[PATCH 3.16 151/245] tcp: fix "old stuff" D-SACK causing SACK to be treated as D-SACK

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3.16.83-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Pengcheng Yang <yangpc@xxxxxxxxxx>

commit c9655008e7845bcfdaac10a1ed8554ec167aea88 upstream.

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@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/ipv4/tcp_input.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1713,8 +1713,11 @@ tcp_sacktag_write_queue(struct sock *sk,
 		}
 
 		/* 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++;
 	}




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