jxl-grid on 32-bit platforms has an out-of-bounds writes due to integer overflow

Summary

On 32-bit platforms, decoding a crafted image may lead to out-of-bounds writes due to integer overflow in length calculation.

Details & PoC

The test listed below fail under miri with command cargo +nightly miri test --release -p jxl-grid

Or you can use Address Sanitizer, which ignores Rust-specific UB like aliasing but still flags out-of-bounds accesses:

RUSTFLAGS=-Zsanitizer=address cargo +nightly test -Zbuild-std -p jxl-grid --release --target x86_64-unknown-linux-gnu

The following tests should be appended to crates/jxl-grid/src/test/subgrids.rs:

 1mod miri_ub {
 2    use super::*;
 3 
 4    // `AlignedGrid::with_alloc_tracker` computes `width * height` unchecked. In release, overflow
 5    // can create a tiny backing buffer for huge logical dimensions.
 6    #[test]
 7    fn aligned_grid_dimension_product_overflows() {
 8        let width = usize::MAX / 2 + 1;
 9        let mut grid = AlignedGrid::<u8>::with_alloc_tracker(width, 2, None).unwrap();
10        let mut subgrid = grid.as_subgrid_mut();
11        *subgrid.get_mut(0, 1) = 1;
12        std::hint::black_box(grid);
13    }
14}

This issue can be reachable through decoding a crafted image in two ways:

1. Huge actual frame
   A frame such as 65536 x 65536 passes the current frame area limit (2^32 <= 2^40) but overflows usize element count on 32-bit. Rendering then allocates too-small AlignedGrids in modular/VarDCT/filter paths and later writes through mutable subgrids.

2. Huge canvas plus tiny cropped frame
   This is the more practical “small payload, huge logical output” case. A bitstream-controlled frame crop can be tiny, but if the canvas/default requested region is huge, composition can allocate an output grid sized to the canvas/ROI at crates/jxl-render/src/blend.rs. That is bitstream frame cropping, not API crop. With a 32-bit target and a full requested image region whose area overflows, this can happen through ordinary render_frame().

Impact

On 32-bit platforms this can cause out-of-bounds writes with attacker-controlled data when decoding a crafted JPEG XL image. This could allow arbitrary code execution.