Re: [PATCH net] tcp: Fix error ts_recent time during three-way handshake

From: Wang Hai
Date: Tue Feb 18 2025 - 21:16:56 EST




On 2025/2/18 21:35, Eric Dumazet wrote:
On Tue, Feb 18, 2025 at 12:00 PM Wang Hai <wanghai38@xxxxxxxxxx> wrote:

If two ack packets from a connection enter tcp_check_req at the same time
through different cpu, it may happen that req->ts_recent is updated with
with a more recent time and the skb with an older time creates a new sock,
which will cause the tcp_validate_incoming check to fail.

cpu1 cpu2
tcp_check_req
tcp_check_req
req->ts_recent = tmp_opt.rcv_tsval = t1
req->ts_recent = tmp_opt.rcv_tsval = t2

newsk->ts_recent = req->ts_recent = t2 // t1 < t2
tcp_child_process
tcp_rcv_state_process
tcp_validate_incoming
tcp_paws_check
if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win) // failed

In tcp_check_req, restore ts_recent to this skb's to fix this bug.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Wang Hai <wanghai38@xxxxxxxxxx>
---
net/ipv4/tcp_minisocks.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index b089b08e9617..0208455f9eb8 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -878,6 +878,10 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
sock_rps_save_rxhash(child, skb);
tcp_synack_rtt_meas(child, req);
*req_stolen = !own_req;
+ if (own_req && tcp_sk(child)->rx_opt.tstamp_ok &&
+ unlikely(tcp_sk(child)->rx_opt.ts_recent != tmp_opt.rcv_tsval))
+ tcp_sk(child)->rx_opt.ts_recent = tmp_opt.rcv_tsval;
+
return inet_csk_complete_hashdance(sk, child, req, own_req);

Have you seen the comment at line 818 ?

/* TODO: We probably should defer ts_recent change once
* we take ownership of @req.
*/

Plan was clear and explained. Why implement something else (and buggy) ?

Hi Eric,

Currently we have a real problem, so we want to solve it. This bug causes the upper layers to be unable to be notified to call accept after the successful three-way handshake.

Skb from cpu1 that fails at tcp_paws_check (which it could have succeeded) will not be able to enter the TCP_ESTABLISHED state, and therefore parent->sk_data_ready(parent) will not be triggered, and skb from cpu2 can complete the three-way handshake, but there is also no way to call parent->sk_data_ready(parent) to notify the upper layer, which will result
in the upper layer not being able to sense and call accept to obtain the nsk.

cpu1 cpu2
tcp_check_req
tcp_check_req
req->ts_recent = tmp_opt.rcv_tsval = t1
req->ts_recent=tmp_opt.rcv_tsval= t2

newsk->ts_recent = req->ts_recent = t2 // t1 < t2
tcp_child_process
tcp_rcv_state_process
tcp_validate_incoming
tcp_paws_check // failed
parent->sk_data_ready(parent); // will not be called
tcp_v4_do_rcv
tcp_rcv_state_process // Complete the three-way handshake
// missing parent->sk_data_ready(parent);