pallet-xqvm execute: unbounded sample alloc + step_limit 0 is unlimited (validator OOM / hang)

Title: pallet-xqvm execute: unbounded sample alloc + step_limit 0 is unlimited (validator OOM / hang)

Summary

pallet-xqvm::execute is a signed, permissionless extrinsic. Weight is pre-charged from the caller-supplied step_limit, then the interpreter runs.

Two independent metering holes:

1. Memory is not metered. BSMX / SSMX / XSMX take a stack i64, convert to usize, and allocate a dense Vec<i64> of that length in one instruction (xquad/xqvm/src/vm.rs). WeightPerStep is 1000 ref_time. A 3-instruction program (PUSH huge; BSMX; HALT) pays for a handful of steps and can request gigabytes.

2. Zero step limit is treated as infinity. The pallet only checks step_limit <= MaxStepLimit. 0 passes. Vm::set_step_limit maps 0 to u64::MAX. Weight charged is execute_base + WeightPerStep * 0. The node then runs until halt, OOM, or process kill. Substrate will not stop the extrinsic mid-loop because announced weight was tiny.

The crate has xqvm::verifier::verify. The pallet only Program::decodes (magic/CRC). It never calls the verifier.

Impact

Any account can stall or OOM every authoring/importing validator. Liveness of any runtime that includes this pallet.

Toy: PUSH 268435456; BSMX; HALT. Three steps. ~2GB per validator. Or step_limit=0: fee for zero steps, loop forever if the program does not halt.

Suggested fix

• Reject step_limit == 0 on the consensus path; never map 0 to u64::MAX there.
• Cap every dense allocation (BSMX/SSMX/XSMX, vec growth, ITER copies); charge proof-size/memory weight.
• Run verifier::verify before Vm::run.
• Prefer DispatchResultWithPostInfo and fail closed if an opcode would allocate above remaining weight.

Files

• xquad/xqvm/src/vm.rs (BSMX/SSMX/XSMX alloc; set_step_limit(0) -> u64::MAX)
• quip-validator/pallets/xqvm/src/lib.rs (execute, weight = WeightPerStep * step_limit)
• quip-validator/runtime/src/configs/mod.rs (WeightPerStep)

Start with the cited allocation and step-limit paths in xquad/xqvm/src/vm.rs, then trace execute and its weight calculation in quip-validator/pallets/xqvm/src/lib.rs and WeightPerStep in quip-validator/runtime/src/configs/mod.rs. Reproduce the two toy cases described in the issue and inspect verifier::verify. Done means consensus execution rejects zero limits, bounds memory-consuming operations, and prevents the reported OOM or nonterminating behavior.