Re: [PATCH] block: not create too many partitions

[Christoph Hellwig <hch@lst.de>]

On Fri, Mar 26, 2021 at 04:59:54PM +0800, Ming Lei wrote:
> Commit a33df75c6328 ("block: use an xarray for disk->part_tbl") drops
> check on max supported partitions number, and allows partition with
> bigger partition number to be added. However, ->bd_partno is defined
> as u8, so partition index of xarray table may not match with ->bd_partno.
> Then delete_partition() may delete one unmatched partition, and caused
> use-after-free.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Reported-by: syzbot+8fede7e30c7cee0de139@syzkaller.appspotmail.com
> Fixes: a33df75c6328 ("block: use an xarray for disk->part_tbl")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> Another fix is to define ->bd_partno as u32, not sure if we need to
> support so many partitions.
> 
>  block/partitions/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/block/partitions/core.c b/block/partitions/core.c
> index 1a7558917c47..933d47105b64 100644
> --- a/block/partitions/core.c
> +++ b/block/partitions/core.c
> @@ -322,6 +322,10 @@ static struct block_device *add_partition(struct gendisk *disk, int partno,
>  	const char *dname;
>  	int err;
>  
> +	/* disk_max_parts() is zero during initialization, ignore if so */
> +	if (disk_max_parts(disk) && (partno + 1) > disk_max_parts(disk))
> +		return ERR_PTR(-EINVAL);

disk->minors is set in __alloc_disk_node, so AFAICS it can't ever be 0
when add_partition is called.  So I think this should be just:

	if (partno >= disk_max_parts(disk))
		return ERR_PTR(-EINVAL);

otherwise this looks good.