- package light
-
- import (
- "bytes"
- "context"
- "errors"
- "fmt"
- "time"
-
- "github.com/tendermint/tendermint/light/provider"
- "github.com/tendermint/tendermint/types"
- )
-
- // The detector component of the light client detects and handles attacks on the light client.
- // More info here:
- // tendermint/docs/architecture/adr-047-handling-evidence-from-light-client.md
-
- // detectDivergence is a second wall of defense for the light client.
- //
- // It takes the target verified header and compares it with the headers of a set of
- // witness providers that the light client is connected to. If a conflicting header
- // is returned it verifies and examines the conflicting header against the verified
- // trace that was produced from the primary. If successful, it produces two sets of evidence
- // and sends them to the opposite provider before halting.
- //
- // If there are no conflictinge headers, the light client deems the verified target header
- // trusted and saves it to the trusted store.
- func (c *Client) detectDivergence(ctx context.Context, primaryTrace []*types.LightBlock, now time.Time) error {
- if primaryTrace == nil || len(primaryTrace) < 2 {
- return errors.New("nil or single block primary trace")
- }
- var (
- headerMatched bool
- lastVerifiedHeader = primaryTrace[len(primaryTrace)-1].SignedHeader
- witnessesToRemove = make([]int, 0)
- )
- c.logger.Debug("running detector against trace", "endBlockHeight", lastVerifiedHeader.Height,
- "endBlockHash", lastVerifiedHeader.Hash, "length", len(primaryTrace))
-
- c.providerMutex.Lock()
- defer c.providerMutex.Unlock()
-
- if len(c.witnesses) == 0 {
- return ErrNoWitnesses
- }
-
- // launch one goroutine per witness to retrieve the light block of the target height
- // and compare it with the header from the primary
- errc := make(chan error, len(c.witnesses))
- for i, witness := range c.witnesses {
- go c.compareNewHeaderWithWitness(ctx, errc, lastVerifiedHeader, witness, i)
- }
-
- // handle errors from the header comparisons as they come in
- for i := 0; i < cap(errc); i++ {
- err := <-errc
-
- switch e := err.(type) {
- case nil: // at least one header matched
- headerMatched = true
- case errConflictingHeaders:
- // We have conflicting headers. This could possibly imply an attack on the light client.
- // First we need to verify the witness's header using the same skipping verification and then we
- // need to find the point that the headers diverge and examine this for any evidence of an attack.
- //
- // We combine these actions together, verifying the witnesses headers and outputting the trace
- // which captures the bifurcation point and if successful provides the information to create valid evidence.
- err := c.handleConflictingHeaders(ctx, primaryTrace, e.Block, e.WitnessIndex, now)
- if err != nil {
- // return information of the attack
- return err
- }
- // if attempt to generate conflicting headers failed then remove witness
- witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
-
- case errBadWitness:
- c.logger.Info("witness returned an error during header comparison, removing...",
- "witness", c.witnesses[e.WitnessIndex], "err", err)
- witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
- default:
- if errors.Is(e, context.Canceled) || errors.Is(e, context.DeadlineExceeded) {
- return e
- }
- c.logger.Info("error in light block request to witness", "err", err)
- }
- }
-
- // remove witnesses that have misbehaved
- if err := c.removeWitnesses(witnessesToRemove); err != nil {
- return err
- }
-
- // 1. If we had at least one witness that returned the same header then we
- // conclude that we can trust the header
- if headerMatched {
- return nil
- }
-
- // 2. Else all witnesses have either not responded, don't have the block or sent invalid blocks.
- return ErrFailedHeaderCrossReferencing
- }
-
- // compareNewHeaderWithWitness takes the verified header from the primary and compares it with a
- // header from a specified witness. The function can return one of three errors:
- //
- // 1: errConflictingHeaders -> there may have been an attack on this light client
- // 2: errBadWitness -> the witness has either not responded, doesn't have the header or has given us an invalid one
- // Note: In the case of an invalid header we remove the witness
- // 3: nil -> the hashes of the two headers match
- func (c *Client) compareNewHeaderWithWitness(ctx context.Context, errc chan error, h *types.SignedHeader,
- witness provider.Provider, witnessIndex int) {
-
- lightBlock, err := c.getLightBlock(ctx, witness, h.Height)
- switch err {
- // no error means we move on to checking the hash of the two headers
- case nil:
- break
-
- // the witness hasn't been helpful in comparing headers, we mark the response and continue
- // comparing with the rest of the witnesses
- case provider.ErrNoResponse, provider.ErrLightBlockNotFound, context.DeadlineExceeded, context.Canceled:
- errc <- err
- return
-
- // the witness' head of the blockchain is lower than the height of the primary. This could be one of
- // two things:
- // 1) The witness is lagging behind
- // 2) The primary may be performing a lunatic attack with a height and time in the future
- case provider.ErrHeightTooHigh:
- // The light client now asks for the latest header that the witness has
- var isTargetHeight bool
- isTargetHeight, lightBlock, err = c.getTargetBlockOrLatest(ctx, h.Height, witness)
- if err != nil {
- if c.providerShouldBeRemoved(err) {
- errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
- } else {
- errc <- err
- }
- return
- }
-
- // if the witness caught up and has returned a block of the target height then we can
- // break from this switch case and continue to verify the hashes
- if isTargetHeight {
- break
- }
-
- // witness' last header is below the primary's header. We check the times to see if the blocks
- // have conflicting times
- if !lightBlock.Time.Before(h.Time) {
- errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
- return
- }
-
- // the witness is behind. We wait for a period WAITING = 2 * DRIFT + LAG.
- // This should give the witness ample time if it is a participating member
- // of consensus to produce a block that has a time that is after the primary's
- // block time. If not the witness is too far behind and the light client removes it
- time.Sleep(2*c.maxClockDrift + c.maxBlockLag)
- isTargetHeight, lightBlock, err = c.getTargetBlockOrLatest(ctx, h.Height, witness)
- if err != nil {
- if c.providerShouldBeRemoved(err) {
- errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
- } else {
- errc <- err
- }
- return
- }
- if isTargetHeight {
- break
- }
-
- // the witness still doesn't have a block at the height of the primary.
- // Check if there is a conflicting time
- if !lightBlock.Time.Before(h.Time) {
- errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
- return
- }
-
- // Following this request response procedure, the witness has been unable to produce a block
- // that can somehow conflict with the primary's block. We thus conclude that the witness
- // is too far behind and thus we return a no response error.
- //
- // NOTE: If the clock drift / lag has been miscalibrated it is feasible that the light client has
- // drifted too far ahead for any witness to be able provide a comparable block and thus may allow
- // for a malicious primary to attack it
- errc <- provider.ErrNoResponse
- return
-
- default:
- // all other errors (i.e. invalid block, closed connection or unreliable provider) we mark the
- // witness as bad and remove it
- errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
- return
- }
-
- if !bytes.Equal(h.Header.Hash(), lightBlock.Header.Hash()) {
- errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
- }
-
- c.logger.Debug("matching header received by witness", "height", h.Height, "witness", witnessIndex)
- errc <- nil
- }
-
- // sendEvidence sends evidence to a provider on a best effort basis.
- func (c *Client) sendEvidence(ctx context.Context, ev *types.LightClientAttackEvidence, receiver provider.Provider) {
- err := receiver.ReportEvidence(ctx, ev)
- if err != nil {
- c.logger.Error("failed to report evidence to provider", "ev", ev, "provider", receiver)
- }
- }
-
- // handleConflictingHeaders handles the primary style of attack, which is where a primary and witness have
- // two headers of the same height but with different hashes
- func (c *Client) handleConflictingHeaders(
- ctx context.Context,
- primaryTrace []*types.LightBlock,
- challendingBlock *types.LightBlock,
- witnessIndex int,
- now time.Time,
- ) error {
- supportingWitness := c.witnesses[witnessIndex]
- witnessTrace, primaryBlock, err := c.examineConflictingHeaderAgainstTrace(
- ctx,
- primaryTrace,
- challendingBlock,
- supportingWitness,
- now,
- )
- if err != nil {
- c.logger.Info("error validating witness's divergent header", "witness", supportingWitness, "err", err)
- return nil
- }
-
- // We are suspecting that the primary is faulty, hence we hold the witness as the source of truth
- // and generate evidence against the primary that we can send to the witness
- commonBlock, trustedBlock := witnessTrace[0], witnessTrace[len(witnessTrace)-1]
- evidenceAgainstPrimary := newLightClientAttackEvidence(primaryBlock, trustedBlock, commonBlock)
- c.logger.Error("ATTEMPTED ATTACK DETECTED. Sending evidence againt primary by witness", "ev", evidenceAgainstPrimary,
- "primary", c.primary, "witness", supportingWitness)
- c.sendEvidence(ctx, evidenceAgainstPrimary, supportingWitness)
-
- if primaryBlock.Commit.Round != witnessTrace[len(witnessTrace)-1].Commit.Round {
- c.logger.Info("The light client has detected, and prevented, an attempted amnesia attack." +
- " We think this attack is pretty unlikely, so if you see it, that's interesting to us." +
- " Can you let us know by opening an issue through https://github.com/tendermint/tendermint/issues/new?")
- }
-
- // This may not be valid because the witness itself is at fault. So now we reverse it, examining the
- // trace provided by the witness and holding the primary as the source of truth. Note: primary may not
- // respond but this is okay as we will halt anyway.
- primaryTrace, witnessBlock, err := c.examineConflictingHeaderAgainstTrace(
- ctx,
- witnessTrace,
- primaryBlock,
- c.primary,
- now,
- )
- if err != nil {
- c.logger.Info("error validating primary's divergent header", "primary", c.primary, "err", err)
- return ErrLightClientAttack
- }
-
- // We now use the primary trace to create evidence against the witness and send it to the primary
- commonBlock, trustedBlock = primaryTrace[0], primaryTrace[len(primaryTrace)-1]
- evidenceAgainstWitness := newLightClientAttackEvidence(witnessBlock, trustedBlock, commonBlock)
- c.logger.Error("Sending evidence against witness by primary", "ev", evidenceAgainstWitness,
- "primary", c.primary, "witness", supportingWitness)
- c.sendEvidence(ctx, evidenceAgainstWitness, c.primary)
- // We return the error and don't process anymore witnesses
- return ErrLightClientAttack
- }
-
- // examineConflictingHeaderAgainstTrace takes a trace from one provider and a divergent header that
- // it has received from another and preforms verifySkipping at the heights of each of the intermediate
- // headers in the trace until it reaches the divergentHeader. 1 of 2 things can happen.
- //
- // 1. The light client verifies a header that is different to the intermediate header in the trace. This
- // is the bifurcation point and the light client can create evidence from it
- // 2. The source stops responding, doesn't have the block or sends an invalid header in which case we
- // return the error and remove the witness
- //
- // CONTRACT:
- // 1. Trace can not be empty len(trace) > 0
- // 2. The last block in the trace can not be of a lower height than the target block
- // trace[len(trace)-1].Height >= targetBlock.Height
- // 3. The
- func (c *Client) examineConflictingHeaderAgainstTrace(
- ctx context.Context,
- trace []*types.LightBlock,
- targetBlock *types.LightBlock,
- source provider.Provider, now time.Time,
- ) ([]*types.LightBlock, *types.LightBlock, error) {
-
- var (
- previouslyVerifiedBlock, sourceBlock *types.LightBlock
- sourceTrace []*types.LightBlock
- err error
- )
-
- if targetBlock.Height < trace[0].Height {
- return nil, nil, fmt.Errorf("target block has a height lower than the trusted height (%d < %d)",
- targetBlock.Height, trace[0].Height)
- }
-
- for idx, traceBlock := range trace {
- // this case only happens in a forward lunatic attack. We treat the block with the
- // height directly after the targetBlock as the divergent block
- if traceBlock.Height > targetBlock.Height {
- // sanity check that the time of the traceBlock is indeed less than that of the targetBlock. If the trace
- // was correctly verified we should expect monotonically increasing time. This means that if the block at
- // the end of the trace has a lesser time than the target block then all blocks in the trace should have a
- // lesser time
- if traceBlock.Time.After(targetBlock.Time) {
- return nil, nil,
- errors.New("sanity check failed: expected traceblock to have a lesser time than the target block")
- }
-
- // before sending back the divergent block and trace we need to ensure we have verified
- // the final gap between the previouslyVerifiedBlock and the targetBlock
- if previouslyVerifiedBlock.Height != targetBlock.Height {
- sourceTrace, err = c.verifySkipping(ctx, source, previouslyVerifiedBlock, targetBlock, now)
- if err != nil {
- return nil, nil, fmt.Errorf("verifySkipping of conflicting header failed: %w", err)
- }
- }
- return sourceTrace, traceBlock, nil
- }
-
- // get the corresponding block from the source to verify and match up against the traceBlock
- if traceBlock.Height == targetBlock.Height {
- sourceBlock = targetBlock
- } else {
- sourceBlock, err = c.getLightBlock(ctx, source, traceBlock.Height)
- if err != nil {
- return nil, nil, fmt.Errorf("failed to examine trace: %w", err)
- }
- }
-
- // The first block in the trace MUST be the same to the light block that the source produces
- // else we cannot continue with verification.
- if idx == 0 {
- if shash, thash := sourceBlock.Hash(), traceBlock.Hash(); !bytes.Equal(shash, thash) {
- return nil, nil, fmt.Errorf("trusted block is different to the source's first block (%X = %X)",
- thash, shash)
- }
- previouslyVerifiedBlock = sourceBlock
- continue
- }
-
- // we check that the source provider can verify a block at the same height of the
- // intermediate height
- sourceTrace, err = c.verifySkipping(ctx, source, previouslyVerifiedBlock, sourceBlock, now)
- if err != nil {
- return nil, nil, fmt.Errorf("verifySkipping of conflicting header failed: %w", err)
- }
- // check if the headers verified by the source has diverged from the trace
- if shash, thash := sourceBlock.Hash(), traceBlock.Hash(); !bytes.Equal(shash, thash) {
- // Bifurcation point found!
- return sourceTrace, traceBlock, nil
- }
-
- // headers are still the same. update the previouslyVerifiedBlock
- previouslyVerifiedBlock = sourceBlock
- }
-
- // We have reached the end of the trace. This should never happen. This can only happen if one of the stated
- // prerequisites to this function were not met. Namely that either trace[len(trace)-1].Height < targetBlock.Height
- // or that trace[i].Hash() != targetBlock.Hash()
- return nil, nil, errNoDivergence
-
- }
-
- // getTargetBlockOrLatest gets the latest height, if it is greater than the target height then it queries
- // the target heght else it returns the latest. returns true if it successfully managed to acquire the target
- // height.
- func (c *Client) getTargetBlockOrLatest(
- ctx context.Context,
- height int64,
- witness provider.Provider,
- ) (bool, *types.LightBlock, error) {
- lightBlock, err := c.getLightBlock(ctx, witness, 0)
- if err != nil {
- return false, nil, err
- }
-
- if lightBlock.Height == height {
- // the witness has caught up to the height of the provider's signed header. We
- // can resume with checking the hashes.
- return true, lightBlock, nil
- }
-
- if lightBlock.Height > height {
- // the witness has caught up. We recursively call the function again. However in order
- // to avoud a wild goose chase where the witness sends us one header below and one header
- // above the height we set a timeout to the context
- lightBlock, err := c.getLightBlock(ctx, witness, height)
- return true, lightBlock, err
- }
-
- return false, lightBlock, nil
- }
-
- // newLightClientAttackEvidence determines the type of attack and then forms the evidence filling out
- // all the fields such that it is ready to be sent to a full node.
- func newLightClientAttackEvidence(conflicted, trusted, common *types.LightBlock) *types.LightClientAttackEvidence {
- ev := &types.LightClientAttackEvidence{ConflictingBlock: conflicted}
- // We use the common height to indicate the form of the attack.
- // if this is an equivocation or amnesia attack, i.e. the validator sets are the same, then we
- // return the height of the conflicting block as the common height. If instead it is a lunatic
- // attack and the validator sets are not the same then we send the height of the common header.
- if ev.ConflictingHeaderIsInvalid(trusted.Header) {
- ev.CommonHeight = common.Height
- ev.Timestamp = common.Time
- ev.TotalVotingPower = common.ValidatorSet.TotalVotingPower()
- } else {
- ev.CommonHeight = trusted.Height
- ev.Timestamp = trusted.Time
- ev.TotalVotingPower = trusted.ValidatorSet.TotalVotingPower()
- }
- ev.ByzantineValidators = ev.GetByzantineValidators(common.ValidatorSet, trusted.SignedHeader)
- return ev
- }
|