From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id ECB3D1DA53; Mon, 14 Oct 2024 14:42:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728916965; cv=none; b=LKRzgQqBJl/SgZg754XourQB/CbR2sIW3ZgmpjCDF7Hc+VngW6rbqHWwaOjwq7lCi8lHyf0DwUrOBFwhs7Ya2BCjaMEydEOgDramPpWHqAbRbq+uKS0YLoPqiy22TqlVS8CQlkw+BuytxmVZUv3bvfhEImX0rJEQ+6RHFkt2Nyc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728916965; c=relaxed/simple; bh=bA+qTZi4AvJN7+LCBB3wIpJ2396go4gFclqYU3hhap4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=uSI4xMWfC/YElo/ISh4e/nic+hoT38y9qt1caBef1Kl+cDMcV6MUyZSv47T51eHVDjf7o7CJDdJifJ/eAE7rhFVQWo9Q65P28ET+NGZRiKVAiovpPj+JnA7woxc7zpFujFe95MV7LQBqu3vmNnFs/JWc5Yl3DOTBk7HHYf8YEqQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=E2x4t1qf; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="E2x4t1qf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 227F4C4CEC3; Mon, 14 Oct 2024 14:42:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1728916964; bh=bA+qTZi4AvJN7+LCBB3wIpJ2396go4gFclqYU3hhap4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=E2x4t1qfP4Vs8AQrDWGobPaVJK3pCdqix7D7AsHA308goUsmxkGNuUgXoju012tnX 3mUgabQvfvzlPZHLTJylDIIv0K0UYYqlOaaL0bnkIP/a6+s5CU1uaE7n6+aa/q8KfQ uOf16ncf8/JvEyEPaTNxjva54sfKVYKcjyJL+9+o= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Geumhwan Yu , Neal Cardwell , Yuchung Cheng , Eric Dumazet , Jakub Kicinski , Sasha Levin Subject: [PATCH 6.6 115/213] tcp: fix to allow timestamp undo if no retransmits were sent Date: Mon, 14 Oct 2024 16:20:21 +0200 Message-ID: <20241014141047.458164268@linuxfoundation.org> X-Mailer: git-send-email 2.47.0 In-Reply-To: <20241014141042.954319779@linuxfoundation.org> References: <20241014141042.954319779@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Neal Cardwell [ Upstream commit e37ab7373696e650d3b6262a5b882aadad69bb9e ] Fix the TCP loss recovery undo logic in tcp_packet_delayed() so that it can trigger undo even if TSQ prevents a fast recovery episode from reaching tcp_retransmit_skb(). Geumhwan Yu recently reported that after this commit from 2019: commit bc9f38c8328e ("tcp: avoid unconditional congestion window undo on SYN retransmit") ...and before this fix we could have buggy scenarios like the following: + Due to reordering, a TCP connection receives some SACKs and enters a spurious fast recovery. + TSQ prevents all invocations of tcp_retransmit_skb(), because many skbs are queued in lower layers of the sending machine's network stack; thus tp->retrans_stamp remains 0. + The connection receives a TCP timestamp ECR value echoing a timestamp before the fast recovery, indicating that the fast recovery was spurious. + The connection fails to undo the spurious fast recovery because tp->retrans_stamp is 0, and thus tcp_packet_delayed() returns false, due to the new logic in the 2019 commit: commit bc9f38c8328e ("tcp: avoid unconditional congestion window undo on SYN retransmit") This fix tweaks the logic to be more similar to the tcp_packet_delayed() logic before bc9f38c8328e, except that we take care not to be fooled by the FLAG_SYN_ACKED code path zeroing out tp->retrans_stamp (the bug noted and fixed by Yuchung in bc9f38c8328e). Note that this returns the high-level behavior of tcp_packet_delayed() to again match the comment for the function, which says: "Nothing was retransmitted or returned timestamp is less than timestamp of the first retransmission." Note that this comment is in the original 2005-04-16 Linux git commit, so this is evidently long-standing behavior. Fixes: bc9f38c8328e ("tcp: avoid unconditional congestion window undo on SYN retransmit") Reported-by: Geumhwan Yu Diagnosed-by: Geumhwan Yu Signed-off-by: Neal Cardwell Signed-off-by: Yuchung Cheng Signed-off-by: Eric Dumazet Link: https://patch.msgid.link/20241001200517.2756803-2-ncardwell.sw@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- net/ipv4/tcp_input.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index b565c8a8e7bad..31e4f7bce00ce 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2452,8 +2452,22 @@ static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp, */ static inline bool tcp_packet_delayed(const struct tcp_sock *tp) { - return tp->retrans_stamp && - tcp_tsopt_ecr_before(tp, tp->retrans_stamp); + const struct sock *sk = (const struct sock *)tp; + + if (tp->retrans_stamp && + tcp_tsopt_ecr_before(tp, tp->retrans_stamp)) + return true; /* got echoed TS before first retransmission */ + + /* Check if nothing was retransmitted (retrans_stamp==0), which may + * happen in fast recovery due to TSQ. But we ignore zero retrans_stamp + * in TCP_SYN_SENT, since when we set FLAG_SYN_ACKED we also clear + * retrans_stamp even if we had retransmitted the SYN. + */ + if (!tp->retrans_stamp && /* no record of a retransmit/SYN? */ + sk->sk_state != TCP_SYN_SENT) /* not the FLAG_SYN_ACKED case? */ + return true; /* nothing was retransmitted */ + + return false; } /* Undo procedures. */ -- 2.43.0