TCP to honor updated window size during handshake.

In passive open cases, we transition to Established state after
initializing endpoint's sender and receiver. With this we lose out
on any updates coming from the ACK that completes the handshake.
This change ensures that we uniformly transition to Established in all
cases and does minor cleanups.

Fixes #2938

PiperOrigin-RevId: 316567014

2 files changed, 31 insertions, 29 deletions

diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index ad197e8db..7679fe169 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -222,12 +222,6 @@ func (l *listenContext) createConnectingEndpoint(s *segment, iss seqnum.Value, i
 
 	n.initGSO()
 
-	// Create sender and receiver.
-	//
-	// The receiver at least temporarily has a zero receive window scale,
-	// but the caller may change it (before starting the protocol loop).
-	n.snd = newSender(n, iss, irs, s.window, rcvdSynOpts.MSS, rcvdSynOpts.WS)
-	n.rcv = newReceiver(n, irs, seqnum.Size(n.initialReceiveWindow()), 0, seqnum.Size(n.receiveBufferSize()))
 	// Bootstrap the auto tuning algorithm. Starting at zero will result in
 	// a large step function on the first window adjustment causing the
 	// window to grow to a really large value.
@@ -295,7 +289,7 @@ func (l *listenContext) createEndpointAndPerformHandshake(s *segment, opts *head
 	}
 
 	// Perform the 3-way handshake.
-	h := newPassiveHandshake(ep, ep.rcv.rcvWnd, isn, irs, opts, deferAccept)
+	h := newPassiveHandshake(ep, seqnum.Size(ep.initialReceiveWindow()), isn, irs, opts, deferAccept)
 	if err := h.execute(); err != nil {
 		ep.mu.Unlock()
 		ep.Close()
@@ -536,6 +530,9 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 			return
 		}
 
+		iss := s.ackNumber - 1
+		irs := s.sequenceNumber - 1
+
 		// Since SYN cookies are in use this is potentially an ACK to a
 		// SYN-ACK we sent but don't have a half open connection state
 		// as cookies are being used to protect against a potential SYN
@@ -546,7 +543,7 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 		// when under a potential syn flood attack.
 		//
 		// Validate the cookie.
-		data, ok := ctx.isCookieValid(s.id, s.ackNumber-1, s.sequenceNumber-1)
+		data, ok := ctx.isCookieValid(s.id, iss, irs)
 		if !ok || int(data) >= len(mssTable) {
 			e.stack.Stats().TCP.ListenOverflowInvalidSynCookieRcvd.Increment()
 			e.stack.Stats().DroppedPackets.Increment()
@@ -571,7 +568,7 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 			rcvdSynOptions.TSEcr = s.parsedOptions.TSEcr
 		}
 
-		n, err := ctx.createConnectingEndpoint(s, s.ackNumber-1, s.sequenceNumber-1, rcvdSynOptions, &waiter.Queue{})
+		n, err := ctx.createConnectingEndpoint(s, iss, irs, rcvdSynOptions, &waiter.Queue{})
 		if err != nil {
 			e.stack.Stats().TCP.FailedConnectionAttempts.Increment()
 			e.stats.FailedConnectionAttempts.Increment()
@@ -589,10 +586,17 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 		n.tsOffset = 0
 
 		// Switch state to connected.
-		// We do not use transitionToStateEstablishedLocked here as there is
-		// no handshake state available when doing a SYN cookie based accept.
 		n.isConnectNotified = true
-		n.setEndpointState(StateEstablished)
+		n.transitionToStateEstablishedLocked(&handshake{
+			ep:          n,
+			iss:         iss,
+			ackNum:      irs + 1,
+			rcvWnd:      seqnum.Size(n.initialReceiveWindow()),
+			sndWnd:      s.window,
+			rcvWndScale: e.rcvWndScaleForHandshake(),
+			sndWndScale: rcvdSynOptions.WS,
+			mss:         rcvdSynOptions.MSS,
+		})
 
 		// Do the delivery in a separate goroutine so
 		// that we don't block the listen loop in case
diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 7da93dcc4..91ee3b0be 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -995,24 +995,22 @@ func (e *endpoint) completeWorkerLocked() {
 
 // transitionToStateEstablisedLocked transitions a given endpoint
 // to an established state using the handshake parameters provided.
-// It also initializes sender/receiver if required.
+// It also initializes sender/receiver.
 func (e *endpoint) transitionToStateEstablishedLocked(h *handshake) {
-	if e.snd == nil {
-		// Transfer handshake state to TCP connection. We disable
-		// receive window scaling if the peer doesn't support it
-		// (indicated by a negative send window scale).
-		e.snd = newSender(e, h.iss, h.ackNum-1, h.sndWnd, h.mss, h.sndWndScale)
-	}
-	if e.rcv == nil {
-		rcvBufSize := seqnum.Size(e.receiveBufferSize())
-		e.rcvListMu.Lock()
-		e.rcv = newReceiver(e, h.ackNum-1, h.rcvWnd, h.effectiveRcvWndScale(), rcvBufSize)
-		// Bootstrap the auto tuning algorithm. Starting at zero will
-		// result in a really large receive window after the first auto
-		// tuning adjustment.
-		e.rcvAutoParams.prevCopied = int(h.rcvWnd)
-		e.rcvListMu.Unlock()
-	}
+	// Transfer handshake state to TCP connection. We disable
+	// receive window scaling if the peer doesn't support it
+	// (indicated by a negative send window scale).
+	e.snd = newSender(e, h.iss, h.ackNum-1, h.sndWnd, h.mss, h.sndWndScale)
+
+	rcvBufSize := seqnum.Size(e.receiveBufferSize())
+	e.rcvListMu.Lock()
+	e.rcv = newReceiver(e, h.ackNum-1, h.rcvWnd, h.effectiveRcvWndScale(), rcvBufSize)
+	// Bootstrap the auto tuning algorithm. Starting at zero will
+	// result in a really large receive window after the first auto
+	// tuning adjustment.
+	e.rcvAutoParams.prevCopied = int(h.rcvWnd)
+	e.rcvListMu.Unlock()
+
 	e.setEndpointState(StateEstablished)
 }