Re: [PATCH v3] md/raid5: reject raid4/5 arrays with too few disks

[sashiko-bot@kernel.org]

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

Pre-existing issues:
- [High] Division by zero in `raid5_run()` when assembling an array with an active reshape and insufficient data disks.
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commit e2bb0701d8414cbf17eed6287965f8022ffe5eab
Author: Chen Cheng <chencheng@fnnas.com>

md/raid5: reject raid4/5 arrays with too few disks

raid4 and raid5 require at least two disks, while raid6 requires at least
four disks. setup_conf() only rejects raid6 arrays with fewer than four
disks. As a result, an invalid raid4 or raid5 array can be assembled with
no data disks and later reach raid5_set_limits().

Reported-by: syzbot+de94ddbfff0c9e6fe030@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/6a778c82.01d0871a.3a0d52.006a.GAE@google.com/
Fixes: f63f17350e53 ("md/raid5: use the atomic queue limit update APIs")

> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index b91545ce090de..e1f20035703ba 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7657,9 +7657,11 @@ static struct r5conf *setup_conf(struct mddev *mddev)
>  			mdname(mddev), mddev->new_layout);
>  		return ERR_PTR(-EIO);
>  	}
> -	if (mddev->new_level == 6 && mddev->raid_disks < 4) {
> -		pr_warn("md/raid:%s: not enough configured devices (%d, minimum 4)\n",
> -			mdname(mddev), mddev->raid_disks);
> +	if ((mddev->new_level != 6 && mddev->raid_disks < 2) ||
> +	    (mddev->new_level == 6 && mddev->raid_disks < 4)) {
> +		pr_warn("md/raid:%s: not enough configured devices (%d, minimum %d)\n",
> +			mdname(mddev), mddev->raid_disks,
> +			mddev->new_level == 6 ? 4 : 2);

[Severity: High]
This isn't a bug introduced by this patch, but does this fully prevent
invalid array geometries from causing panics during array assembly?

In raid5_run(), if there is an active reshape and the array is assembled with
insufficient data disks, we calculate new_data_disks and divide by it before
setup_conf() is called to validate the array geometry:

drivers/md/raid5.c:raid5_run() {
    if (mddev->reshape_position != MaxSector) {
        ...
        new_data_disks = mddev->raid_disks - max_degraded;
        if (sector_div(here_new, chunk_sectors * new_data_disks)) {
            ...
        }
        ...
        sector_div(here_old, chunk_sectors * (old_disks-max_degraded));
    }
    ...
    if (mddev->private == NULL)
        conf = setup_conf(mddev);
}

If a crafted superblock sets raid_disks <= max_degraded and reshape_position
is not MaxSector, these divisions by zero are reached before the new checks
in setup_conf() can reject the array.

Could these checks be moved earlier in the assembly path to prevent the
divide-by-zero panics?

>  		return ERR_PTR(-EINVAL);
>  	}
>

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Sashiko AI review · https://sashiko.dev/#/patchset/20260911041521.2433222-1-chencheng@fnnas.com?part=1