabdallah@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This commit is contained in:
Ali Abdallah 2021-05-20 12:53:11 +02:00 committed by Pablo Neira Ayuso
parent c23a9fd209
commit c4edc3ccbc
1 changed files with 36 additions and 17 deletions
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53
net/netfilter/nf_conntrack_proto_tcp.c
View File

@ -823,6 +823,22 @@ static noinline bool tcp_new(struct nf_conn *ct, const struct sk_buff *skb,
return true;
}
static bool tcp_can_early_drop(const struct nf_conn *ct)
{
switch (ct->proto.tcp.state) {
case TCP_CONNTRACK_FIN_WAIT:
case TCP_CONNTRACK_LAST_ACK:
case TCP_CONNTRACK_TIME_WAIT:
case TCP_CONNTRACK_CLOSE:
case TCP_CONNTRACK_CLOSE_WAIT:
return true;
default:
break;
}
return false;
}
/* Returns verdict for packet, or -1 for invalid. */
int nf_conntrack_tcp_packet(struct nf_conn *ct,
struct sk_buff *skb,
@ -1030,9 +1046,28 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
if (index != TCP_RST_SET)
break;
if (ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET) {
/* If we are closing, tuple might have been re-used already.
* last_index, last_ack, and all other ct fields used for
* sequence/window validation are outdated in that case.
*
* As the conntrack can already be expired by GC under pressure,
* just skip validation checks.
*/
if (tcp_can_early_drop(ct))
goto in_window;
/* td_maxack might be outdated if we let a SYN through earlier */
if ((ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET) &&
ct->proto.tcp.last_index != TCP_SYN_SET) {
u32 seq = ntohl(th->seq);
/* If we are not in established state and SEQ=0 this is most
* likely an answer to a SYN we let go through above (last_index
* can be updated due to out-of-order ACKs).
*/
if (seq == 0 && !nf_conntrack_tcp_established(ct))
break;
if (before(seq, ct->proto.tcp.seen[!dir].td_maxack)) {
/* Invalid RST */
spin_unlock_bh(&ct->lock);
@ -1165,22 +1200,6 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
return NF_ACCEPT;
}
static bool tcp_can_early_drop(const struct nf_conn *ct)
{
switch (ct->proto.tcp.state) {
case TCP_CONNTRACK_FIN_WAIT:
case TCP_CONNTRACK_LAST_ACK:
case TCP_CONNTRACK_TIME_WAIT:
case TCP_CONNTRACK_CLOSE:
case TCP_CONNTRACK_CLOSE_WAIT:
return true;
default:
break;
}
return false;
}
#if IS_ENABLED(CONFIG_NF_CT_NETLINK)
#include <linux/netfilter/nfnetlink.h>