Re: [PATCH v4 04/25] md/raid5: reject zero-sector reshape chunks

Thank you for your contribution! Sashiko AI review found 1 potential issue(=
s) to consider:

Pre-existing issues:
- [High] Writing large unsigned values to the `chunk_size` or `layout` sysf=
s attributes causes integer truncation to negative values, completely bypas=
sing validation and leading to arbitrary geometry corruption or kernel BUGs.
--

--- Patch [4]: [PATCH v4 04/25] md/raid5: reject zero-sector reshape chunks=
 ---
commit 6759d75e360239d90630b8d047daa1d3cb528597
Author: Yu Kuai <[email protected]>

md/raid5: reject zero-sector reshape chunks

Sashiko reported that RAID5 can accept a reshape chunk size that becomes
zero sectors. chunk_size_store() stores the sysfs byte value as n >> 9, so
writing a value below 512 bytes sets mddev->new_chunk_sectors to zero.

> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 0c5c9fb0606ee..4276b125f94cb 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -8479,6 +8479,8 @@ static int check_reshape(struct mddev *mddev)
>  		return 0; /* nothing to do */
>  	if (has_failed(conf))
>  		return -EINVAL;
> +	if (!mddev->new_chunk_sectors)
> +		return -EINVAL;

[Severity: High]
This is a pre-existing issue, but does this check adequately protect against
negative values?

In drivers/md/md.c:chunk_size_store(), user-supplied unsigned values are
shifted and truncated into the signed int field new_chunk_sectors without
overflow checks:

    mddev->new_chunk_sectors =3D n >> 9;

If a large value is written, new_chunk_sectors can become negative. This
would bypass the !mddev->new_chunk_sectors check introduced here.

Additionally, in raid5_check_reshape(), validation is skipped for negative
values:

    if (new_chunk > 0) {
        ...
    }

Would it be better for this check to reject all values <=3D 0, to prevent
negative chunk sizes from corrupting I/O geometry?

A similar truncation issue exists for new_layout in
drivers/md/md.c:layout_store(). A negative layout value bypasses validation
in raid5_check_reshape() and eventually triggers a BUG() in
raid5_compute_sector() if it hits the default case.

--=20
Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20260801172519.2982=
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