tendermint/light/detector.go

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 detect 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 and is used
// only in the case of skipping verification which employs the trust level mechanism.
//
// 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
			supportingWitness := c.witnesses[e.WitnessIndex]
			witnessTrace, primaryBlock, err := c.examineConflictingHeaderAgainstTrace(
				ctx,
				primaryTrace,
				e.Block.SignedHeader,
				supportingWitness,
				now,
			)
			if err != nil {
				c.logger.Info("Error validating witness's divergent header", "witness", supportingWitness, "err", err)
				witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
				continue
			}
			// 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 else if it is a lunatic attack and the validator sets
			// are not the same then we send the height of the common header.
			commonHeight := primaryBlock.Height
			if isInvalidHeader(witnessTrace[len(witnessTrace)-1].Header, primaryBlock.Header) {
				// height of the common header
				commonHeight = witnessTrace[0].Height
			}

			// 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
			ev := &types.LightClientAttackEvidence{
				ConflictingBlock: primaryBlock,
				CommonHeight:     commonHeight, // the first block in the bisection is common to both providers
			}
			c.logger.Error("Attack detected. Sending evidence againt primary by witness", "ev", ev,
				"primary", c.primary, "witness", supportingWitness)
			c.sendEvidence(ctx, ev, supportingWitness)

			// 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.SignedHeader,
				c.primary,
				now,
			)
			if err != nil {
				c.logger.Info("Error validating primary's divergent header", "primary", c.primary, "err", err)
				continue
			}
			// 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 else if it is a lunatic attack and the validator sets
			// are not the same then we send the height of the common header.
			commonHeight = primaryBlock.Height
			if isInvalidHeader(primaryTrace[len(primaryTrace)-1].Header, witnessBlock.Header) {
				// height of the common header
				commonHeight = primaryTrace[0].Height
			}

			// We now use the primary trace to create evidence against the witness and send it to the primary
			ev = &types.LightClientAttackEvidence{
				ConflictingBlock: witnessBlock,
				CommonHeight:     commonHeight, // the first block in the bisection is common to both providers
			}
			c.logger.Error("Sending evidence against witness by primary", "ev", ev,
				"primary", c.primary, "witness", supportingWitness)
			c.sendEvidence(ctx, ev, c.primary)
			// We return the error and don't process anymore witnesses
			return e

		case errBadWitness:
			c.logger.Info("Witness returned an error during header comparison", "witness", c.witnesses[e.WitnessIndex],
				"err", err)
			// if witness sent us an invalid header, then remove it. If it didn't respond or couldn't find the block, then we
			// ignore it and move on to the next witness
			if _, ok := e.Reason.(provider.ErrBadLightBlock); ok {
				c.logger.Info("Witness sent us invalid header / vals -> removing it", "witness", c.witnesses[e.WitnessIndex])
				witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
			}
		}
	}

	for _, idx := range witnessesToRemove {
		c.removeWitness(idx)
	}

	// 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 := witness.LightBlock(ctx, h.Height)
	if err != nil {
		errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
		return
	}

	if !bytes.Equal(h.Hash(), lightBlock.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)
	}
}

// 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
func (c *Client) examineConflictingHeaderAgainstTrace(
	ctx context.Context,
	trace []*types.LightBlock,
	divergentHeader *types.SignedHeader,
	source provider.Provider, now time.Time) ([]*types.LightBlock, *types.LightBlock, error) {

	var previouslyVerifiedBlock *types.LightBlock

	for idx, traceBlock := range trace {
		// The first block in the trace MUST be the same to the light block that the source produces
		// else we cannot continue with verification.
		sourceBlock, err := source.LightBlock(ctx, traceBlock.Height)
		if err != nil {
			return nil, nil, err
		}

		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
		trace, 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 trace, traceBlock, nil
		}

		// headers are still the same. update the previouslyVerifiedBlock
		previouslyVerifiedBlock = sourceBlock
	}

	// We have reached the end of the trace without observing a divergence. The last header  is thus different
	// from the divergent header that the source originally sent us, then we return an error.
	return nil, nil, fmt.Errorf("source provided different header to the original header it provided (%X != %X)",
		previouslyVerifiedBlock.Hash(), divergentHeader.Hash())

}

// isInvalidHeader takes a trusted header and matches it againt a conflicting header
// to determine whether the conflicting header was the product of a valid state transition
// or not. If it is then all the deterministic fields of the header should be the same.
// If not, it is an invalid header and constitutes a lunatic attack.
func isInvalidHeader(trusted, conflicting *types.Header) bool {
	return !bytes.Equal(trusted.ValidatorsHash, conflicting.ValidatorsHash) ||
		!bytes.Equal(trusted.NextValidatorsHash, conflicting.NextValidatorsHash) ||
		!bytes.Equal(trusted.ConsensusHash, conflicting.ConsensusHash) ||
		!bytes.Equal(trusted.AppHash, conflicting.AppHash) ||
		!bytes.Equal(trusted.LastResultsHash, conflicting.LastResultsHash)
}