[PATCH nf-next] netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

[PATCH nf-next] netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

[Minghao Zhang]

Commit be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST
matches exact sequence") keeps an established conntrack entry in
ESTABLISHED when an in-window RST does not match the expected sequence
number exactly, so the endpoint can validate the RST with a challenge
ACK.

The timeout selection nevertheless uses the CLOSE timeout for every RST
packet.  The bug is that timeout selection is based on the packet type,
not on the state transition result: even when RST validation keeps
new_state in ESTABLISHED, the timeout is still forced to
TCP_CONNTRACK_CLOSE.

Linux TCP independently rate limits challenge ACKs per socket.  A second
non-exact RST can therefore arrive after the first challenge ACK has
restored the timeout but before the rate limit expires.  The second RST
lowers the timeout to 10 seconds again while the endpoint suppresses the
second challenge ACK, allowing the conntrack entry to expire while both
TCP endpoints remain established.

Using the ESTABLISHED timeout for non-exact RSTs would avoid this short
expiration window, but it could also retain stale entries for the
five-day default because conntrack cannot reliably match the endpoint's
exact TCP state.

Use the UNACK timeout for RST packets that do not move the conntrack
entry to TCP_CONNTRACK_CLOSE.  Exact-match RSTs and accepted RST packet
trains still use the CLOSE timeout because their state transition result
is CLOSE.  This avoids the exploitable 10-second expiration window for
non-exact RSTs while preserving the short timeout for RSTs that
conntrack accepts as closing the flow.

Fixes: be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST matches exact sequence")
Suggested-by: Florian Westphal <fw@strlen.de>
Reported-by: Minghao Zhang <zhangmh25@mails.tsinghua.edu.cn>
Reported-by: Jianjun Chen <jianjun@tsinghua.edu.cn>
Signed-off-by: Minghao Zhang <zhangmh25@mails.tsinghua.edu.cn>
---
 net/netfilter/nf_conntrack_proto_tcp.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 0c1d086e9..849471101 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1280,8 +1280,11 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
 	if (ct->proto.tcp.retrans >= tn->tcp_max_retrans &&
 	    timeouts[new_state] > timeouts[TCP_CONNTRACK_RETRANS])
 		timeout = timeouts[TCP_CONNTRACK_RETRANS];
-	else if (unlikely(index == TCP_RST_SET))
+	else if (unlikely(index == TCP_RST_SET &&
+			  new_state == TCP_CONNTRACK_CLOSE))
 		timeout = timeouts[TCP_CONNTRACK_CLOSE];
+	else if (unlikely(index == TCP_RST_SET))
+		timeout = timeouts[TCP_CONNTRACK_UNACK];
 	else if ((ct->proto.tcp.seen[0].flags | ct->proto.tcp.seen[1].flags) &
 		 IP_CT_TCP_FLAG_DATA_UNACKNOWLEDGED &&
 		 timeouts[new_state] > timeouts[TCP_CONNTRACK_UNACK])
-- 
2.43.0

Re: [PATCH nf-next] netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

[Florian Westphal]

Minghao Zhang <zhangmh25@mails.tsinghua.edu.cn> wrote:
> Commit be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST
> matches exact sequence") keeps an established conntrack entry in
> ESTABLISHED when an in-window RST does not match the expected sequence
> number exactly, so the endpoint can validate the RST with a challenge
> ACK.
> 
> The timeout selection nevertheless uses the CLOSE timeout for every RST
> packet.  The bug is that timeout selection is based on the packet type,
> not on the state transition result: even when RST validation keeps
> new_state in ESTABLISHED, the timeout is still forced to
> TCP_CONNTRACK_CLOSE.
> 
> Linux TCP independently rate limits challenge ACKs per socket.  A second
> non-exact RST can therefore arrive after the first challenge ACK has
> restored the timeout but before the rate limit expires.  The second RST
> lowers the timeout to 10 seconds again while the endpoint suppresses the
> second challenge ACK, allowing the conntrack entry to expire while both
> TCP endpoints remain established.
> 
> Using the ESTABLISHED timeout for non-exact RSTs would avoid this short
> expiration window, but it could also retain stale entries for the
> five-day default because conntrack cannot reliably match the endpoint's
> exact TCP state.
> 
> Use the UNACK timeout for RST packets that do not move the conntrack
> entry to TCP_CONNTRACK_CLOSE.  Exact-match RSTs and accepted RST packet
> trains still use the CLOSE timeout because their state transition result
> is CLOSE.  This avoids the exploitable 10-second expiration window for
> non-exact RSTs while preserving the short timeout for RSTs that
> conntrack accepts as closing the flow.

Hmm.

>  		timeout = timeouts[TCP_CONNTRACK_RETRANS];
> -	else if (unlikely(index == TCP_RST_SET))
> +	else if (unlikely(index == TCP_RST_SET &&
> +			  new_state == TCP_CONNTRACK_CLOSE))
>  		timeout = timeouts[TCP_CONNTRACK_CLOSE];

Is this needed?  If new state is TCP_CONNTRACK_CLOSE, the existing
else branch will:
        else
                timeout = timeouts[new_state];

> +	else if (unlikely(index == TCP_RST_SET))
> +		timeout = timeouts[TCP_CONNTRACK_UNACK];

AFAICS this is enough:
-       else if (unlikely(index == TCP_RST_SET))
-               timeout = timeouts[TCP_CONNTRACK_CLOSE];
+       else if (unlikely(index == TCP_RST_SET && new_state != TCP_CONNTRACK_ESTABLISHED))
+               timeout = timeouts[TCP_CONNTRACK_UNACK];

Could you test this?  Thanks!

Re: Re: [PATCH nf-next] netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

[zhangmh25]

 Hi Florian,


  Thanks, yes, the explicit CLOSE branch was redundant.


  I tested the narrower version which only uses UNACK when the RST does not
  close the conntrack entry:


          else if (unlikely(index == TCP_RST_SET &&
                            new_state == TCP_CONNTRACK_ESTABLISHED))
                  timeout = timeouts[TCP_CONNTRACK_UNACK];


  With this version, non-exact in-window RSTs stay in ESTABLISHED and get the
  UNACK timeout, while exact RSTs still fall through to timeouts[new_state] and
  therefore keep using the CLOSE timeout.


  Using new_state != TCP_CONNTRACK_ESTABLISHED would also catch the exact-RST
  case and make it use UNACK instead of CLOSE, so I used the ESTABLISHED-only
  condition in v2.


  I have sent v2 with this simplification.


  Thanks,
  Minghao Zhang


> -----Original Messages-----
> From: "Florian Westphal" <fw@strlen.de>
> Send time:Wednesday, 29/07/2026 23:09:03
> To: "Minghao Zhang" <zhangmh25@mails.tsinghua.edu.cn>
> Cc: netfilter-devel@vger.kernel.org, pablo@netfilter.org, "Jianjun Chen" <jianjun@tsinghua.edu.cn>
> Subject: Re: [PATCH nf-next] netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets
> 
> Minghao Zhang <zhangmh25@mails.tsinghua.edu.cn> wrote:
> > Commit be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST
> > matches exact sequence") keeps an established conntrack entry in
> > ESTABLISHED when an in-window RST does not match the expected sequence
> > number exactly, so the endpoint can validate the RST with a challenge
> > ACK.
> > 
> > The timeout selection nevertheless uses the CLOSE timeout for every RST
> > packet.  The bug is that timeout selection is based on the packet type,
> > not on the state transition result: even when RST validation keeps
> > new_state in ESTABLISHED, the timeout is still forced to
> > TCP_CONNTRACK_CLOSE.
> > 
> > Linux TCP independently rate limits challenge ACKs per socket.  A second
> > non-exact RST can therefore arrive after the first challenge ACK has
> > restored the timeout but before the rate limit expires.  The second RST
> > lowers the timeout to 10 seconds again while the endpoint suppresses the
> > second challenge ACK, allowing the conntrack entry to expire while both
> > TCP endpoints remain established.
> > 
> > Using the ESTABLISHED timeout for non-exact RSTs would avoid this short
> > expiration window, but it could also retain stale entries for the
> > five-day default because conntrack cannot reliably match the endpoint's
> > exact TCP state.
> > 
> > Use the UNACK timeout for RST packets that do not move the conntrack
> > entry to TCP_CONNTRACK_CLOSE.  Exact-match RSTs and accepted RST packet
> > trains still use the CLOSE timeout because their state transition result
> > is CLOSE.  This avoids the exploitable 10-second expiration window for
> > non-exact RSTs while preserving the short timeout for RSTs that
> > conntrack accepts as closing the flow.
> 
> Hmm.
> 
> >  		timeout = timeouts[TCP_CONNTRACK_RETRANS];
> > -	else if (unlikely(index == TCP_RST_SET))
> > +	else if (unlikely(index == TCP_RST_SET &&
> > +			  new_state == TCP_CONNTRACK_CLOSE))
> >  		timeout = timeouts[TCP_CONNTRACK_CLOSE];
> 
> Is this needed?  If new state is TCP_CONNTRACK_CLOSE, the existing
> else branch will:
>         else
>                 timeout = timeouts[new_state];
> 
> > +	else if (unlikely(index == TCP_RST_SET))
> > +		timeout = timeouts[TCP_CONNTRACK_UNACK];
> 
> AFAICS this is enough:
> -       else if (unlikely(index == TCP_RST_SET))
> -               timeout = timeouts[TCP_CONNTRACK_CLOSE];
> +       else if (unlikely(index == TCP_RST_SET && new_state != TCP_CONNTRACK_ESTABLISHED))
> +               timeout = timeouts[TCP_CONNTRACK_UNACK];
> 
> Could you test this?  Thanks!

[PATCH v2 nf-next] netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

[Minghao Zhang]

Commit be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST
matches exact sequence") keeps an established conntrack entry in
ESTABLISHED when an in-window RST does not match the expected sequence
number exactly, so the endpoint can validate the RST with a challenge
ACK.

The timeout selection nevertheless uses the CLOSE timeout for every RST
packet.  The bug is that timeout selection is based on the packet type,
not on the state transition result: even when RST validation keeps
new_state in ESTABLISHED, the timeout is still forced to
TCP_CONNTRACK_CLOSE.

Linux TCP independently rate limits challenge ACKs per socket.  A second
non-exact RST can therefore arrive after the first challenge ACK has
restored the timeout but before the rate limit expires.  The second RST
lowers the timeout to 10 seconds again while the endpoint suppresses the
second challenge ACK, allowing the conntrack entry to expire while both
TCP endpoints remain established.

Using the ESTABLISHED timeout for such RSTs would avoid this short
expiration window, but it could also retain stale entries for the
five-day default because conntrack cannot reliably match the endpoint's
exact TCP state.

Use the UNACK timeout for RST packets that leave the conntrack entry in
TCP_CONNTRACK_ESTABLISHED.  Exact-match RSTs and accepted RST packet
trains still fall through to timeouts[new_state], which preserves the
CLOSE timeout when conntrack accepts the RST as closing the flow.

This avoids the exploitable 10-second expiration window for non-exact
RSTs while preserving the short timeout for RSTs that conntrack accepts
as closing the flow.

Fixes: be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST matches exact sequence")
Suggested-by: Florian Westphal <fw@strlen.de>
Reported-by: Minghao Zhang <zhangmh25@mails.tsinghua.edu.cn>
Reported-by: Jianjun Chen <jianjun@tsinghua.edu.cn>
Signed-off-by: Minghao Zhang <zhangmh25@mails.tsinghua.edu.cn>
---
v2:
 - Simplify the timeout selection as suggested by Pablo.
 - Use the UNACK timeout only when an RST leaves the conntrack entry in
   TCP_CONNTRACK_ESTABLISHED.  RST packets that move the entry to
   TCP_CONNTRACK_CLOSE now fall through to timeouts[new_state].

 net/netfilter/nf_conntrack_proto_tcp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 0c1d086e9..79fba5b8b 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1280,8 +1280,9 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
 	if (ct->proto.tcp.retrans >= tn->tcp_max_retrans &&
 	    timeouts[new_state] > timeouts[TCP_CONNTRACK_RETRANS])
 		timeout = timeouts[TCP_CONNTRACK_RETRANS];
-	else if (unlikely(index == TCP_RST_SET))
-		timeout = timeouts[TCP_CONNTRACK_CLOSE];
+	else if (unlikely(index == TCP_RST_SET &&
+			  new_state == TCP_CONNTRACK_ESTABLISHED))
+		timeout = timeouts[TCP_CONNTRACK_UNACK];
 	else if ((ct->proto.tcp.seen[0].flags | ct->proto.tcp.seen[1].flags) &
 		 IP_CT_TCP_FLAG_DATA_UNACKNOWLEDGED &&
 		 timeouts[new_state] > timeouts[TCP_CONNTRACK_UNACK])
-- 
2.43.0