tests/fuzzers: added bn marshaling fuzzers (#32053)

Adds marshaling fuzzing for G1 and G2 to oss-fuzz. 

Also aligns the behavior of the google library to that of gnark and
cloudflare, which only ever read the first 64 / 128 bytes of the input,
regardless of how long the input is

crypto/bn256/google/bn256.go

@@ -128,7 +128,7 @@ func (e *G1) Marshal() []byte {
 func (e *G1) Unmarshal(m []byte) ([]byte, error) {
 	// Each value is a 256-bit number.
 	const numBytes = 256 / 8
-	if len(m) != 2*numBytes {
+	if len(m) < 2*numBytes {
 		return nil, errors.New("bn256: not enough data")
 	}
 	// Unmarshal the points and check their caps
@@ -253,7 +253,7 @@ func (n *G2) Marshal() []byte {
 func (e *G2) Unmarshal(m []byte) ([]byte, error) {
 	// Each value is a 256-bit number.
 	const numBytes = 256 / 8
-	if len(m) != 4*numBytes {
+	if len(m) < 4*numBytes {
 		return nil, errors.New("bn256: not enough data")
 	}
 	// Unmarshal the points and check their caps

oss-fuzz.sh

@@ -152,6 +152,14 @@ compile_fuzzer github.com/ethereum/go-ethereum/tests/fuzzers/bn256 \
   FuzzPair fuzzBn256Pair \
   $repo/tests/fuzzers/bn256/bn256_test.go
 
+compile_fuzzer github.com/ethereum/go-ethereum/tests/fuzzers/bn256 \
+  FuzzUnmarshalG1 fuzzBn256UnmarshalG1 \
+  $repo/tests/fuzzers/bn256/bn256_test.go
+
+compile_fuzzer github.com/ethereum/go-ethereum/tests/fuzzers/bn256 \
+  FuzzUnmarshalG2 fuzzBn256UnmarshalG2 \
+  $repo/tests/fuzzers/bn256/bn256_test.go
+
 compile_fuzzer github.com/ethereum/go-ethereum/tests/fuzzers/txfetcher \
   Fuzz fuzzTxfetcher \
   $repo/tests/fuzzers/txfetcher/txfetcher_test.go

tests/fuzzers/bn256/bn256_fuzz.go

@@ -161,6 +161,60 @@ func fuzzPair(data []byte) int {
 	return 1
 }
 
+func fuzzUnmarshalG1(input []byte) int {
+	rc := new(cloudflare.G1)
+	_, errC := rc.Unmarshal(input)
+
+	rg := new(google.G1)
+	_, errG := rg.Unmarshal(input)
+
+	rs := new(gnark.G1)
+	_, errS := rs.Unmarshal(input)
+
+	if errC != nil && errG != nil && errS != nil {
+		return 0 // bad input
+	}
+	if errC == nil && errG == nil && errS == nil {
+		//make sure we unmarshalled the same points:
+		if !bytes.Equal(rc.Marshal(), rg.Marshal()) {
+			panic("marshaling mismatch: cloudflare/google")
+		}
+		if !bytes.Equal(rc.Marshal(), rs.Marshal()) {
+			panic("marshaling mismatch: cloudflare/gnark")
+		}
+		return 1
+	} else {
+		panic(fmt.Sprintf("error missmatch: cf: %v g: %v gn: %v", errC, errG, errS))
+	}
+}
+
+func fuzzUnmarshalG2(input []byte) int {
+	rc := new(cloudflare.G2)
+	_, errC := rc.Unmarshal(input)
+
+	rg := new(google.G2)
+	_, errG := rg.Unmarshal(input)
+
+	rs := new(gnark.G2)
+	_, errS := rs.Unmarshal(input)
+
+	if errC != nil && errG != nil && errS != nil {
+		return 0 // bad input
+	}
+	if errC == nil && errG == nil && errS == nil {
+		//make sure we unmarshalled the same points:
+		if !bytes.Equal(rc.Marshal(), rg.Marshal()) {
+			panic("marshaling mismatch: cloudflare/google")
+		}
+		if !bytes.Equal(rc.Marshal(), rs.Marshal()) {
+			panic("marshaling mismatch: cloudflare/gnark")
+		}
+		return 1
+	} else {
+		panic(fmt.Sprintf("error missmatch: cf: %v g: %v gn: %v", errC, errG, errS))
+	}
+}
+
 // normalizeGTToGnark scales a Cloudflare/Google GT element by `s`
 // so that it can be compared with a gnark GT point.
 //

tests/fuzzers/bn256/bn256_test.go

@@ -35,3 +35,15 @@ func FuzzPair(f *testing.F) {
 		fuzzPair(data)
 	})
 }
+
+func FuzzUnmarshalG1(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data []byte) {
+		fuzzUnmarshalG1(data)
+	})
+}
+
+func FuzzUnmarshalG2(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data []byte) {
+		fuzzUnmarshalG2(data)
+	})
+}