From 43c209f48e0aa9024705583cc6f0fafa7d6380ca Mon Sep 17 00:00:00 2001
From: Kevin Krakauer <krakauer@google.com>
Date: Mon, 13 Jul 2020 11:59:26 -0700
Subject: [PATCH] garbage collect connections

As in Linux, we must periodically clean up unused connections.

PiperOrigin-RevId: 321003353
---
 pkg/tcpip/stack/BUILD                         |  14 +
 pkg/tcpip/stack/conntrack.go                  | 273 +++++++++++++++---
 pkg/tcpip/stack/iptables.go                   |  42 ++-
 pkg/tcpip/stack/iptables_state.go             |  40 +++
 pkg/tcpip/stack/iptables_types.go             |  11 +
 pkg/tcpip/stack/stack.go                      |   1 +
 .../transport/tcpconntrack/tcp_conntrack.go   |   5 +
 7 files changed, 345 insertions(+), 41 deletions(-)
 create mode 100644 pkg/tcpip/stack/iptables_state.go

diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index e65c731c2..800bf3f08 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -27,6 +27,18 @@ go_template_instance(
     },
 )
 
+go_template_instance(
+    name = "tuple_list",
+    out = "tuple_list.go",
+    package = "stack",
+    prefix = "tuple",
+    template = "//pkg/ilist:generic_list",
+    types = {
+        "Element": "*tuple",
+        "Linker": "*tuple",
+    },
+)
+
 go_library(
     name = "stack",
     srcs = [
@@ -35,6 +47,7 @@ go_library(
         "forwarder.go",
         "icmp_rate_limit.go",
         "iptables.go",
+        "iptables_state.go",
         "iptables_targets.go",
         "iptables_types.go",
         "linkaddrcache.go",
@@ -50,6 +63,7 @@ go_library(
         "stack_global_state.go",
         "stack_options.go",
         "transport_demuxer.go",
+        "tuple_list.go",
     ],
     visibility = ["//visibility:public"],
     deps = [
diff --git a/pkg/tcpip/stack/conntrack.go b/pkg/tcpip/stack/conntrack.go
index af9c325ca..d39baf620 100644
--- a/pkg/tcpip/stack/conntrack.go
+++ b/pkg/tcpip/stack/conntrack.go
@@ -15,9 +15,12 @@
 package stack
 
 import (
+	"encoding/binary"
 	"sync"
+	"time"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/hash/jenkins"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/transport/tcpconntrack"
 )
@@ -30,6 +33,10 @@ import (
 //
 // Currently, only TCP tracking is supported.
 
+// Our hash table has 16K buckets.
+// TODO(gvisor.dev/issue/170): These should be tunable.
+const numBuckets = 1 << 14
+
 // Direction of the tuple.
 type direction int
 
@@ -48,7 +55,12 @@ const (
 
 // tuple holds a connection's identifying and manipulating data in one
 // direction. It is immutable.
+//
+// +stateify savable
 type tuple struct {
+	// tupleEntry is used to build an intrusive list of tuples.
+	tupleEntry
+
 	tupleID
 
 	// conn is the connection tracking entry this tuple belongs to.
@@ -61,6 +73,8 @@ type tuple struct {
 // tupleID uniquely identifies a connection in one direction. It currently
 // contains enough information to distinguish between any TCP or UDP
 // connection, and will need to be extended to support other protocols.
+//
+// +stateify savable
 type tupleID struct {
 	srcAddr    tcpip.Address
 	srcPort    uint16
@@ -83,6 +97,8 @@ func (ti tupleID) reply() tupleID {
 }
 
 // conn is a tracked connection.
+//
+// +stateify savable
 type conn struct {
 	// original is the tuple in original direction. It is immutable.
 	original tuple
@@ -98,22 +114,67 @@ type conn struct {
 	tcbHook Hook
 
 	// mu protects tcb.
-	mu sync.Mutex
+	mu sync.Mutex `state:"nosave"`
 
 	// tcb is TCB control block. It is used to keep track of states
 	// of tcp connection and is protected by mu.
 	tcb tcpconntrack.TCB
+
+	// lastUsed is the last time the connection saw a relevant packet, and
+	// is updated by each packet on the connection. It is protected by mu.
+	lastUsed time.Time `state:".(unixTime)"`
+}
+
+// timedOut returns whether the connection timed out based on its state.
+func (cn *conn) timedOut(now time.Time) bool {
+	const establishedTimeout = 5 * 24 * time.Hour
+	const defaultTimeout = 120 * time.Second
+	cn.mu.Lock()
+	defer cn.mu.Unlock()
+	if cn.tcb.State() == tcpconntrack.ResultAlive {
+		// Use the same default as Linux, which doesn't delete
+		// established connections for 5(!) days.
+		return now.Sub(cn.lastUsed) > establishedTimeout
+	}
+	// Use the same default as Linux, which lets connections in most states
+	// other than established remain for <= 120 seconds.
+	return now.Sub(cn.lastUsed) > defaultTimeout
 }
 
 // ConnTrack tracks all connections created for NAT rules. Most users are
 // expected to only call handlePacket and createConnFor.
+//
+// ConnTrack keeps all connections in a slice of buckets, each of which holds a
+// linked list of tuples. This gives us some desirable properties:
+// - Each bucket has its own lock, lessening lock contention.
+// - The slice is large enough that lists stay short (<10 elements on average).
+//   Thus traversal is fast.
+// - During linked list traversal we reap expired connections. This amortizes
+//   the cost of reaping them and makes reapUnused faster.
+//
+// Locks are ordered by their location in the buckets slice. That is, a
+// goroutine that locks buckets[i] can only lock buckets[j] s.t. i < j.
+//
+// +stateify savable
 type ConnTrack struct {
-	// mu protects conns.
-	mu sync.RWMutex
+	// seed is a one-time random value initialized at stack startup
+	// and is used in the calculation of hash keys for the list of buckets.
+	// It is immutable.
+	seed uint32
 
-	// conns maintains a map of tuples needed for connection tracking for
-	// iptables NAT rules. It is protected by mu.
-	conns map[tupleID]tuple
+	// mu protects the buckets slice, but not buckets' contents. Only take
+	// the write lock if you are modifying the slice or saving for S/R.
+	mu sync.RWMutex `state:"nosave"`
+
+	// buckets is protected by mu.
+	buckets []bucket
+}
+
+// +stateify savable
+type bucket struct {
+	// mu protects tuples.
+	mu     sync.Mutex `state:"nosave"`
+	tuples tupleList
 }
 
 // packetToTupleID converts packet to a tuple ID. It fails when pkt lacks a valid
@@ -143,8 +204,9 @@ func packetToTupleID(pkt *PacketBuffer) (tupleID, *tcpip.Error) {
 // newConn creates new connection.
 func newConn(orig, reply tupleID, manip manipType, hook Hook) *conn {
 	conn := conn{
-		manip:   manip,
-		tcbHook: hook,
+		manip:    manip,
+		tcbHook:  hook,
+		lastUsed: time.Now(),
 	}
 	conn.original = tuple{conn: &conn, tupleID: orig}
 	conn.reply = tuple{conn: &conn, tupleID: reply, direction: dirReply}
@@ -162,14 +224,28 @@ func (ct *ConnTrack) connFor(pkt *PacketBuffer) (*conn, direction) {
 		return nil, dirOriginal
 	}
 
-	ct.mu.Lock()
-	defer ct.mu.Unlock()
+	bucket := ct.bucket(tid)
+	now := time.Now()
 
-	tuple, ok := ct.conns[tid]
-	if !ok {
-		return nil, dirOriginal
+	ct.mu.RLock()
+	defer ct.mu.RUnlock()
+	ct.buckets[bucket].mu.Lock()
+	defer ct.buckets[bucket].mu.Unlock()
+
+	// Iterate over the tuples in a bucket, cleaning up any unused
+	// connections we find.
+	for other := ct.buckets[bucket].tuples.Front(); other != nil; other = other.Next() {
+		// Clean up any timed-out connections we happen to find.
+		if ct.reapTupleLocked(other, bucket, now) {
+			// The tuple expired.
+			continue
+		}
+		if tid == other.tupleID {
+			return other.conn, other.direction
+		}
 	}
-	return tuple.conn, tuple.direction
+
+	return nil, dirOriginal
 }
 
 // createConnFor creates a new conn for pkt.
@@ -197,13 +273,31 @@ func (ct *ConnTrack) createConnFor(pkt *PacketBuffer, hook Hook, rt RedirectTarg
 	}
 	conn := newConn(tid, replyTID, manip, hook)
 
-	// Add the changed tuple to the map.
-	// TODO(gvisor.dev/issue/170): Need to support collisions using linked
-	// list.
-	ct.mu.Lock()
-	defer ct.mu.Unlock()
-	ct.conns[tid] = conn.original
-	ct.conns[replyTID] = conn.reply
+	// Lock the buckets in the correct order.
+	tupleBucket := ct.bucket(tid)
+	replyBucket := ct.bucket(replyTID)
+	ct.mu.RLock()
+	defer ct.mu.RUnlock()
+	if tupleBucket < replyBucket {
+		ct.buckets[tupleBucket].mu.Lock()
+		ct.buckets[replyBucket].mu.Lock()
+	} else if tupleBucket > replyBucket {
+		ct.buckets[replyBucket].mu.Lock()
+		ct.buckets[tupleBucket].mu.Lock()
+	} else {
+		// Both tuples are in the same bucket.
+		ct.buckets[tupleBucket].mu.Lock()
+	}
+
+	// Add the tuple to the map.
+	ct.buckets[tupleBucket].tuples.PushFront(&conn.original)
+	ct.buckets[replyBucket].tuples.PushFront(&conn.reply)
+
+	// Unlocking can happen in any order.
+	ct.buckets[tupleBucket].mu.Unlock()
+	if tupleBucket != replyBucket {
+		ct.buckets[replyBucket].mu.Unlock()
+	}
 
 	return conn
 }
@@ -297,35 +391,134 @@ func (ct *ConnTrack) handlePacket(pkt *PacketBuffer, hook Hook, gso *GSO, r *Rou
 	// other tcp states.
 	conn.mu.Lock()
 	defer conn.mu.Unlock()
-	var st tcpconntrack.Result
-	tcpHeader := header.TCP(pkt.TransportHeader)
-	if conn.tcb.IsEmpty() {
+
+	// Mark the connection as having been used recently so it isn't reaped.
+	conn.lastUsed = time.Now()
+	// Update connection state.
+	if tcpHeader := header.TCP(pkt.TransportHeader); conn.tcb.IsEmpty() {
 		conn.tcb.Init(tcpHeader)
 		conn.tcbHook = hook
+	} else if hook == conn.tcbHook {
+		conn.tcb.UpdateStateOutbound(tcpHeader)
 	} else {
-		switch hook {
-		case conn.tcbHook:
-			st = conn.tcb.UpdateStateOutbound(tcpHeader)
-		default:
-			st = conn.tcb.UpdateStateInbound(tcpHeader)
+		conn.tcb.UpdateStateInbound(tcpHeader)
+	}
+}
+
+// bucket gets the conntrack bucket for a tupleID.
+func (ct *ConnTrack) bucket(id tupleID) int {
+	h := jenkins.Sum32(ct.seed)
+	h.Write([]byte(id.srcAddr))
+	h.Write([]byte(id.dstAddr))
+	shortBuf := make([]byte, 2)
+	binary.LittleEndian.PutUint16(shortBuf, id.srcPort)
+	h.Write([]byte(shortBuf))
+	binary.LittleEndian.PutUint16(shortBuf, id.dstPort)
+	h.Write([]byte(shortBuf))
+	binary.LittleEndian.PutUint16(shortBuf, uint16(id.transProto))
+	h.Write([]byte(shortBuf))
+	binary.LittleEndian.PutUint16(shortBuf, uint16(id.netProto))
+	h.Write([]byte(shortBuf))
+	ct.mu.RLock()
+	defer ct.mu.RUnlock()
+	return int(h.Sum32()) % len(ct.buckets)
+}
+
+// reapUnused deletes timed out entries from the conntrack map. The rules for
+// reaping are:
+// - Most reaping occurs in connFor, which is called on each packet. connFor
+//   cleans up the bucket the packet's connection maps to. Thus calls to
+//   reapUnused should be fast.
+// - Each call to reapUnused traverses a fraction of the conntrack table.
+//   Specifically, it traverses len(ct.buckets)/fractionPerReaping.
+// - After reaping, reapUnused decides when it should next run based on the
+//   ratio of expired connections to examined connections. If the ratio is
+//   greater than maxExpiredPct, it schedules the next run quickly. Otherwise it
+//   slightly increases the interval between runs.
+// - maxFullTraversal caps the time it takes to traverse the entire table.
+//
+// reapUnused returns the next bucket that should be checked and the time after
+// which it should be called again.
+func (ct *ConnTrack) reapUnused(start int, prevInterval time.Duration) (int, time.Duration) {
+	// TODO(gvisor.dev/issue/170): This can be more finely controlled, as
+	// it is in Linux via sysctl.
+	const fractionPerReaping = 128
+	const maxExpiredPct = 50
+	const maxFullTraversal = 60 * time.Second
+	const minInterval = 10 * time.Millisecond
+	const maxInterval = maxFullTraversal / fractionPerReaping
+
+	now := time.Now()
+	checked := 0
+	expired := 0
+	var idx int
+	ct.mu.RLock()
+	defer ct.mu.RUnlock()
+	for i := 0; i < len(ct.buckets)/fractionPerReaping; i++ {
+		idx = (i + start) % len(ct.buckets)
+		ct.buckets[idx].mu.Lock()
+		for tuple := ct.buckets[idx].tuples.Front(); tuple != nil; tuple = tuple.Next() {
+			checked++
+			if ct.reapTupleLocked(tuple, idx, now) {
+				expired++
+			}
 		}
+		ct.buckets[idx].mu.Unlock()
 	}
+	// We already checked buckets[idx].
+	idx++
 
-	// Delete conn if tcp connection is closed.
-	if st == tcpconntrack.ResultClosedByPeer || st == tcpconntrack.ResultClosedBySelf || st == tcpconntrack.ResultReset {
-		ct.deleteConn(conn)
+	// If half or more of the connections are expired, the table has gotten
+	// stale. Reschedule quickly.
+	expiredPct := 0
+	if checked != 0 {
+		expiredPct = expired * 100 / checked
 	}
+	if expiredPct > maxExpiredPct {
+		return idx, minInterval
+	}
+	if interval := prevInterval + minInterval; interval <= maxInterval {
+		// Increment the interval between runs.
+		return idx, interval
+	}
+	// We've hit the maximum interval.
+	return idx, maxInterval
 }
 
-// deleteConn deletes the connection.
-func (ct *ConnTrack) deleteConn(conn *conn) {
-	if conn == nil {
-		return
+// reapTupleLocked tries to remove tuple and its reply from the table. It
+// returns whether the tuple's connection has timed out.
+//
+// Preconditions: ct.mu is locked for reading and bucket is locked.
+func (ct *ConnTrack) reapTupleLocked(tuple *tuple, bucket int, now time.Time) bool {
+	if !tuple.conn.timedOut(now) {
+		return false
 	}
 
-	ct.mu.Lock()
-	defer ct.mu.Unlock()
+	// To maintain lock order, we can only reap these tuples if the reply
+	// appears later in the table.
+	replyBucket := ct.bucket(tuple.reply())
+	if bucket > replyBucket {
+		return true
+	}
 
-	delete(ct.conns, conn.original.tupleID)
-	delete(ct.conns, conn.reply.tupleID)
+	// Don't re-lock if both tuples are in the same bucket.
+	differentBuckets := bucket != replyBucket
+	if differentBuckets {
+		ct.buckets[replyBucket].mu.Lock()
+	}
+
+	// We have the buckets locked and can remove both tuples.
+	if tuple.direction == dirOriginal {
+		ct.buckets[replyBucket].tuples.Remove(&tuple.conn.reply)
+	} else {
+		ct.buckets[replyBucket].tuples.Remove(&tuple.conn.original)
+	}
+	ct.buckets[bucket].tuples.Remove(tuple)
+
+	// Don't re-unlock if both tuples are in the same bucket.
+	if differentBuckets {
+		ct.buckets[replyBucket].mu.Unlock()
+	}
+
+	return true
 }
diff --git a/pkg/tcpip/stack/iptables.go b/pkg/tcpip/stack/iptables.go
index 974d77c36..f846ea2e5 100644
--- a/pkg/tcpip/stack/iptables.go
+++ b/pkg/tcpip/stack/iptables.go
@@ -16,6 +16,7 @@ package stack
 
 import (
 	"fmt"
+	"time"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
@@ -41,6 +42,9 @@ const (
 // underflow.
 const HookUnset = -1
 
+// reaperDelay is how long to wait before starting to reap connections.
+const reaperDelay = 5 * time.Second
+
 // DefaultTables returns a default set of tables. Each chain is set to accept
 // all packets.
 func DefaultTables() *IPTables {
@@ -112,8 +116,9 @@ func DefaultTables() *IPTables {
 			Output:     []string{TablenameMangle, TablenameNat, TablenameFilter},
 		},
 		connections: ConnTrack{
-			conns: make(map[tupleID]tuple),
+			seed: generateRandUint32(),
 		},
+		reaperDone: make(chan struct{}, 1),
 	}
 }
 
@@ -169,6 +174,12 @@ func (it *IPTables) GetTable(name string) (Table, bool) {
 func (it *IPTables) ReplaceTable(name string, table Table) {
 	it.mu.Lock()
 	defer it.mu.Unlock()
+	// If iptables is being enabled, initialize the conntrack table and
+	// reaper.
+	if !it.modified {
+		it.connections.buckets = make([]bucket, numBuckets)
+		it.startReaper(reaperDelay)
+	}
 	it.modified = true
 	it.tables[name] = table
 }
@@ -249,6 +260,35 @@ func (it *IPTables) Check(hook Hook, pkt *PacketBuffer, gso *GSO, r *Route, addr
 	return true
 }
 
+// beforeSave is invoked by stateify.
+func (it *IPTables) beforeSave() {
+	// Ensure the reaper exits cleanly.
+	it.reaperDone <- struct{}{}
+	// Prevent others from modifying the connection table.
+	it.connections.mu.Lock()
+}
+
+// afterLoad is invoked by stateify.
+func (it *IPTables) afterLoad() {
+	it.startReaper(reaperDelay)
+}
+
+// startReaper starts a goroutine that wakes up periodically to reap timed out
+// connections.
+func (it *IPTables) startReaper(interval time.Duration) {
+	go func() { // S/R-SAFE: reaperDone is signalled when iptables is saved.
+		bucket := 0
+		for {
+			select {
+			case <-it.reaperDone:
+				return
+			case <-time.After(interval):
+				bucket, interval = it.connections.reapUnused(bucket, interval)
+			}
+		}
+	}()
+}
+
 // CheckPackets runs pkts through the rules for hook and returns a map of packets that
 // should not go forward.
 //
diff --git a/pkg/tcpip/stack/iptables_state.go b/pkg/tcpip/stack/iptables_state.go
new file mode 100644
index 000000000..529e02a07
--- /dev/null
+++ b/pkg/tcpip/stack/iptables_state.go
@@ -0,0 +1,40 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"time"
+)
+
+// +stateify savable
+type unixTime struct {
+	second int64
+	nano   int64
+}
+
+// saveLastUsed is invoked by stateify.
+func (cn *conn) saveLastUsed() unixTime {
+	return unixTime{cn.lastUsed.Unix(), cn.lastUsed.UnixNano()}
+}
+
+// loadLastUsed is invoked by stateify.
+func (cn *conn) loadLastUsed(unix unixTime) {
+	cn.lastUsed = time.Unix(unix.second, unix.nano)
+}
+
+// beforeSave is invoked by stateify.
+func (ct *ConnTrack) beforeSave() {
+	ct.mu.Lock()
+}
diff --git a/pkg/tcpip/stack/iptables_types.go b/pkg/tcpip/stack/iptables_types.go
index c528ec381..eb70e3104 100644
--- a/pkg/tcpip/stack/iptables_types.go
+++ b/pkg/tcpip/stack/iptables_types.go
@@ -78,6 +78,8 @@ const (
 )
 
 // IPTables holds all the tables for a netstack.
+//
+// +stateify savable
 type IPTables struct {
 	// mu protects tables, priorities, and modified.
 	mu sync.RWMutex
@@ -97,10 +99,15 @@ type IPTables struct {
 	modified bool
 
 	connections ConnTrack
+
+	// reaperDone can be signalled to stop the reaper goroutine.
+	reaperDone chan struct{}
 }
 
 // A Table defines a set of chains and hooks into the network stack. It is
 // really just a list of rules.
+//
+// +stateify savable
 type Table struct {
 	// Rules holds the rules that make up the table.
 	Rules []Rule
@@ -130,6 +137,8 @@ func (table *Table) ValidHooks() uint32 {
 // contains zero or more matchers, each of which is a specification of which
 // packets this rule applies to. If there are no matchers in the rule, it
 // applies to any packet.
+//
+// +stateify savable
 type Rule struct {
 	// Filter holds basic IP filtering fields common to every rule.
 	Filter IPHeaderFilter
@@ -142,6 +151,8 @@ type Rule struct {
 }
 
 // IPHeaderFilter holds basic IP filtering data common to every rule.
+//
+// +stateify savable
 type IPHeaderFilter struct {
 	// Protocol matches the transport protocol.
 	Protocol tcpip.TransportProtocolNumber
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index cdcfb8321..0aa815447 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -425,6 +425,7 @@ type Stack struct {
 	handleLocal bool
 
 	// tables are the iptables packet filtering and manipulation rules.
+	// TODO(gvisor.dev/issue/170): S/R this field.
 	tables *IPTables
 
 	// resumableEndpoints is a list of endpoints that need to be resumed if the
diff --git a/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go b/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
index 12bc1b5b5..558b06df0 100644
--- a/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
+++ b/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
@@ -106,6 +106,11 @@ func (t *TCB) UpdateStateOutbound(tcp header.TCP) Result {
 	return st
 }
 
+// State returns the current state of the TCB.
+func (t *TCB) State() Result {
+	return t.state
+}
+
 // IsAlive returns true as long as the connection is established(Alive)
 // or connecting state.
 func (t *TCB) IsAlive() bool {