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

Ben Hutchings
2020-04-23T23:16:16Z

From: Pengcheng Yang <yangpc@wangsu.com>

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@wangsu.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
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++;
}