bit-shift-safety

BIT_SHIFT_SAFETY Skill

Trigger Pattern: Always (Sui Move) -- Move VM aborts on shift >= bit width Inject Into: Breadth agents, depth-edge-case

For every bit shift operation in the protocol:

BACKGROUND: The Move VM (shared by Sui and Aptos) aborts the entire transaction if a bit shift operand equals or exceeds the bit width of the type. For u64, shifting by 64 or more aborts. For u128, shifting by 128 or more aborts. This is NOT a revert with an error code -- it is a VM abort that cannot be caught. On Sui, a PTB (Programmable Transaction Block) abort means ALL commands in the PTB fail, and shared objects that were locked for the transaction are released without state changes.

1. Shift Operation Inventory

Enumerate ALL bit shift operations (<< and >>) across all modules:

Grep pattern: Search all .move files for << and >> operators.

Type-to-width mapping:

2. Shift Amount Source Classification

For each shift operation, classify the shift amount source:

2a. Literal Shifts (Low Risk)

let x = value << 32;  // Literal: always safe if < bit width

Check: Is the literal < bit width of the type? If yes -> SAFE. If no -> compilation may succeed but runtime aborts.

2b. Parameter-Derived Shifts (Medium Risk)

public fun shift_by(value: u64, amount: u8): u64 {
    value << (amount as u8)  // Parameter: caller controls shift amount
}

Check: Is the amount parameter validated before the shift? Common patterns:

• assert!(amount < 64, E_SHIFT_OVERFLOW) -- explicit validation
• amount % 64 -- wrapping (changes semantics but prevents abort)
• No validation -- FINDING

2c. Computed Shifts (High Risk)

let shift = calculate_precision(decimals);  // Computed: depends on runtime state
let result = base << shift;

Check: Can the computation ever produce a value >= bit width? Trace the computation to its inputs. If any input is user-controlled or state-derived -> HIGH RISK.

3. Abort Impact Analysis

For each unvalidated shift operation, assess the abort impact:

Sui-specific abort consequences:

• PTB abort: All commands in the Programmable Transaction Block fail atomically. If the shift is in command 3 of a 5-command PTB, commands 1-2 are also rolled back.
• Shared object locking: If the aborting function locks shared objects (accessed via &mut reference), those objects are temporarily unavailable during consensus. Repeated aborts can cause transient unavailability. This is NOT permanent locking -- Sui releases locks after the transaction fails.
• Gas consumption: The sender pays gas for the aborted transaction up to the abort point.
• Griefing vector: If an attacker can trigger the abort via a public function with a user-controlled shift amount, they can grief other users by causing their PTBs to abort. This is especially impactful when the aborting function is called as part of a common user flow (deposit, swap, claim).

3a. Griefing Scenario Modeling

For each unvalidated shift in a public/entry function:

Scenario: Shift Abort Griefing
1. Attacker calls {FUNCTION} with shift amount = {BIT_WIDTH}
2. Move VM aborts the transaction
3. Impact on other users: {IMPACT}
   - If function modifies shared state: other PTBs depending on that state must retry
   - If function is part of a multi-step user flow: user loses gas + must restart
4. Attacker cost: gas for one failed transaction
5. Severity: {based on impact}

4. Common Vulnerable Patterns

4a. Decimal Conversion Shifts

// VULNERABLE: decimals comes from token metadata, could be >= 64
let scale = 1u64 << decimals;

Fix pattern: assert!(decimals < 64, E_INVALID_DECIMALS) or use math::pow(10, decimals) instead.

4b. Bit Packing / Unpacking

// VULNERABLE if position is not bounds-checked
let field = (packed >> position) & mask;

Check: Is position derived from user input or configuration? If yes and no validation -> FINDING.

4c. Fixed-Point Arithmetic

// Common in DeFi: fixed-point multiplication with shift
let result = (a * b) >> PRECISION_BITS;

Check: Is PRECISION_BITS a constant? If yes and < bit width -> SAFE. If computed -> trace source.

4d. Loop-Based Shifts

let mut i = 0;
while (i < n) {
    value = value << 1;  // Safe per iteration, but after 64 iterations value = 0 (not abort)
    i = i + 1;
};

Note: Shifting by 1 repeatedly does NOT abort (shift amount is always 1). But the value overflows silently to 0 after bit-width iterations. Check if this silent overflow causes logic errors.

5. Validation Pattern Verification

For each shift operation that IS validated, verify the validation is correct:

Common validation errors:

• Off-by-one: <= bit_width instead of < bit_width
• Wrong bit width: validating against 64 for a u128 shift (allows 64-127 to abort)
• Missing cast: amount is u64 but shift operand must be u8 -- does the cast truncate?

6. Cross-Function Shift Propagation

Trace shift amounts across function boundaries:

entry_function(user_input: u64)
  -> helper_a(derived_value)  // derived_value = user_input * 2
    -> helper_b(shift_amount) // shift_amount = derived_value + offset
      -> actual_shift: value << shift_amount  // Is shift_amount < bit_width?

For each chain: Can ANY combination of valid inputs to the entry function produce a shift amount >= bit width at the actual shift site? Document the full trace.

Finding Template

**ID**: [BS-N]
**Severity**: [HIGH if public function, MEDIUM if restricted caller, LOW if constant shift]
**Step Execution**: check1,2,3,4,5,6 | X(reasons) | ?(uncertain)
**Rules Applied**: [R4:Y, R10:Y, ...]
**Depth Evidence**: [BOUNDARY:shift=bit_width], [TRACE:user_input->shift_amount->abort]
**Location**: module::function:LineN
**Title**: Unvalidated bit shift in [function] causes VM abort on [condition]
**Description**: [Specific shift operation, source of shift amount, why it can reach bit width]
**Impact**: [Transaction abort, shared object locking, griefing potential, gas waste]

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Step Execution Checklist (MANDATORY)

CRITICAL: You MUST report completion status for ALL sections. Findings with incomplete sections will be flagged for depth review.

Cross-Reference Markers

After Section 1 (Shift Inventory):

• IF zero shift operations found -> mark skill as N/A, skip remaining sections
• IF shifts found in math/fixed-point libraries -> prioritize Section 4c

After Section 3 (Abort Impact):

• IF abort affects shared objects -> cross-reference with ABILITY_ANALYSIS Section 2b (shared object analysis)
• IF abort is in a hot potato consumption path -> cross-reference with ABILITY_ANALYSIS Section 5 (hot potato enforcement) -- abort before consumption = permanent PTB failure

After Section 6 (Cross-Function Propagation):

• IF any chain reaches bit width with valid inputs -> FINDING (minimum Medium)
• Tag: [BOUNDARY:shift={bit_width}], [TRACE:input_path->abort_site]