refactor: optimize public call stack item hashing #7330
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,102 @@ | ||
| use crate::abis::{call_context::CallContext, function_data::FunctionData, gas::Gas}; | ||
| use crate::address::AztecAddress; | ||
| use crate::constants::{GENERATOR_INDEX__CALL_STACK_ITEM, PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH}; | ||
| use crate::traits::{Hash, Empty, Serialize, Deserialize}; | ||
| use crate::utils::reader::Reader; | ||
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| /** | ||
| * A compressed version of the PublicCallStackItem struct used to compute the "hash" | ||
| * of a PublicCallStackItem. | ||
| * | ||
| * Historically, we have been zeroing most values in the PublicCallStackItem struct | ||
| * to compute the hash involved when adding a PublicCallStackItem to the PublicCallStack. | ||
| * | ||
| * This struct is used to store the values that we did not zero out, and allow us to hash | ||
| * only these, thereby skipping a lot of computation and saving us a lot of constraints | ||
| * | ||
| * Essentially this struct exists such that we don't have a `hash` function in the | ||
| * PublicCallStackItem struct that practically throws away some values of the struct | ||
| * without clearly indicating that it does so. | ||
| */ | ||
| struct PublicCallStackItemCompressed { | ||
|
There was a problem hiding this comment. Could you please add here a comment describing wtf is this and why we need it? |
||
| contract_address: AztecAddress, | ||
| call_context: CallContext, | ||
| function_data: FunctionData, | ||
| args_hash: Field, | ||
| returns_hash: Field, | ||
| revert_code: u8, | ||
| start_gas_left: Gas, | ||
| end_gas_left: Gas, | ||
| } | ||
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|
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| impl Eq for PublicCallStackItemCompressed { | ||
| fn eq(self, other: PublicCallStackItemCompressed) -> bool { | ||
| (self.contract_address == other.contract_address) | ||
| & (self.call_context == other.call_context) | ||
| & (self.function_data == other.function_data) | ||
| & (self.args_hash == other.args_hash) | ||
| & (self.returns_hash == other.returns_hash) | ||
| & (self.revert_code == other.revert_code) | ||
| & (self.start_gas_left == other.start_gas_left) | ||
| & (self.end_gas_left == other.end_gas_left) | ||
| } | ||
| } | ||
|
|
||
| impl Hash for PublicCallStackItemCompressed { | ||
| fn hash(self) -> Field { | ||
| std::hash::pedersen_hash_with_separator(self.serialize(), GENERATOR_INDEX__CALL_STACK_ITEM) | ||
| } | ||
| } | ||
|
|
||
| impl Empty for PublicCallStackItemCompressed { | ||
| fn empty() -> Self { | ||
| PublicCallStackItemCompressed { | ||
| contract_address: AztecAddress::empty(), | ||
| call_context: CallContext::empty(), | ||
| function_data: FunctionData::empty(), | ||
| args_hash: 0, | ||
| returns_hash: 0, | ||
| revert_code: 0, | ||
| start_gas_left: Gas::empty(), | ||
| end_gas_left: Gas::empty(), | ||
| } | ||
| } | ||
| } | ||
|
|
||
| impl Serialize<PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH> for PublicCallStackItemCompressed { | ||
| fn serialize(self) -> [Field; PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH] { | ||
| let mut fields: BoundedVec<Field, PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH> = BoundedVec::new(); | ||
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| fields.push(self.contract_address.to_field()); | ||
| fields.extend_from_array(self.call_context.serialize()); | ||
| fields.extend_from_array(self.function_data.serialize()); | ||
| fields.push(self.args_hash); | ||
| fields.push(self.returns_hash); | ||
| fields.push(self.revert_code as Field); | ||
| fields.extend_from_array(self.start_gas_left.serialize()); | ||
| fields.extend_from_array(self.end_gas_left.serialize()); | ||
|
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| assert_eq(fields.len(), PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH); | ||
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| fields.storage | ||
| } | ||
| } | ||
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| impl Deserialize<PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH> for PublicCallStackItemCompressed { | ||
| fn deserialize(fields: [Field; PUBLIC_CALL_STACK_ITEM_COMPRESSED_LENGTH]) -> PublicCallStackItemCompressed { | ||
| let mut reader = Reader::new(fields); | ||
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| let item = PublicCallStackItemCompressed { | ||
| contract_address: reader.read_struct(AztecAddress::deserialize), | ||
| call_context: reader.read_struct(CallContext::deserialize), | ||
| function_data: reader.read_struct(FunctionData::deserialize), | ||
| args_hash: reader.read(), | ||
| returns_hash: reader.read(), | ||
| revert_code: reader.read() as u8, | ||
| start_gas_left: reader.read_struct(Gas::deserialize), | ||
| end_gas_left: reader.read_struct(Gas::deserialize), | ||
| }; | ||
| reader.finish(); | ||
| item | ||
| } | ||
| } | ||
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If my understanding is correct, I think this is exploitable. Imagine a public => public call. The AVM, when fullfilling the CALL opcode in the parent, asks the prover to provide a call stack item for the result of the call.
The prover, then spawns a child AVM, runs the function, and provides a call stack item.
Now the parent AVM exposes in its public inputs the hash of the nested call stack item, extracts the return values from the nested call stack item and writes the return values in the memory of the parent public function.
A malicious prover can, if the kernel does not check the return values when comparing the parent_function_avm_public_inputs.call_requests.hash vs the child_function_avm_public_inputs, provide to the parent AVM fake return values that will never be checked by the kernel to be equal to the ones actually returned by the child function. The parent function will think the child function returned something that wasn't returned
I think, the check
request_hash <=> actual hashshould check:In the case of private => public, since no data is exposed back to the caller, then only the intent needs to be checked, that is achieved in master, by this check:
In the case of public => public, it's what I expressed above, both the intent and any data from the result that changes behavior of the caller
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Ooops. It's me that told Lasse that we didn't have to include the return_args hash. Because I think the public call request will be for the private functions only. It’s now being used between public calls because AVM can’t simulate the entire call request at once yet. But you are right it's an exploit at the moment and I guess we can include it now and remove it later!
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It shouldn't be too complicated i think, It's just adding a few fields to the
PublicCallStackItemCompressedstruct and adding to it the logic ofis_execution_requestto it (maybe we could call itis_request_from_private?).There was a problem hiding this comment.
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Will throw them back in, should be fairly straight forward with the struct in place.
So additions would be: