Do not resolve routes immediately

When a response needs to be sent to an incoming packet, the stack should
consult its neighbour table to determine the remote address's link
address.

When an entry does not exist in the stack's neighbor table, the stack
should queue the packet while link resolution completes. See comments.

PiperOrigin-RevId: 336185457

1 files changed, 33 insertions, 18 deletions

diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index 37c169a5d..7be35c78b 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -440,8 +440,6 @@ func (e *endpoint) handleICMP(r *stack.Route, pkt *stack.PacketBuffer, hasFragme
 			return
 		}
 
-		remoteLinkAddr := r.RemoteLinkAddress
-
 		// As per RFC 4291 section 2.7, multicast addresses must not be used as
 		// source addresses in IPv6 packets.
 		localAddr := r.LocalAddress
@@ -456,9 +454,6 @@ func (e *endpoint) handleICMP(r *stack.Route, pkt *stack.PacketBuffer, hasFragme
 		}
 		defer r.Release()
 
-		// Use the link address from the source of the original packet.
-		r.ResolveWith(remoteLinkAddr)
-
 		replyPkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
 			ReserveHeaderBytes: int(r.MaxHeaderLength()) + header.ICMPv6EchoMinimumSize,
 			Data:               pkt.Data,
@@ -742,14 +737,7 @@ func (*icmpReasonPortUnreachable) isICMPReason() {}
 
 // returnError takes an error descriptor and generates the appropriate ICMP
 // error packet for IPv6 and sends it.
-func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tcpip.Error {
-	stats := r.Stats().ICMP
-	sent := stats.V6PacketsSent
-	if !r.Stack().AllowICMPMessage() {
-		sent.RateLimited.Increment()
-		return nil
-	}
-
+func (p *protocol) returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tcpip.Error {
 	// Only send ICMP error if the address is not a multicast v6
 	// address and the source is not the unspecified address.
 	//
@@ -780,6 +768,26 @@ func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tc
 		return nil
 	}
 
+	// Even if we were able to receive a packet from some remote, we may not have
+	// a route to it - the remote may be blocked via routing rules. We must always
+	// consult our routing table and find a route to the remote before sending any
+	// packet.
+	route, err := p.stack.FindRoute(r.NICID(), r.LocalAddress, r.RemoteAddress, ProtocolNumber, false /* multicastLoop */)
+	if err != nil {
+		return err
+	}
+	defer route.Release()
+	// From this point on, the incoming route should no longer be used; route
+	// must be used to send the ICMP error.
+	r = nil
+
+	stats := p.stack.Stats().ICMP
+	sent := stats.V6PacketsSent
+	if !p.stack.AllowICMPMessage() {
+		sent.RateLimited.Increment()
+		return nil
+	}
+
 	network, transport := pkt.NetworkHeader().View(), pkt.TransportHeader().View()
 
 	if pkt.TransportProtocolNumber == header.ICMPv6ProtocolNumber {
@@ -806,11 +814,11 @@ func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tc
 	//    packet that caused the error) as possible without making
 	//    the error message packet exceed the minimum IPv6 MTU
 	//    [IPv6].
-	mtu := int(r.MTU())
+	mtu := int(route.MTU())
 	if mtu > header.IPv6MinimumMTU {
 		mtu = header.IPv6MinimumMTU
 	}
-	headerLen := int(r.MaxHeaderLength()) + header.ICMPv6ErrorHeaderSize
+	headerLen := int(route.MaxHeaderLength()) + header.ICMPv6ErrorHeaderSize
 	available := int(mtu) - headerLen
 	if available < header.IPv6MinimumSize {
 		return nil
@@ -843,9 +851,16 @@ func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tc
 	default:
 		panic(fmt.Sprintf("unsupported ICMP type %T", reason))
 	}
-	icmpHdr.SetChecksum(header.ICMPv6Checksum(icmpHdr, r.LocalAddress, r.RemoteAddress, newPkt.Data))
-	err := r.WritePacket(nil /* gso */, stack.NetworkHeaderParams{Protocol: header.ICMPv6ProtocolNumber, TTL: r.DefaultTTL(), TOS: stack.DefaultTOS}, newPkt)
-	if err != nil {
+	icmpHdr.SetChecksum(header.ICMPv6Checksum(icmpHdr, route.LocalAddress, route.RemoteAddress, newPkt.Data))
+	if err := route.WritePacket(
+		nil, /* gso */
+		stack.NetworkHeaderParams{
+			Protocol: header.ICMPv6ProtocolNumber,
+			TTL:      route.DefaultTTL(),
+			TOS:      stack.DefaultTOS,
+		},
+		newPkt,
+	); err != nil {
 		sent.Dropped.Increment()
 		return err
 	}