From 07965467ee1d521871594bbd330083d964b85002 Mon Sep 17 00:00:00 2001
From: Aayush <arajasek94@gmail.com>
Date: Wed, 5 Jul 2023 11:01:38 -0400
Subject: [PATCH] feat: fvm: apply a max length when decoding events

---
 chain/types/event.go | 26 --------------------------
 chain/vm/fvm.go      |  4 ++--
 chain/vm/fvm_util.go | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 41 insertions(+), 28 deletions(-)
 create mode 100644 chain/vm/fvm_util.go

diff --git a/chain/types/event.go b/chain/types/event.go
index 91b0e95d3..106a120e2 100644
--- a/chain/types/event.go
+++ b/chain/types/event.go
@@ -1,11 +1,6 @@
 package types
 
 import (
-	"bytes"
-	"fmt"
-
-	cbg "github.com/whyrusleeping/cbor-gen"
-
 	"github.com/filecoin-project/go-state-types/abi"
 )
 
@@ -38,24 +33,3 @@ type EventEntry struct {
 }
 
 type FilterID [32]byte // compatible with EthHash
-
-// DecodeEvents decodes a CBOR list of CBOR-encoded events.
-func DecodeEvents(input []byte) ([]Event, error) {
-	r := bytes.NewReader(input)
-	typ, len, err := cbg.NewCborReader(r).ReadHeader()
-	if err != nil {
-		return nil, fmt.Errorf("failed to read events: %w", err)
-	}
-	if typ != cbg.MajArray {
-		return nil, fmt.Errorf("expected a CBOR list, was major type %d", typ)
-	}
-	events := make([]Event, 0, len)
-	for i := 0; i < int(len); i++ {
-		var evt Event
-		if err := evt.UnmarshalCBOR(r); err != nil {
-			return nil, fmt.Errorf("failed to parse event: %w", err)
-		}
-		events = append(events, evt)
-	}
-	return events, nil
-}
diff --git a/chain/vm/fvm.go b/chain/vm/fvm.go
index 7c79972c7..0efaa6302 100644
--- a/chain/vm/fvm.go
+++ b/chain/vm/fvm.go
@@ -458,7 +458,7 @@ func (vm *FVM) ApplyMessage(ctx context.Context, cmsg types.ChainMsg) (*ApplyRet
 	}
 
 	if vm.returnEvents && len(ret.EventsBytes) > 0 {
-		applyRet.Events, err = types.DecodeEvents(ret.EventsBytes)
+		applyRet.Events, err = decodeEvents(ret.EventsBytes)
 		if err != nil {
 			return nil, fmt.Errorf("failed to decode events returned by the FVM: %w", err)
 		}
@@ -514,7 +514,7 @@ func (vm *FVM) ApplyImplicitMessage(ctx context.Context, cmsg *types.Message) (*
 	}
 
 	if vm.returnEvents && len(ret.EventsBytes) > 0 {
-		applyRet.Events, err = types.DecodeEvents(ret.EventsBytes)
+		applyRet.Events, err = decodeEvents(ret.EventsBytes)
 		if err != nil {
 			return nil, fmt.Errorf("failed to decode events returned by the FVM: %w", err)
 		}
diff --git a/chain/vm/fvm_util.go b/chain/vm/fvm_util.go
new file mode 100644
index 000000000..9f3dfd869
--- /dev/null
+++ b/chain/vm/fvm_util.go
@@ -0,0 +1,39 @@
+package vm
+
+import (
+	"bytes"
+	"fmt"
+
+	cbg "github.com/whyrusleeping/cbor-gen"
+
+	"github.com/filecoin-project/lotus/chain/types"
+)
+
+const MaxEventSliceLength = 6_000_000
+
+// DecodeEvents decodes a CBOR list of CBOR-encoded events.
+func decodeEvents(input []byte) ([]types.Event, error) {
+	r := bytes.NewReader(input)
+	typ, length, err := cbg.NewCborReader(r).ReadHeader()
+	if err != nil {
+		return nil, fmt.Errorf("failed to read events: %w", err)
+	}
+
+	if length > MaxEventSliceLength {
+		log.Errorf("extremely long event slice (len %d) returned, not decoding", length)
+		return nil, nil
+	}
+
+	if typ != cbg.MajArray {
+		return nil, fmt.Errorf("expected a CBOR list, was major type %d", typ)
+	}
+	events := make([]types.Event, 0, length)
+	for i := 0; i < int(length); i++ {
+		var evt types.Event
+		if err := evt.UnmarshalCBOR(r); err != nil {
+			return nil, fmt.Errorf("failed to parse event: %w", err)
+		}
+		events = append(events, evt)
+	}
+	return events, nil
+}