Detect another invalid SSZ input found through fuzzing

The first offset of an SSZ object should always have a fixed constant
value. Otherwise, some unused bytes may appear between the fixed portion
and the dynamic portion.

Please note that this fix shutds down the minimal forward compatibility
currently supported by the SSZ format (and thus, the expected behavior
must be clarified in the SSZ spec).

beacon_chain/ssz.nim

@@ -28,6 +28,9 @@ import
 
 # ################### Helper functions ###################################
 
+when hasSerializationTracing:
+  import stew/byteutils
+
 export
   serialization, types, bytes_reader
 

beacon_chain/ssz/bytes_reader.nim

@@ -225,6 +225,11 @@ func readSszValue*[T](input: openarray[byte], val: var T) {.raisesssz.} =
           startOffset = readOffsetUnchecked(boundingOffsets[0])
           endOffset = if boundingOffsets[1] == -1: input.len
                       else: readOffsetUnchecked(boundingOffsets[1])
+
+        when boundingOffsets.isFirstOffset:
+          if startOffset != minimallyExpectedSize:
+            raise newException(MalformedSszError, "SSZ object dynamic portion starts at invalid offset")
+
         trs "VAR FIELD ", startOffset, "-", endOffset
         if startOffset > endOffset:
           raise newException(MalformedSszError, "SSZ field offsets are not monotonically increasing")

beacon_chain/ssz/types.nim

@@ -165,9 +165,10 @@ proc fieldInfos*(RecordType: type): seq[tuple[name: string,
 
 func getFieldBoundingOffsetsImpl(RecordType: type,
                                  fieldName: static string):
-     tuple[fieldOffset, nextFieldOffset: int] {.compileTime.} =
-  result = (-1, -1)
+     tuple[fieldOffset, nextFieldOffset: int, isFirstOffset: bool] {.compileTime.} =
+  result = (-1, -1, false)
   var fieldBranchKey: string
+  var isFirstOffset = true
 
   for f in fieldInfos(RecordType):
     if fieldName == f.name:
@@ -177,6 +178,7 @@ func getFieldBoundingOffsetsImpl(RecordType: type,
         return
       else:
         fieldBranchKey = f.branchKey
+      result.isFirstOffset = isFirstOffset
 
     elif result[0] != -1 and
          f.fixedSize == 0 and
@@ -185,9 +187,12 @@ func getFieldBoundingOffsetsImpl(RecordType: type,
       result[1] = f.offset
       return
 
+    if f.fixedSize == 0:
+      isFirstOffset = false
+
 func getFieldBoundingOffsets*(RecordType: type,
                               fieldName: static string):
-     tuple[fieldOffset, nextFieldOffset: int] {.compileTime.} =
+     tuple[fieldOffset, nextFieldOffset: int, isFirstOffset: bool] {.compileTime.} =
   ## Returns the start and end offsets of a field.
   ##
   ## For fixed-size fields, the start offset points to the first

scripts/run_ssz_fuzzing_test.nims

@@ -3,7 +3,7 @@ import strformat, confutils
 
 const
   gitRoot = thisDir() / ".."
-  fixturesDir = gitRoot / "vendor" / "nim-eth2-scenarios" / "tests-v0.11.2" / "mainnet" / "phase0" / "ssz_static"
+  fixturesDir = gitRoot / "vendor" / "nim-eth2-scenarios" / "tests-v0.11.3" / "mainnet" / "phase0" / "ssz_static"
 
   fuzzingTestsDir = gitRoot / "tests" / "fuzzing"
   fuzzingCorpusesDir = fuzzingTestsDir / "corpus"
@@ -44,3 +44,4 @@ cli do (testname {.argument.}: string,
   let testProgram = fuzzingTestsDir / &"ssz_decode_{testname}.nim"
 
   exec &"""nim "{fuzzNims}" "{fuzzer}" "{testProgram}" "{corpusDir}"  """
+

tests/fuzzing/ssz_fuzzing.nim

@@ -1,5 +1,5 @@
 import
-  testutils/fuzzing, faststreams/inputs,
+  testutils/fuzzing, faststreams/inputs, serialization/testing/tracing,
   ../../beacon_chain/ssz,
   ../../beacon_chain/spec/[datatypes, crypto, digest, datatypes]
 
@@ -20,6 +20,11 @@ template sszFuzzingTest*(T: type) =
       let reEncoded = SSZ.encode(decoded)
 
       if payload != reEncoded:
+        when hasSerializationTracing:
+          # Run deserialization again to produce a seriazation trace
+          # (this is useful for comparing with the initial deserialization)
+          discard SSZ.decode(reEncoded, T)
+
         echo "Payload with len = ", payload.len
         echo payload
         echo "Re-encoided payload with len = ", reEncoded.len

tests/official/test_fixture_ssz_generic_types.nim

@@ -223,10 +223,7 @@ proc sszCheck(baseDir, sszType, sszSubType: string) =
     of "FixedTestStruct": checkBasic(FixedTestStruct, dir, expectedHash)
     of "VarTestStruct": checkBasic(VarTestStruct, dir, expectedHash)
     of "ComplexTestStruct": checkBasic(ComplexTestStruct, dir, expectedHash)
-    of "BitsStruct":
-      discard
-      # Compile-time issues
-      # checkBasic(BitsStruct, dir, expectedHash)
+    of "BitsStruct": checkBasic(BitsStruct, dir, expectedHash)
     else:
       raise newException(ValueError, "unknown container in test: " & sszSubType)
   else:

vendor/nim-faststreams

@@ -1 +1 @@
-Subproject commit 5d7cad792ff782672cc46a84cfcae341b55e2437
+Subproject commit 81c24860e2622a15e05c81d15e3d1cc02c460870

vendor/nim-serialization

@@ -1 +1 @@
-Subproject commit 16e6bcd16d71eba932978ebc22908da414f52db7
+Subproject commit 501f94ad61ae22197d565ac39a00783a6e6f6faa

vendor/nim-testutils

@@ -1 +1 @@
-Subproject commit 6382fdf88aa2e2179419348eb5771ba66a4a77e3
+Subproject commit fcc5aa3a532cf42f9f1074f29df36fd69b3d2920