blockchain: refuse a v2 header committing to a height below the BLAKE2b activation

blockchain/blake2b_test.go

@@ -249,3 +249,35 @@ func TestBlake2bHeaderContext(t *testing.T) {
 		t.Fatalf("classic header without BLAKE2b configured: %v", err)
 	}
 }
+
+// TestCheckHeaderV2Sanity pins the context-free rule Knots applies in
+// CheckBlockHeader: a v2 header committing to the activation height is
+// accepted, one committing to the block below it is refused (bad-version-
+// sha256d), and a network without BLAKE2b configured takes no v2 header.
+func TestCheckHeaderV2Sanity(t *testing.T) {
+	params := &chaincfg.MainNetParams
+	v2 := mustHeader(t, activationHeaderRaw)
+	if v2.Height != 961640 {
+		t.Fatalf("the activation header commits to %d", v2.Height)
+	}
+
+	if err := CheckHeaderV2Sanity(v2, params); err != nil {
+		t.Fatalf("v2 header committing to the activation height: %v", err)
+	}
+	below := *v2
+	below.Height = 961639
+	if code, _ := ruleCode(CheckHeaderV2Sanity(&below, params)); code != ErrBadHeaderV2 {
+		t.Fatalf("v2 header committing to the block below the activation: got %v, want ErrBadHeaderV2", code)
+	}
+
+	classic := chaincfg.MainNetParams.GenesisBlock.Header
+	if err := CheckHeaderV2Sanity(&classic, params); err != nil {
+		t.Fatalf("classic header: %v", err)
+	}
+
+	none := chaincfg.MainNetParams
+	none.Blake2bHeight = 0
+	if code, _ := ruleCode(CheckHeaderV2Sanity(v2, &none)); code != ErrUnsupportedProofOfWork {
+		t.Fatalf("v2 header without BLAKE2b configured: got %v, want ErrUnsupportedProofOfWork", code)
+	}
+}

blockchain/blake2bpow.go

@@ -73,6 +73,30 @@ func checkHeaderV2Context(header *wire.BlockHeader, height int32,
 	return nil
 }
 
+// CheckHeaderV2Sanity is the context-free part of Knots' CheckBlockHeader for
+// BLAKE2b headers: a v2 header must commit to a height at which BLAKE2b proof
+// of work is required (Knots' bad-version-sha256d), and a network without
+// BLAKE2b proof of work configured takes no v2 header. It needs only the
+// header, so it runs where the block's real height is not known yet (an
+// orphan, a header out of order); checkHeaderV2Context then checks the
+// committed height against the real one.
+func CheckHeaderV2Sanity(header *wire.BlockHeader, params *chaincfg.Params) error {
+	if !header.IsV2() {
+		return nil
+	}
+	if !params.Blake2bPowDeployed() {
+		return ruleError(ErrUnsupportedProofOfWork, "BLAKE2b (header v2) "+
+			"proof of work is not configured for network "+params.Name)
+	}
+	if !params.IsBlake2bHeight(header.Height) {
+		str := fmt.Sprintf("header v2 commits to height %d: blocks require "+
+			"SHA256d proof of work until height %d", header.Height,
+			params.Blake2bHeight)
+		return ruleError(ErrBadHeaderV2, str)
+	}
+	return nil
+}
+
 // applyBlake2bTargetShift is Knots' ApplyBlake2bTargetShift: the target of the
 // first block mined under BLAKE2b, from the one the usual rules give.
 func applyBlake2bTargetShift(bits uint32, params *chaincfg.Params) uint32 {

blockchain/process.go

@@ -169,6 +169,10 @@ func (b *BlockChain) ProcessBlock(block *btcutil.Block, flags BehaviorFlags) (bo
 	if err != nil {
 		return false, false, err
 	}
+	err = CheckHeaderV2Sanity(&block.MsgBlock().Header, b.chainParams)
+	if err != nil {
+		return false, false, err
+	}
 
 	// Find the previous checkpoint and perform some additional checks based
 	// on the checkpoint.  This provides a few nice properties such as

blockchain/validate.go

@@ -1394,6 +1394,10 @@ func (b *BlockChain) CheckConnectBlockTemplate(block *btcutil.Block) error {
 	if err != nil {
 		return err
 	}
+	err = CheckHeaderV2Sanity(&header, b.chainParams)
+	if err != nil {
+		return err
+	}
 
 	err = b.checkBlockContext(block, tip, flags)
 	if err != nil {

rpcserver.go

@@ -3720,6 +3720,11 @@ func verifyChain(s *rpcServer, level, depth int32) error {
 		if level > 0 {
 			err := blockchain.CheckBlockSanity(block,
 				s.cfg.ChainParams.PowLimit, s.cfg.TimeSource)
+			if err == nil {
+				err = blockchain.CheckHeaderV2Sanity(
+					&block.MsgBlock().Header, s.cfg.ChainParams,
+				)
+			}
 			if err != nil {
 				rpcsLog.Errorf("Verify is unable to validate "+
 					"block at hash %v height %d: %v",