fix: use checked arithmetic in bounds_check! to prevent overflow bypass

[ludfjig]

The bounds check silently wraps on usize overflow in release builds, bypassing the check entirely. Not exploitable today since callers validate offsets before reaching the macro, but any future caller that doesn't could trigger it.

[jsturtevant]

Making the use of our helper functions easier to do the right thing properly is 💯

[Copilot]

Pull request overview

This PR hardens Hyperlight host shared-memory access by making the bounds_check! macro overflow-safe, preventing usize addition wraparound from bypassing bounds checks in release builds.

Changes:

• Update bounds_check! to use checked_add so offset + size overflow is treated as out-of-bounds.
• Add a regression test that exercises overflow offsets/sizes across several ExclusiveSharedMemory and HostSharedMemory APIs.

Comments suppressed due to low confidence (2)

src/hyperlight_host/src/mem/shared_mem.rs:57

• bounds_check! is used by both read and write/copy/fill paths in this file, but the error message hard-codes "Cannot read value...". This can be misleading when the failing operation is a write or fill; consider making the message operation-agnostic (e.g., "Cannot access memory...") or allowing the call site to supply an operation label.

        if $offset.checked_add($size).is_none_or(|end| end > $mem_size) {
            return Err(new_error!(
                "Cannot read value from offset {} with size {} in memory of size {}",
                $offset,
                $size,
                $mem_size
            ));

src/hyperlight_host/src/mem/shared_mem.rs:56

• bounds_check! references $offset, $size, and $mem_size multiple times (in the check and again in the formatted error). If a caller ever passes expressions with side effects, they could be evaluated more than once. Consider binding the macro inputs to local let variables inside the macro expansion so each argument is evaluated exactly once.

    ($offset:expr, $size:expr, $mem_size:expr) => {
        if $offset.checked_add($size).is_none_or(|end| end > $mem_size) {
            return Err(new_error!(
                "Cannot read value from offset {} with size {} in memory of size {}",
                $offset,
                $size,
                $mem_size