pkg/link/spoofcheck.go

// Copyright 2021 CNI 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 link

import (
	"context"
	"fmt"
	"os"
	"time"

	"github.com/networkplumbing/go-nft/nft"
	"github.com/networkplumbing/go-nft/nft/schema"
)

const (
	natTableName            = "nat"
	preRoutingBaseChainName = "PREROUTING"
)

type NftConfigurer interface {
	Apply(*nft.Config) (*nft.Config, error)
	Read(filterCommands ...string) (*nft.Config, error)
}

type SpoofChecker struct {
	iface      string
	macAddress string
	refID      string
	configurer NftConfigurer
	rulestore  *nft.Config
}

type defaultNftConfigurer struct{}

func (dnc defaultNftConfigurer) Apply(cfg *nft.Config) (*nft.Config, error) {
	const timeout = 55 * time.Second
	ctxWithTimeout, cancelFunc := context.WithTimeout(context.Background(), timeout)
	defer cancelFunc()
	return nft.ApplyConfigEcho(ctxWithTimeout, cfg)
}

func (dnc defaultNftConfigurer) Read(filterCommands ...string) (*nft.Config, error) {
	const timeout = 55 * time.Second
	ctxWithTimeout, cancelFunc := context.WithTimeout(context.Background(), timeout)
	defer cancelFunc()
	return nft.ReadConfigContext(ctxWithTimeout, filterCommands...)
}

func NewSpoofChecker(iface, macAddress, refID string) *SpoofChecker {
	return NewSpoofCheckerWithConfigurer(iface, macAddress, refID, defaultNftConfigurer{})
}

func NewSpoofCheckerWithConfigurer(iface, macAddress, refID string, configurer NftConfigurer) *SpoofChecker {
	return &SpoofChecker{iface, macAddress, refID, configurer, nil}
}

// Setup applies nftables configuration to restrict traffic
// from the provided interface. Only traffic with the mentioned mac address
// is allowed to pass, all others are blocked.
// The configuration follows the format libvirt and ebtables implemented, allowing
// extensions to the rules in the future.
// refID is used to label the rules with a unique comment, identifying the rule-set.
//
// In order to take advantage of the nftables configuration change atomicity, the
// following steps are taken to apply the configuration:
// - Declare the table and chains (they will be created in case not present).
// - Apply the rules, while first flushing the iface/mac specific regular chain rules.
// Two transactions are used because the flush succeeds only if the table/chain it targets
// exists. This avoids the need to query the existing state and acting upon it (a raceful pattern).
// Although two transactions are taken place, only the 2nd one where the rules
// are added has a real impact on the system.
func (sc *SpoofChecker) Setup() error {
	baseConfig := nft.NewConfig()

	baseConfig.AddTable(&schema.Table{Family: schema.FamilyBridge, Name: natTableName})

	baseConfig.AddChain(sc.baseChain())
	ifaceChain := sc.ifaceChain()
	baseConfig.AddChain(ifaceChain)
	macChain := sc.macChain(ifaceChain.Name)
	baseConfig.AddChain(macChain)

	if _, err := sc.configurer.Apply(baseConfig); err != nil {
		return fmt.Errorf("failed to setup spoof-check: %v", err)
	}

	rulesConfig := nft.NewConfig()

	rulesConfig.FlushChain(ifaceChain)
	rulesConfig.FlushChain(macChain)

	rulesConfig.AddRule(sc.matchIfaceJumpToChainRule(preRoutingBaseChainName, ifaceChain.Name))
	rulesConfig.AddRule(sc.jumpToChainRule(ifaceChain.Name, macChain.Name))
	rulesConfig.AddRule(sc.matchMacRule(macChain.Name))
	rulesConfig.AddRule(sc.dropRule(macChain.Name))

	rulestore, err := sc.configurer.Apply(rulesConfig)
	if err != nil {
		return fmt.Errorf("failed to setup spoof-check: %v", err)
	}
	sc.rulestore = rulestore

	return nil
}

func (sc *SpoofChecker) findPreroutingRule(ruleToFind *schema.Rule) ([]*schema.Rule, error) {
	ruleset := sc.rulestore
	if ruleset == nil {
		chain, err := sc.configurer.Read(listChainBridgeNatPrerouting()...)
		if err != nil {
			return nil, err
		}
		ruleset = chain
	}
	return ruleset.LookupRule(ruleToFind), nil
}

// Teardown removes the interface and mac-address specific chains and their rules.
// The table and base-chain are expected to survive while the base-chain rule that matches the
// interface is removed.
func (sc *SpoofChecker) Teardown() error {
	ifaceChain := sc.ifaceChain()
	expectedRuleToFind := sc.matchIfaceJumpToChainRule(preRoutingBaseChainName, ifaceChain.Name)
	// It is safer to exclude the statement matching, avoiding cases where a current statement includes
	// additional default entries (e.g. counters).
	ruleToFindExcludingStatements := *expectedRuleToFind
	ruleToFindExcludingStatements.Expr = nil

	rules, ifaceMatchRuleErr := sc.findPreroutingRule(&ruleToFindExcludingStatements)
	if ifaceMatchRuleErr == nil && len(rules) > 0 {
		c := nft.NewConfig()
		for _, rule := range rules {
			c.DeleteRule(rule)
		}
		if _, err := sc.configurer.Apply(c); err != nil {
			ifaceMatchRuleErr = fmt.Errorf("failed to delete iface match rule: %v", err)
		}
		// Drop the cache, it should contain deleted rule(s) now
		sc.rulestore = nil
	} else {
		fmt.Fprintf(os.Stderr, "spoofcheck/teardown: unable to detect iface match rule for deletion: %+v", expectedRuleToFind)
	}

	regularChainsConfig := nft.NewConfig()
	regularChainsConfig.DeleteChain(ifaceChain)
	regularChainsConfig.DeleteChain(sc.macChain(ifaceChain.Name))

	var regularChainsErr error
	if _, err := sc.configurer.Apply(regularChainsConfig); err != nil {
		regularChainsErr = fmt.Errorf("failed to delete regular chains: %v", err)
	}

	if ifaceMatchRuleErr != nil || regularChainsErr != nil {
		return fmt.Errorf("failed to teardown spoof-check: %v, %v", ifaceMatchRuleErr, regularChainsErr)
	}
	return nil
}

func (sc *SpoofChecker) matchIfaceJumpToChainRule(chain, toChain string) *schema.Rule {
	return &schema.Rule{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Chain:  chain,
		Expr: []schema.Statement{
			{Match: &schema.Match{
				Op:    schema.OperEQ,
				Left:  schema.Expression{RowData: []byte(`{"meta":{"key":"iifname"}}`)},
				Right: schema.Expression{String: &sc.iface},
			}},
			{Verdict: schema.Verdict{Jump: &schema.ToTarget{Target: toChain}}},
		},
		Comment: ruleComment(sc.refID),
	}
}

func (sc *SpoofChecker) jumpToChainRule(chain, toChain string) *schema.Rule {
	return &schema.Rule{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Chain:  chain,
		Expr: []schema.Statement{
			{Verdict: schema.Verdict{Jump: &schema.ToTarget{Target: toChain}}},
		},
		Comment: ruleComment(sc.refID),
	}
}

func (sc *SpoofChecker) matchMacRule(chain string) *schema.Rule {
	return &schema.Rule{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Chain:  chain,
		Expr: []schema.Statement{
			{Match: &schema.Match{
				Op: schema.OperEQ,
				Left: schema.Expression{Payload: &schema.Payload{
					Protocol: schema.PayloadProtocolEther,
					Field:    schema.PayloadFieldEtherSAddr,
				}},
				Right: schema.Expression{String: &sc.macAddress},
			}},
			{Verdict: schema.Verdict{SimpleVerdict: schema.SimpleVerdict{Return: true}}},
		},
		Comment: ruleComment(sc.refID),
	}
}

func (sc *SpoofChecker) dropRule(chain string) *schema.Rule {
	return &schema.Rule{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Chain:  chain,
		Expr: []schema.Statement{
			{Verdict: schema.Verdict{SimpleVerdict: schema.SimpleVerdict{Drop: true}}},
		},
		Comment: ruleComment(sc.refID),
	}
}

func (sc *SpoofChecker) baseChain() *schema.Chain {
	chainPriority := -300
	return &schema.Chain{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Name:   preRoutingBaseChainName,
		Type:   schema.TypeFilter,
		Hook:   schema.HookPreRouting,
		Prio:   &chainPriority,
		Policy: schema.PolicyAccept,
	}
}

func (sc *SpoofChecker) ifaceChain() *schema.Chain {
	ifaceChainName := "cni-br-iface-" + sc.refID
	return &schema.Chain{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Name:   ifaceChainName,
	}
}

func (sc *SpoofChecker) macChain(ifaceChainName string) *schema.Chain {
	macChainName := ifaceChainName + "-mac"
	return &schema.Chain{
		Family: schema.FamilyBridge,
		Table:  natTableName,
		Name:   macChainName,
	}
}

func ruleComment(id string) string {
	const refIDPrefix = "macspoofchk-"
	return refIDPrefix + id
}

func listChainBridgeNatPrerouting() []string {
	return []string{"chain", "bridge", natTableName, preRoutingBaseChainName}
}