tcp: improve SEG.SEQ validation for RST segments

[tuexen]

Summary

A RST segment can be sent in response to
(a) received segment or
(b) by the upper layer protocol.
The SEG.SEQ validation consists of two checks: the in-window check of SEG.SEQ and the exact match check of SEG.SEQ.
For the in-window check, the left edge of the window needs to be based on tp->last_ack_sent to cover the delayed ACK case, where as the right edge needs to be based on tp->rcv_nxt + tp->rcv_wnd. This both assumes that tp->rcv_wnd is not zero. For the special case of tp->rcv_wnd being zero, add checks against tp->last_ack_sent for (a) and on tp->rcv_nxt for (b). This applies to all TCP stacks.
When the exact match of SEG.SEQ is performed, it should be based on tp->last_ack_sent for (a) and on tp->rcv_nxt for (b). To cover both, check for both. Add this to the base stack, since the RACK and BBR stack already did this.

This fixes PR 296594.

[rscheff]

Looks like you do modify the BBR/RACK stack too, unlike the high level description. In both places the same checks used to be done in a mirror image - and this again mirrors each other.

[tuexen]

In D58594#1344809, @rscheff wrote:

Looks like you do modify the BBR/RACK stack too, unlike the high level description. In both places the same checks used to be done in a mirror image - and this again mirrors each other.

My description was bad. The in-window check is fix in both stacks, only the exact match check was already correct for the RACK and BBR stacks.