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tailscale/tka/tka.go

589 lines
19 KiB
Go

// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package tka (WIP) implements the Tailnet Key Authority.
package tka
import (
"bytes"
"errors"
"fmt"
"os"
"sort"
)
// Authority is a Tailnet Key Authority. This type is the main coupling
// point to the rest of the tailscale client.
//
// Authority objects can either be created from an existing, non-empty
// tailchonk (via tka.Open()), or created from scratch using tka.Bootstrap()
// or tka.Create().
type Authority struct {
head AUM
oldestAncestor AUM
state State
storage Chonk
}
// A chain describes a linear sequence of updates from Oldest to Head,
// resulting in some State at Head.
type chain struct {
Oldest AUM
Head AUM
state State
// Set to true if the AUM chain intersects with the active
// chain from a previous run.
chainsThroughActive bool
}
// computeChainCandidates returns all possible chains based on AUMs stored
// in the given tailchonk. A chain is defined as a unique (oldest, newest)
// AUM tuple. chain.state is not yet populated in returned chains.
//
// If lastKnownOldest is provided, any chain that includes the given AUM
// has the chainsThroughActive field set to true. This bit is leveraged
// in computeActiveAncestor() to filter out irrelevant chains when determining
// the active ancestor from a list of distinct chains.
func computeChainCandidates(storage Chonk, lastKnownOldest *AUMHash, maxIter int) ([]chain, error) {
heads, err := storage.Heads()
if err != nil {
return nil, fmt.Errorf("reading heads: %v", err)
}
candidates := make([]chain, len(heads))
for i := range heads {
// Oldest is iteratively computed below.
candidates[i] = chain{Oldest: heads[i], Head: heads[i]}
}
// Not strictly necessary, but simplifies checks in tests.
sort.Slice(candidates, func(i, j int) bool {
ih, jh := candidates[i].Oldest.Hash(), candidates[j].Oldest.Hash()
return bytes.Compare(ih[:], jh[:]) < 0
})
// candidates.Oldest needs to be computed by working backwards from
// head as far as we can.
iterAgain := true // if theres still work to be done.
for i := 0; iterAgain; i++ {
if i >= maxIter {
return nil, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
}
iterAgain = false
for j := range candidates {
parent, hasParent := candidates[j].Oldest.Parent()
if hasParent {
parent, err := storage.AUM(parent)
if err != nil {
if err == os.ErrNotExist {
continue
}
return nil, fmt.Errorf("reading parent: %v", err)
}
candidates[j].Oldest = parent
if lastKnownOldest != nil && *lastKnownOldest == parent.Hash() {
candidates[j].chainsThroughActive = true
}
iterAgain = true
}
}
}
return candidates, nil
}
// pickNextAUM returns the AUM which should be used as the next
// AUM in the chain, possibly applying fork resolution logic.
//
// In other words: given an AUM with 3 children like this:
//
// / - 1
// P - 2
// \ - 3
//
// pickNextAUM will determine and return the correct branch.
//
// This method takes ownership of the provided slice.
func pickNextAUM(state State, candidates []AUM) AUM {
switch len(candidates) {
case 0:
panic("pickNextAUM called with empty candidate set")
case 1:
return candidates[0]
}
// Oooof, we have some forks in the chain. We need to pick which
// one to use by applying the Fork Resolution Algorithm ✨
//
// The rules are this:
// 1. The child with the highest signature weight is chosen.
// 2. If equal, the child which is a RemoveKey AUM is chosen.
// 3. If equal, the child with the lowest AUM hash is chosen.
sort.Slice(candidates, func(j, i int) bool {
// Rule 1.
iSigWeight, jSigWeight := candidates[i].Weight(state), candidates[j].Weight(state)
if iSigWeight != jSigWeight {
return iSigWeight < jSigWeight
}
// Rule 2.
if iKind, jKind := candidates[i].MessageKind, candidates[j].MessageKind; iKind != jKind &&
(iKind == AUMRemoveKey || jKind == AUMRemoveKey) {
return jKind == AUMRemoveKey
}
// Rule 3.
iHash, jHash := candidates[i].Hash(), candidates[j].Hash()
return bytes.Compare(iHash[:], jHash[:]) > 0
})
return candidates[0]
}
// advanceByPrimary computes the next AUM to advance with based on
// deterministic fork-resolution rules. All nodes should apply this logic
// when computing the primary chain, hence achieving consensus on what the
// primary chain (and hence, the shared state) is.
//
// This method returns the chosen AUM & the state obtained by applying that
// AUM.
//
// The return value for next is nil if there are no children AUMs, hence
// the provided state is at head (up to date).
func advanceByPrimary(state State, candidates []AUM) (next *AUM, out State, err error) {
if len(candidates) == 0 {
return nil, state, nil
}
aum := pickNextAUM(state, candidates)
if state, err = state.applyVerifiedAUM(aum); err != nil {
return nil, State{}, fmt.Errorf("advancing state: %v", err)
}
return &aum, state, nil
}
// fastForwardWithAdvancer iteratively advances the current state by calling
// the given advancer to get+apply the next update. This process is repeated
// until the given termination function returns true or there is no more
// progress possible.
//
// The last-processed AUM, and the state computed after applying the last AUM,
// are returned.
func fastForwardWithAdvancer(
storage Chonk, maxIter int, startState State,
advancer func(state State, candidates []AUM) (next *AUM, out State, err error),
done func(curAUM AUM, curState State) bool,
) (AUM, State, error) {
if startState.LastAUMHash == nil {
return AUM{}, State{}, errors.New("invalid initial state")
}
nextAUM, err := storage.AUM(*startState.LastAUMHash)
if err != nil {
return AUM{}, State{}, fmt.Errorf("reading next: %v", err)
}
curs := nextAUM
state := startState
for i := 0; i < maxIter; i++ {
if done != nil && done(curs, state) {
return curs, state, nil
}
children, err := storage.ChildAUMs(curs.Hash())
if err != nil {
return AUM{}, State{}, fmt.Errorf("getting children of %X: %v", curs.Hash(), err)
}
next, nextState, err := advancer(state, children)
if err != nil {
return AUM{}, State{}, fmt.Errorf("advance %X: %v", curs.Hash(), err)
}
if next == nil {
// There were no more children, we are at 'head'.
return curs, state, nil
}
curs = *next
state = nextState
}
return AUM{}, State{}, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
}
// fastForward iteratively advances the current state based on known AUMs until
// the given termination function returns true or there is no more progress possible.
//
// The last-processed AUM, and the state computed after applying the last AUM,
// are returned.
func fastForward(storage Chonk, maxIter int, startState State, done func(curAUM AUM, curState State) bool) (AUM, State, error) {
return fastForwardWithAdvancer(storage, maxIter, startState, advanceByPrimary, done)
}
// computeStateAt returns the State at wantHash.
func computeStateAt(storage Chonk, maxIter int, wantHash AUMHash) (State, error) {
// TODO(tom): This is going to get expensive for really long
// chains. We should make nodes emit a checkpoint every
// X updates or something.
topAUM, err := storage.AUM(wantHash)
if err != nil {
return State{}, err
}
// Iterate backwards till we find a starting point to compute
// the state from.
//
// Valid starting points are either a checkpoint AUM, or a
// genesis AUM.
var (
curs = topAUM
state State
path = make(map[AUMHash]struct{}, 32) // 32 chosen arbitrarily.
)
for i := 0; true; i++ {
if i > maxIter {
return State{}, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
}
path[curs.Hash()] = struct{}{}
// Checkpoints encapsulate the state at that point, dope.
if curs.MessageKind == AUMCheckpoint {
state = curs.State.cloneForUpdate(&curs)
break
}
parent, hasParent := curs.Parent()
if !hasParent {
// This is a 'genesis' update: there are none before it, so
// this AUM can be applied to the empty state to determine
// the state at this AUM.
//
// It is only valid for NoOp, AddKey, and Checkpoint AUMs
// to be a genesis update. Checkpoint was handled earlier.
if mk := curs.MessageKind; mk == AUMNoOp || mk == AUMAddKey {
var err error
if state, err = (State{}).applyVerifiedAUM(curs); err != nil {
return State{}, fmt.Errorf("applying genesis (%+v): %v", curs, err)
}
break
}
return State{}, fmt.Errorf("invalid genesis update: %+v", curs)
}
// If we got here, the current state is dependent on the previous.
// Keep iterating backwards till thats not the case.
if curs, err = storage.AUM(parent); err != nil {
return State{}, fmt.Errorf("reading parent: %v", err)
}
}
// We now know some starting point state. Iterate forward till we
// are at the AUM we want state for.
//
// We want to fast forward based on the path we took above, which
// (in the case of a non-primary fork) may differ from a regular
// fast-forward (which follows standard fork-resolution rules). As
// such, we use a custom advancer here.
advancer := func(state State, candidates []AUM) (next *AUM, out State, err error) {
for _, c := range candidates {
if _, inPath := path[c.Hash()]; inPath {
if state, err = state.applyVerifiedAUM(c); err != nil {
return nil, State{}, fmt.Errorf("advancing state: %v", err)
}
return &c, state, nil
}
}
return nil, State{}, errors.New("no candidate matching path")
}
_, state, err = fastForwardWithAdvancer(storage, maxIter, state, advancer, func(curs AUM, _ State) bool {
return curs.Hash() == wantHash
})
// fastForward only terminates before the done condition if it
// doesnt have any later AUMs to process. This cant be the case
// as we've already iterated through them above so they must exist,
// but we check anyway to be super duper sure.
if err == nil && *state.LastAUMHash != wantHash {
// TODO(tom): Error instead of panic before GA.
panic("unexpected fastForward outcome")
}
return state, err
}
// computeActiveAncestor determines which ancestor AUM to use as the
// ancestor of the valid chain.
//
// If all the chains end up having the same ancestor, then thats the
// only possible ancestor, ezpz. However if there are multiple distinct
// ancestors, that means there are distinct chains, and we need some
// hint to choose what to use. For that, we rely on the chainsThroughActive
// bit, which signals to us that that ancestor was part of the
// chain in a previous run.
func computeActiveAncestor(storage Chonk, chains []chain) (AUMHash, error) {
// Dedupe possible ancestors, tracking if they were part of
// the active chain on a previous run.
ancestors := make(map[AUMHash]bool, len(chains))
for _, c := range chains {
ancestors[c.Oldest.Hash()] = c.chainsThroughActive
}
if len(ancestors) == 1 {
// There's only one. DOPE.
for k, _ := range ancestors {
return k, nil
}
}
// Theres more than one, so we need to use the ancestor that was
// part of the active chain in a previous iteration.
// Note that there can only be one distinct ancestor that was
// formerly part of the active chain, because AUMs can only have
// one parent and would have converged to a common ancestor.
for k, chainsThroughActive := range ancestors {
if chainsThroughActive {
return k, nil
}
}
return AUMHash{}, errors.New("multiple distinct chains")
}
// computeActiveChain bootstraps the runtime state of the Authority when
// starting entirely off stored state.
//
// TODO(tom): Don't look at head states, just iterate forward from
// the ancestor.
//
// The algorithm is as follows:
// 1. Determine all possible 'head' (like in git) states.
// 2. Filter these possible chains based on whether the ancestor was
// formerly (in a previous run) part of the chain.
// 3. Compute the state of the state machine at this ancestor. This is
// needed for fast-forward, as each update operates on the state of
// the update preceeding it.
// 4. Iteratively apply updates till we reach head ('fast forward').
func computeActiveChain(storage Chonk, lastKnownOldest *AUMHash, maxIter int) (chain, error) {
chains, err := computeChainCandidates(storage, lastKnownOldest, maxIter)
if err != nil {
return chain{}, fmt.Errorf("computing candidates: %v", err)
}
// Find the right ancestor.
oldestHash, err := computeActiveAncestor(storage, chains)
if err != nil {
return chain{}, fmt.Errorf("computing ancestor: %v", err)
}
ancestor, err := storage.AUM(oldestHash)
if err != nil {
return chain{}, err
}
// At this stage we know the ancestor AUM, so we have excluded distinct
// chains but we might still have forks (so we don't know the head AUM).
//
// We iterate forward from the ancestor AUM, handling any forks as we go
// till we arrive at a head.
out := chain{Oldest: ancestor, Head: ancestor}
if out.state, err = computeStateAt(storage, maxIter, oldestHash); err != nil {
return chain{}, fmt.Errorf("bootstrapping state: %v", err)
}
out.Head, out.state, err = fastForward(storage, maxIter, out.state, nil)
if err != nil {
return chain{}, fmt.Errorf("fast forward: %v", err)
}
return out, nil
}
// aumVerify verifies if an AUM is well-formed, correctly signed, and
// can be accepted for storage.
func aumVerify(aum AUM, state State, isGenesisAUM bool) error {
if err := aum.StaticValidate(); err != nil {
return fmt.Errorf("invalid: %v", err)
}
if !isGenesisAUM {
if err := checkParent(aum, state); err != nil {
return err
}
}
if len(aum.Signatures) == 0 {
return errors.New("unsigned AUM")
}
sigHash := aum.SigHash()
for i, sig := range aum.Signatures {
key, err := state.GetKey(sig.KeyID)
if err != nil {
return fmt.Errorf("bad keyID on signature %d: %v", i, err)
}
if err := sig.Verify(sigHash, key); err != nil {
return fmt.Errorf("signature %d: %v", i, err)
}
}
return nil
}
func checkParent(aum AUM, state State) error {
parent, hasParent := aum.Parent()
if !hasParent {
return errors.New("aum has no parent")
}
if state.LastAUMHash == nil {
return errors.New("cannot check update parent hash against a state with no previous AUM")
}
if *state.LastAUMHash != parent {
return fmt.Errorf("aum with parent %x cannot be applied to a state with parent %x", state.LastAUMHash, parent)
}
return nil
}
// Head returns the AUM digest of the latest update applied to the state
// machine.
func (a *Authority) Head() AUMHash {
return *a.state.LastAUMHash
}
// Open initializes an existing TKA from the given tailchonk.
//
// Only use this if the current node has initialized an Authority before.
// If a TKA exists on other nodes but theres nothing locally, use Bootstrap().
// If no TKA exists anywhere and you are creating it for the first
// time, use New().
func Open(storage Chonk) (*Authority, error) {
a, err := storage.LastActiveAncestor()
if err != nil {
return nil, fmt.Errorf("reading last ancestor: %v", err)
}
c, err := computeActiveChain(storage, a, 2000)
if err != nil {
return nil, fmt.Errorf("active chain: %v", err)
}
return &Authority{
head: c.Head,
oldestAncestor: c.Oldest,
storage: storage,
state: c.state,
}, nil
}
// Create initializes a brand-new TKA, generating a genesis update
// and committing it to the given storage.
//
// The given signer must also be present in state as a trusted key.
//
// Do not use this to initialize a TKA that already exists, use Open()
// or Bootstrap() instead.
func Create(storage Chonk, state State, signer Signer) (*Authority, AUM, error) {
// Generate & sign a checkpoint, our genesis update.
genesis := AUM{
MessageKind: AUMCheckpoint,
State: &state,
}
if err := genesis.StaticValidate(); err != nil {
// This serves as an easy way to validate the given state.
return nil, AUM{}, fmt.Errorf("invalid state: %v", err)
}
if err := signer.SignAUM(&genesis); err != nil {
return nil, AUM{}, fmt.Errorf("signing failed: %v", err)
}
a, err := Bootstrap(storage, genesis)
return a, genesis, err
}
// Bootstrap initializes a TKA based on the given checkpoint.
//
// Call this when setting up a new nodes' TKA, but other nodes
// with initialized TKA's exist.
//
// Pass the returned genesis AUM from Create(), or a later checkpoint AUM.
//
// TODO(tom): We should test an authority bootstrapped from a later checkpoint
// works fine with sync and everything.
func Bootstrap(storage Chonk, bootstrap AUM) (*Authority, error) {
heads, err := storage.Heads()
if err != nil {
return nil, fmt.Errorf("reading heads: %v", err)
}
if len(heads) != 0 {
return nil, errors.New("tailchonk is not empty")
}
// Check the AUM is well-formed.
if bootstrap.MessageKind != AUMCheckpoint {
return nil, fmt.Errorf("bootstrap AUMs must be checkpoint messages, got %v", bootstrap.MessageKind)
}
if bootstrap.State == nil {
return nil, errors.New("bootstrap AUM is missing state")
}
if err := aumVerify(bootstrap, *bootstrap.State, true); err != nil {
return nil, fmt.Errorf("invalid bootstrap: %v", err)
}
// Everything looks good, write it to storage.
if err := storage.CommitVerifiedAUMs([]AUM{bootstrap}); err != nil {
return nil, fmt.Errorf("commit: %v", err)
}
if err := storage.SetLastActiveAncestor(bootstrap.Hash()); err != nil {
return nil, fmt.Errorf("set ancestor: %v", err)
}
return Open(storage)
}
// Inform is called to tell the authority about new updates. Updates
// should be ordered oldest to newest. An error is returned if any
// of the updates could not be processed.
func (a *Authority) Inform(updates []AUM) error {
stateAt := make(map[AUMHash]State, len(updates)+1)
toCommit := make([]AUM, 0, len(updates))
for i, update := range updates {
hash := update.Hash()
if _, err := a.storage.AUM(hash); err == nil {
// Already have this AUM.
continue
}
parent, hasParent := update.Parent()
if !hasParent {
return fmt.Errorf("update %d: missing parent", i)
}
state, hasState := stateAt[parent]
var err error
if !hasState {
if state, err = computeStateAt(a.storage, 2000, parent); err != nil {
return fmt.Errorf("update %d computing state: %v", i, err)
}
stateAt[parent] = state
}
if err := aumVerify(update, state, false); err != nil {
return fmt.Errorf("update %d invalid: %v", i, err)
}
if stateAt[hash], err = state.applyVerifiedAUM(update); err != nil {
return fmt.Errorf("update %d cannot be applied: %v", i, err)
}
toCommit = append(toCommit, update)
}
if err := a.storage.CommitVerifiedAUMs(toCommit); err != nil {
return fmt.Errorf("commit: %v", err)
}
// TODO(tom): Theres no need to recompute the state from scratch
// in every case. We should detect when updates were
// a linear, non-forking series applied to head, and
// just use the last State we computed.
oldestAncestor := a.oldestAncestor.Hash()
c, err := computeActiveChain(a.storage, &oldestAncestor, 2000)
if err != nil {
return fmt.Errorf("recomputing active chain: %v", err)
}
a.head = c.Head
a.oldestAncestor = c.Oldest
a.state = c.state
return nil
}