From nobody Thu Sep 4 03:31:15 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 119A8C6FA82 for ; Tue, 13 Sep 2022 15:44:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235719AbiIMPof (ORCPT ); Tue, 13 Sep 2022 11:44:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56330 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234280AbiIMPn6 (ORCPT ); Tue, 13 Sep 2022 11:43:58 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B0EA485ABD; Tue, 13 Sep 2022 07:48:03 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 2F9F1B80D87; Tue, 13 Sep 2022 14:32:53 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8A533C433D6; Tue, 13 Sep 2022 14:32:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1663079571; bh=ilTAr5nPNvI1E5aLl32zYpZw9kchp8GWq3gidlQZdjk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Fgru0b1sDAfpfoR72OxUM5oyNNbRVabnRAQM2IFubRybSrWknQtJ/n+L38W2oHQTF rX7IsKqpGdXuwsFQ4W10UmSk0tvMuwepfV9q5PRJ1CelNdfnZlrhloOYFD/b3puJJm ZD0MdvxLtnq6SBnXeEOH4alvALX20OjtvFihifhI= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Nagaraj Arankal , Neal Cardwell , Yuchung Cheng , Eric Dumazet , Paolo Abeni , Sasha Levin Subject: [PATCH 4.19 71/79] tcp: fix early ETIMEDOUT after spurious non-SACK RTO Date: Tue, 13 Sep 2022 16:07:29 +0200 Message-Id: <20220913140352.333113786@linuxfoundation.org> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20220913140348.835121645@linuxfoundation.org> References: <20220913140348.835121645@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: Neal Cardwell [ Upstream commit 686dc2db2a0fdc1d34b424ec2c0a735becd8d62b ] Fix a bug reported and analyzed by Nagaraj Arankal, where the handling of a spurious non-SACK RTO could cause a connection to fail to clear retrans_stamp, causing a later RTO to very prematurely time out the connection with ETIMEDOUT. Here is the buggy scenario, expanding upon Nagaraj Arankal's excellent report: (*1) Send one data packet on a non-SACK connection (*2) Because no ACK packet is received, the packet is retransmitted and we enter CA_Loss; but this retransmission is spurious. (*3) The ACK for the original data is received. The transmitted packet is acknowledged. The TCP timestamp is before the retrans_stamp, so tcp_may_undo() returns true, and tcp_try_undo_loss() returns true without changing state to Open (because tcp_is_sack() is false), and tcp_process_loss() returns without calling tcp_try_undo_recovery(). Normally after undoing a CA_Loss episode, tcp_fastretrans_alert() would see that the connection has returned to CA_Open and fall through and call tcp_try_to_open(), which would set retrans_stamp to 0. However, for non-SACK connections we hold the connection in CA_Loss, so do not fall through to call tcp_try_to_open() and do not set retrans_stamp to 0. So retrans_stamp is (erroneously) still non-zero. At this point the first "retransmission event" has passed and been recovered from. Any future retransmission is a completely new "event". However, retrans_stamp is erroneously still set. (And we are still in CA_Loss, which is correct.) (*4) After 16 minutes (to correspond with tcp_retries2=3D15), a new data packet is sent. Note: No data is transmitted between (*3) and (*4) and we disabled keep alives. The socket's timeout SHOULD be calculated from this point in time, but instead it's calculated from the prior "event" 16 minutes ago (step (*2)). (*5) Because no ACK packet is received, the packet is retransmitted. (*6) At the time of the 2nd retransmission, the socket returns ETIMEDOUT, prematurely, because retrans_stamp is (erroneously) too far in the past (set at the time of (*2)). This commit fixes this bug by ensuring that we reuse in tcp_try_undo_loss() the same careful logic for non-SACK connections that we have in tcp_try_undo_recovery(). To avoid duplicating logic, we factor out that logic into a new tcp_is_non_sack_preventing_reopen() helper and call that helper from both undo functions. Fixes: da34ac7626b5 ("tcp: only undo on partial ACKs in CA_Loss") Reported-by: Nagaraj Arankal Link: https://lore.kernel.org/all/SJ0PR84MB1847BE6C24D274C46A1B9B0EB27A9@SJ= 0PR84MB1847.NAMPRD84.PROD.OUTLOOK.COM/ Signed-off-by: Neal Cardwell Signed-off-by: Yuchung Cheng Reviewed-by: Eric Dumazet Link: https://lore.kernel.org/r/20220903121023.866900-1-ncardwell.kernel@gm= ail.com Signed-off-by: Paolo Abeni Signed-off-by: Sasha Levin --- net/ipv4/tcp_input.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 0be9d5d3c032f..aac5d5b739268 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2372,6 +2372,21 @@ static inline bool tcp_may_undo(const struct tcp_soc= k *tp) return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp)); } =20 +static bool tcp_is_non_sack_preventing_reopen(struct sock *sk) +{ + struct tcp_sock *tp =3D tcp_sk(sk); + + if (tp->snd_una =3D=3D tp->high_seq && tcp_is_reno(tp)) { + /* Hold old state until something *above* high_seq + * is ACKed. For Reno it is MUST to prevent false + * fast retransmits (RFC2582). SACK TCP is safe. */ + if (!tcp_any_retrans_done(sk)) + tp->retrans_stamp =3D 0; + return true; + } + return false; +} + /* People celebrate: "We love our President!" */ static bool tcp_try_undo_recovery(struct sock *sk) { @@ -2394,14 +2409,8 @@ static bool tcp_try_undo_recovery(struct sock *sk) } else if (tp->rack.reo_wnd_persist) { tp->rack.reo_wnd_persist--; } - if (tp->snd_una =3D=3D tp->high_seq && tcp_is_reno(tp)) { - /* Hold old state until something *above* high_seq - * is ACKed. For Reno it is MUST to prevent false - * fast retransmits (RFC2582). SACK TCP is safe. */ - if (!tcp_any_retrans_done(sk)) - tp->retrans_stamp =3D 0; + if (tcp_is_non_sack_preventing_reopen(sk)) return true; - } tcp_set_ca_state(sk, TCP_CA_Open); tp->is_sack_reneg =3D 0; return false; @@ -2437,6 +2446,8 @@ static bool tcp_try_undo_loss(struct sock *sk, bool f= rto_undo) NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSPURIOUSRTOS); inet_csk(sk)->icsk_retransmits =3D 0; + if (tcp_is_non_sack_preventing_reopen(sk)) + return true; if (frto_undo || tcp_is_sack(tp)) { tcp_set_ca_state(sk, TCP_CA_Open); tp->is_sack_reneg =3D 0; --=20 2.35.1