Attackathon _ Fuel Network 32438 - [Smart Contract - Low] Unhandled Bailout During AbstractInstructi

Submitted on Fri Jun 21 2024 18:21:27 GMT-0400 (Atlantic Standard Time) by @anatomist for Attackathon | Fuel Network

Report ID: #32438

Report type: Smart Contract

Report severity: Low

Target: https://github.com/FuelLabs/sway/tree/7b56ec734d4a4fda550313d448f7f20dba818b59

Impacts:

• Incorrect sway optimization leading to incorrect bytecode

Description

Brief/Intro

const_indexing_aggregates_function() process VirtualOp::LW without handling the case if the offset in addr_reg is not aligned to 8, resulting in the old constant being used and leading to incorrect program behavior.

Vulnerability Details

While processing a VirtualOp::LW with non-aligned address offset, const_indexing_aggregates_function() does not clear register information in reg_contents and setup a new definition with record_new_def(). This means the old state of the register stored in reg_contents will still be used, causing incorrect value tracking after the LW instruction. The incorrect tracking may then cause incorrect immediate replacement in other instructions.

VirtualOp::LW(dest, addr_reg, imm) => match reg_contents.get(addr_reg) {
    Some(RegContents::BaseOffset(base_reg, offset))
        if get_def_version(&latest_version, &base_reg.reg) == base_reg.ver
            && ((offset / 8) + imm.value as u64)
                < compiler_constants::TWELVE_BITS =>
    {
        // bail if LW cannot read where this memory is
        if offset % 8 == 0 {
            let new_imm = VirtualImmediate12::new_unchecked(
                (offset / 8) + imm.value as u64,
                "Immediate offset too big for LW",
            );
            let new_lw =
                VirtualOp::LW(dest.clone(), base_reg.reg.clone(), new_imm);
            // The register defined is no more useful for us. Forget anything from its past.
            reg_contents.remove(dest);
            record_new_def(&mut latest_version, dest);
            // Replace the LW with a new one in-place.
            *op = new_lw;
        }
    }
    _ => {
        reg_contents.remove(dest);
        record_new_def(&mut latest_version, dest);
    }
},

Impact Details

As usual, it is hard to come up with a precise impact estimation of incorrect code generation because it depends on what code the user writes. The best case scenario would be contracts that run into those bugs getting bricked, and the worst case scenario would be that incorrect program behaviors lead to loss of funds.

References

• https://github.com/FuelLabs/sway/blob/7b56ec734d4a4fda550313d448f7f20dba818b59/sway-core/src/asm_generation/fuel/optimizations.rs#L155

Proof of concept

Proof of Concept

This test would fail because addi a a i15 incorrectly uses the reg values before lw a a i0

#[test]
fn incorrect_bailout() -> u64 {
    let a = asm(a, b) {
        movi a i32;		// a = 32
        aloc a;			// hp = buf[0;32]

        movi a i1;		// a = 1
        addi b hp i23;	// b = &buf[23]
        sb b a i0;		// buf[23] = a = 1
        addi a hp i9;	// a = &buf[9]
        addi b hp i1;	// b = &buf[1]
        sb b a i7;		// buf[1:9] = a = &buf[9] avoid using sw, which is buggy itself
        srli a a i8;
        sb b a i6;
        srli a a i8;
        sb b a i5;
        srli a a i8;
        sb b a i4;
        srli a a i8;
        sb b a i3;
        srli a a i8;
        sb b a i2;
        srli a a i8;
        sb b a i1;
        srli a a i8;
        sb b a i0;

        addi a hp i1;	// a = &buf[1]
        lw a a i0;		// a = buf[1:9] = &buf[9]
        addi a a i15;	// expected : a = &buf[24]              real : a = &buf[16]
        lw a a i0;		// expected : a = buf[24:32] = 0        real : a = bug[16:24] = 1
        a: u64
    };
    assert(a == 0);
    a
}