[Crash/Fuzzing] IndexError during Attestation SSZ parsing (container empty)

[pventuzelo]

During fuzzing, I triggered an IndexError during parsing of Attestation ssz file with mainnet preset.

Error: unhandled exception: index out of bounds, the container is empty [IndexError]

Reproducing

Download:
IndexError_attestation_empty_container_nimbus_devel.zip

branch: devel
commit: fe30b24abad112a6fed9a6d4d18df1978f2350f6

Load the file using ncli_hash_tree_root:

$ make

$ cd ncli

$ ../env.sh nim c -d:const_preset=mainnet ncli_hash_tree_root

$ ./ncli_hash_tree_root --kind=attestation --file= IndexError_attestation_empty_container_nimbus_devel.ssz
Traceback (most recent call last, using override)
XXX/nim-beacon-chain/vendor/nim-confutils/confutils.nim(981) confutils
XXX/nim-beacon-chain/ncli/ncli_hash_tree_root2.nim(14) CLI
XXX/nim-beacon-chain/beacon_chain/ssz.nim(583) hash_tree_root
XXX/nim-beacon-chain/beacon_chain/ssz.nim(444) hashTreeRootImpl
XXX/nim-beacon-chain/beacon_chain/ssz.nim(570) hash_tree_root
XXX/nim-beacon-chain/beacon_chain/ssz.nim(466) bitlistHashTreeRoot
XXX/nim-beacon-chain/vendor/nimbus-build-system/vendor/Nim/lib/system.nim(2515) X5BX5D
XXX/nim-beacon-chain/vendor/nimbus-build-system/vendor/Nim/lib/system/chcks.nim(23) raiseIndexError2
XXX/nim-beacon-chain/vendor/nimbus-build-system/vendor/Nim/lib/system/fatal.nim(51) sysFatal
XXX/nim-beacon-chain/vendor/nimbus-build-system/vendor/Nim/lib/system/excpt.nim(407) reportUnhandledError
XXX/nim-beacon-chain/vendor/nimbus-build-system/vendor/Nim/lib/system/excpt.nim(358) reportUnhandledErrorAux
Error: unhandled exception: index out of bounds, the container is empty [IndexError]

[tersec]

zcli refuses to even parse this one, so probably it's an SSZ parsing issue that leaks through to SSZ hashing:

$ zcli pretty attestation IndexError_attestation_empty_container_nimbus_devel.ssz 
cannot load input
cannot decode ssz: expected object length is larger than given bytesLen
$ zcli check attestation IndexError_attestation_empty_container_nimbus_devel.ssz 
cannot verify input
expected object length is larger than given bytesLen

https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/ssz/simple-serialize.md#bitlistn notes that:

An additional 1 bit is added to the end, at index e where e is the length of the bitlist (not the limit), so that the length in bits will also be known.

and when it reaches bitlistHashTreeRoot it's just completely empty, not even the trailing 1 bit, which suggests again that the SSZ parsing is where it goes awry.

[tersec]

#936