Re: [PATCH v5 02/11] block: Block Device Filtering Mechanism

Sergei Shtepa
2023-07-17T17:40:17Z

Hi.

On 7/12/23 14:34, Yu Kuai wrote:
> Subject:
> Re: [PATCH v5 02/11] block: Block Device Filtering Mechanism
> From:
> Yu Kuai <yukuai1@huaweicloud.com>
> Date:
> 7/12/23, 14:34
>
> To:
> Yu Kuai <yukuai1@huaweicloud.com>, Sergei Shtepa <sergei.shtepa@veeam.com>, axboe@kernel.dk, hch@infradead.org, corbet@lwn.net, snitzer@kernel.org
> CC:
> viro@zeniv.linux.org.uk, brauner@kernel.org, dchinner@redhat.com, willy@infradead.org, dlemoal@kernel.org, linux@weissschuh.net, jack@suse.cz, ming.lei@redhat.com, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Donald Buczek <buczek@molgen.mpg.de>, "yukuai (C)" <yukuai3@huawei.com>
>
>
> Hi,
>
> 在 2023/07/12 18:04, Yu Kuai 写道:
>> Hi,
>>
>> 在 2023/07/11 10:02, Yu Kuai 写道:
>>
>>>> +static bool submit_bio_filter(struct bio *bio)
>>>> +{
>>>> +    if (bio_flagged(bio, BIO_FILTERED))
>>>> +        return false;
>>>> +
>>>> +    bio_set_flag(bio, BIO_FILTERED);
>>>> +    return bio->bi_bdev->bd_filter->ops->submit_bio(bio);
>>>> +}
>>>> +
>>>>   static void __submit_bio(struct bio *bio)
>>>>   {
>>>> +    /*
>>>> +     * If there is a filter driver attached, check if the BIO needs to go to
>>>> +     * the filter driver first, which can then pass on the bio or consume it.
>>>> +     */
>>>> +    if (bio->bi_bdev->bd_filter && submit_bio_filter(bio))
>>>> +        return;
>>>> +
>>>>       if (unlikely(!blk_crypto_bio_prep(&bio)))
>>>>           return;
>>
>> ...
>>
>>>> +static void __blkfilter_detach(struct block_device *bdev)
>>>> +{
>>>> +    struct blkfilter *flt = bdev->bd_filter;
>>>> +    const struct blkfilter_operations *ops = flt->ops;
>>>> +
>>>> +    bdev->bd_filter = NULL;
>>>> +    ops->detach(flt);
>>>> +    module_put(ops->owner);
>>>> +}
>>>> +
>>>> +void blkfilter_detach(struct block_device *bdev)
>>>> +{
>>>> +    if (bdev->bd_filter) {
>>>> +        blk_mq_freeze_queue(bdev->bd_queue);
>>
>> And this is not sate as well, for bio-based device, q_usage_counter is
>> not grabbed while submit_bio_filter() is called, hence there is a risk
>> of uaf from submit_bio_filter().
>
> And there is another question, can blkfilter_detach() from
> del_gendisk/delete_partiton and ioctl concurrent? I think it's a
> problem.
>

Yes, it looks like if two threads execute the blkfilter_detach() function,
then a problem is possible. The blk_mq_freeze_queue() function does not
block threads.
But for this, it is necessary that the IOCTL for the block device and
its removal are performed simultaneously. Is this possible?

I suppose that using mutex bdev->bd_disk->open_mutex in
blkfilter_ioctl_attach(), blkfilter_ioctl_detach() and
blkfilter_ioctl_ctl() can fix the problem. What do you think?

> Thanks,
> Kuai
>>
>> Thanks,
>> Kuai
>>
>> .
>>
>