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From:   Pengcheng Yang <yangpc@wangsu.com>
To:     edumazet@google.com
Cc:     davem@davemloft.net, kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org,
        ast@kernel.org, daniel@iogearbox.net, kafai@fb.com,
        songliubraving@fb.com, yhs@fb.com, andriin@fb.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pengcheng Yang <yangpc@wangsu.com>
Subject: [PATCH] tcp: fix "old stuff" D-SACK causing SACK to be treated as D-SACK
Date:   Mon, 30 Dec 2019 17:54:41 +0800
Message-Id: <1577699681-14748-1-git-send-email-yangpc@wangsu.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When we receive a D-SACK, where the sequence number satisfies:
	undo_marker <= start_seq < end_seq <= prior_snd_una
we consider this is a valid D-SACK and tcp_is_sackblock_valid()
returns true, then this D-SACK is discarded as "old stuff",
but the variable first_sack_index is not marked as negative
in tcp_sacktag_write_queue().

If this D-SACK also carries a SACK that needs to be processed
(for example, the previous SACK segment was lost), this SACK
will be treated as a D-SACK in the following processing of
tcp_sacktag_write_queue(), which will eventually lead to
incorrect updates of undo_retrans and reordering.

Fixes: fd6dad616d4f ("[TCP]: Earlier SACK block verification & simplify access to them")
Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
---
 net/ipv4/tcp_input.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 88b987c..0238b55 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1727,8 +1727,11 @@ static int tcp_sack_cache_ok(const struct tcp_sock *tp, const struct tcp_sack_bl
 		}
 
 		/* Ignore very old stuff early */
-		if (!after(sp[used_sacks].end_seq, prior_snd_una))
+		if (!after(sp[used_sacks].end_seq, prior_snd_una)) {
+			if (i == 0)
+				first_sack_index = -1;
 			continue;
+		}
 
 		used_sacks++;
 	}
-- 
1.8.0