Reply to spurious TCP packets with a RST.

Spurious TCP packets appear when the system is force-restarting due to
an error. In such a case, open TCP connections are not properly teared
down, and the remote TCP endpoint sends retransmissions to the newly
restarted system. These spurious TCP packets are problematic because
they fill packet buffers and quickly cause packet drops.

This commit enables the firewall to reply to spurious TCP packets with
TCP RST to stop the flow of spurious packets.

We do this with a TCP RST "packet template" which we pre-set at startup
and complete with MAC address, IP address, port, sequence number, and
checksum, when sending the RST.

To do this, we need to:
- add needed 32-bit `ntohs` and `htons`
- add a representation of a TCP header in the firewall compartment

Signed-off-by: Hugo Lefeuvre <hugo.lefeuvre@scisemi.com>
(cherry picked from commit ec54006)

lib/firewall/firewall.cc

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: MIT
 
 #include "firewall.hh"
+#include "../tcpip/checksum-internal.h"
 #include <atomic>
 #include <compartment-macros.h>
 #include <debug.hh>
@@ -32,6 +33,26 @@ namespace
 		  __builtin_bswap16(value)
 #else
 		  value
+#endif
+		    ;
+	}
+	uint32_t constexpr ntohs(uint32_t value)
+	{
+		return
+#ifdef __LITTLE_ENDIAN__
+		  __builtin_bswap32(value)
+#else
+		  value
+#endif
+		    ;
+	}
+	uint32_t constexpr htons(uint32_t value)
+	{
+		return
+#ifdef __LITTLE_ENDIAN__
+		  __builtin_bswap32(value)
+#else
+		  value
 #endif
 		    ;
 	}
@@ -172,13 +193,57 @@ namespace
 		}
 	} __packed;
 
+	static_assert(sizeof(IPv4Header) == 20);
+
 	struct TCPUDPCommonPrefix
 	{
 		uint16_t sourcePort;
 		uint16_t destinationPort;
 	} __packed;
 
-	static_assert(sizeof(IPv4Header) == 20);
+	struct TCPHeader
+	{
+		/**
+		 * Source port.
+		 */
+		uint16_t sourcePort;
+		/**
+		 * Destination port.
+		 */
+		uint16_t destinationPort;
+		/**
+		 * Sequence number.
+		 */
+		uint32_t sequenceNumber;
+		/**
+		 * Acknowledgement number.
+		 */
+		uint32_t acknowledgementNumber;
+		/**
+		 * Reserved bits, data offset, and flags.
+		 */
+		uint16_t reserved : 4, dataOffset : 4, fin : 1, syn : 1, rst : 1,
+		  psh : 1, ack : 1, urg : 1, ece : 1, cwr : 1;
+		/**
+		 * Window size.
+		 */
+		uint16_t windowSize;
+		/**
+		 * Checksum.
+		 */
+		uint16_t checksum;
+		/**
+		 * Urgent pointer.
+		 */
+		uint16_t urgentPointer;
+	} __packed;
+
+	struct FullPacket
+	{
+		EthernetHeader ethernet;
+		IPv4Header     ipv4;
+		TCPHeader      tcp;
+	} __packed;
 
 	/**
 	 * Simple firewall table for IPv4 endpoints.
@@ -320,6 +385,10 @@ namespace
 			               });
 			if (!found)
 			{
+				// Note that a failure to remove the endpoint
+				// is not always a bug. This is meant to happen
+				// if the DNS resolution failed when binding a
+				// socket.
 				Debug::log("Failed to remove endpoint (local: {})", localPort);
 			}
 		}
@@ -361,6 +430,34 @@ namespace
 	uint32_t          dnsServerAddress;
 	_Atomic(uint32_t) dnsIsPermitted;
 
+	/**
+	 * This buffer will be pre-set into a TCP RST packet template during
+	 * the initialization of the firewall.  When the firewall needs to send
+	 * a TCP RST, this template is updated with matching MACs, addresses,
+	 * ports, sequence number, and checksums, and sent.
+	 */
+	static struct FullPacket rstPacketTemplate = {0};
+
+	/**
+	 * Pre-set the RST packet template.
+	 */
+	void init_rst_template()
+	{
+		rstPacketTemplate.ethernet.etherType = EtherType::IPv4;
+		// 5 x 32 bit = 20 bytes
+		rstPacketTemplate.ipv4.versionAndHeaderLength = (4 << 4) | 5;
+		// The RST packet does not have a payload.
+		rstPacketTemplate.ipv4.packetLength =
+		  ntohs(static_cast<uint16_t>(sizeof(IPv4Header) + sizeof(TCPHeader)));
+		// Default TTL as recommended by RFC 1700.
+		rstPacketTemplate.ipv4.timeToLive = 64;
+		rstPacketTemplate.ipv4.protocol   = IPProtocolNumber::TCP;
+		// 5 * 32 bit = 20 bytes (again)
+		rstPacketTemplate.tcp.dataOffset = 5;
+		// Enable TCP RST flag.
+		rstPacketTemplate.tcp.rst = 1;
+	}
+
 	bool packet_filter_ipv4(const uint8_t *data,
 	                        size_t         length,
 	                        uint32_t(IPv4Header::*remoteAddress),
@@ -502,16 +599,118 @@ namespace
 		return true;
 	}
 
+	/**
+	 * If passed packet is an IPv4 TCP packet, reply with a RST to the
+	 * sender. This takes the whole ethernet frame into `data` (and the
+	 * size of the buffer in `length`).
+	 */
+	void try_reset_ipv4_tcp(const uint8_t *data, size_t length)
+	{
+		if (__predict_false(length <
+		                    sizeof(EthernetHeader) + sizeof(IPv4Header)))
+		{
+			Debug::log("Ignoring inbound packet with length {}", length);
+			return;
+		}
+
+		EthernetHeader *ethernetHeader =
+		  reinterpret_cast<EthernetHeader *>(const_cast<uint8_t *>(data));
+		auto *ipv4Header =
+		  reinterpret_cast<const IPv4Header *>(data + sizeof(EthernetHeader));
+		if (ipv4Header->protocol == IPProtocolNumber::TCP)
+		{
+			if (ipv4Header->body_offset() < sizeof(ipv4Header))
+			{
+				Debug::log("Body offset is {} but IPv4 header is {} bytes",
+				           ipv4Header->body_offset(),
+				           sizeof(ipv4Header));
+				return;
+			}
+			if (ipv4Header->body_offset() + sizeof(TCPHeader) > length)
+			{
+				Debug::log("Ignoring inbound packet with length {}", length);
+				return;
+			}
+			const TCPHeader *tcpHeader = reinterpret_cast<const TCPHeader *>(
+			  data + sizeof(EthernetHeader) + ipv4Header->body_offset());
+
+			// Do not send a RST if the received TCP packet is
+			// itself a RST.
+			if (tcpHeader->rst == 1)
+			{
+				Debug::log("Ignoring inbound TCP RST packet.");
+				return;
+			}
+
+			// Create a read-only capability to pass to the TCP/IP
+			// stack when we calculate checksums.
+			CHERI::Capability<uint8_t> rstPacketTemplateROCap{
+			  reinterpret_cast<uint8_t *>(&rstPacketTemplate)};
+			// Remove all permissions except load.  This also
+			// removes global, so that this cannot be captured.
+			rstPacketTemplateROCap.permissions() &=
+			  CHERI::PermissionSet{CHERI::Permission::Load};
+
+			/// Build the RST packet.
+			// Source and destination MACs.
+			std::copy(std::begin(ethernetHeader->source),
+			          std::end(ethernetHeader->source),
+			          std::begin(rstPacketTemplate.ethernet.destination));
+			std::copy(std::begin(ethernetHeader->destination),
+			          std::end(ethernetHeader->destination),
+			          std::begin(rstPacketTemplate.ethernet.source));
+			// Source and destination IPs.
+			rstPacketTemplate.ipv4.sourceAddress =
+			  ipv4Header->destinationAddress;
+			rstPacketTemplate.ipv4.destinationAddress =
+			  ipv4Header->sourceAddress;
+			// IPv4 checksum. The value returned is in network byte
+			// order. Make sure to reset the checksum field's value
+			// before calculation.
+			rstPacketTemplate.ipv4.headerChecksum = 0;
+			rstPacketTemplate.ipv4.headerChecksum =
+			  network_calculate_ipv4_checksum(rstPacketTemplateROCap +
+			                                    sizeof(EthernetHeader),
+			                                  sizeof(IPv4Header));
+			// Source and destination ports.
+			rstPacketTemplate.tcp.sourcePort      = tcpHeader->destinationPort;
+			rstPacketTemplate.tcp.destinationPort = tcpHeader->sourcePort;
+			// Set the sequence number to the ack.
+			rstPacketTemplate.tcp.sequenceNumber =
+			  tcpHeader->acknowledgementNumber;
+			// TCP checksum. The value returned is in network byte
+			// order. No need to reset the field here as it isn't
+			// included in the calculation.
+			rstPacketTemplate.tcp.checksum = network_calculate_tcp_checksum(
+			  rstPacketTemplateROCap,
+			  sizeof(FullPacket),
+			  sizeof(EthernetHeader) + sizeof(IPv4Header) + 16);
+
+			/// Send the RST packet.
+			Debug::log("Sending a RST packet.");
+			LockGuard g{sendLock};
+			auto     &ethernet = lazy_network_interface();
+			// Do not go through the firewall: the packet would be
+			// rejected since the destination is not present in the
+			// table.
+			ethernet.send_frame(
+			  rstPacketTemplateROCap,
+			  sizeof(FullPacket),
+			  [](const uint8_t *data, size_t length) { return true; });
+		}
+	}
+
 	bool packet_filter_ingress(const uint8_t *data, size_t length)
 	{
-		uint32_t stateSnapshot = tcpipRestartState->load();
+		uint32_t stateSnapshot  = tcpipRestartState->load();
+		bool     isOngoingReset = false;
 		if (stateSnapshot != 0 &&
 		    ((stateSnapshot & RestartStateDriverKickedBit) == 0))
 		{
 			// We are in a reset and the driver has not yet been
 			// restarted.
 			Debug::log("Dropping packet due to network stack restart.");
-			return false;
+			isOngoingReset = true;
 		}
 
 		// Not a valid Ethernet frame (64 bytes including four-byte FCS, which
@@ -523,26 +722,42 @@ namespace
 		}
 		EthernetHeader *ethernetHeader =
 		  reinterpret_cast<EthernetHeader *>(const_cast<uint8_t *>(data));
+		bool accept = false;
 		switch (ethernetHeader->etherType)
 		{
 			// For now, testing with v6 disabled.
 			case EtherType::IPv6:
-				return true;
+				accept = true;
+				break;
 			case EtherType::ARP:
 				Debug::log("Saw ARP frame");
-				return true;
+				accept = true;
+				break;
 			case EtherType::IPv4:
-				return packet_filter_ipv4(data + sizeof(EthernetHeader),
-				                          length - sizeof(EthernetHeader),
-				                          &IPv4Header::sourceAddress,
-				                          &TCPUDPCommonPrefix::destinationPort,
-				                          &TCPUDPCommonPrefix::sourcePort,
-				                          false);
-			default:
-				return false;
+				if (!isOngoingReset)
+				{
+					accept =
+					  packet_filter_ipv4(data + sizeof(EthernetHeader),
+					                     length - sizeof(EthernetHeader),
+					                     &IPv4Header::sourceAddress,
+					                     &TCPUDPCommonPrefix::destinationPort,
+					                     &TCPUDPCommonPrefix::sourcePort,
+					                     false);
+				}
+				if (!accept)
+				{
+					// If this is a TCP packet, send a RST
+					// to the source.
+					//
+					// Only reset for IPv4 for now.  Pass
+					// the whole Ethernet packet (unlike
+					// `packet_filter_ipv4`).
+					try_reset_ipv4_tcp(data, length);
+				}
+				break;
 		}
 
-		return false;
+		return accept && !isOngoingReset;
 	}
 
 	std::atomic<uint32_t> receivedCounter;
@@ -787,6 +1002,7 @@ bool ethernet_driver_start(std::atomic<uint8_t> *state)
 	Debug::log("Initialising network interface");
 	auto &ethernet = lazy_network_interface();
 	ethernet.mac_address_set();
+	init_rst_template();
 	// Poke the barrier and make the driver thread start.
 	barrier = 2;
 	barrier.notify_one();