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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
primaryEv := &types.LightClientAttackEvidence{
ConflictingBlock: primaryBlock,
CommonHeight: commonHeight, // the first block in the bisection is common to both providers
}
c.logger.Error("Attempted attack detected. Sending evidence againt primary by witness", "ev", primaryEv,
"primary", c.primary, "witness", supportingWitness)
c.sendEvidence(ctx, primaryEv, 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.SignedHeader,
c.primary,
now,
)
if err != nil {
c.logger.Info("Error validating primary's divergent header", "primary", c.primary, "err", err)
return ErrLightClientAttack
}
// 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
witnessEv := &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", witnessEv,
"primary", c.primary, "witness", supportingWitness)
c.sendEvidence(ctx, witnessEv, c.primary)
// We return the error and don't process anymore witnesses
return ErrLightClientAttack
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)
}