Re: [PATCH] tcp: fix "old stuff" D-SACK causing SACK to be treated as D-SACK

[David Miller]

From: Pengcheng Yang <yangpc@xxxxxxxxxx>
Date: Mon, 30 Dec 2019 17:54:41 +0800

> 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>

Applied and queued up for -stable, thank you.