Re: Block device direct read EIO handling broken?

Damien Le Moal
2019-08-05T22:05:47Z

On 2019/08/06 6:59, Damien Le Moal wrote:
> On 2019/08/06 6:28, Jens Axboe wrote:
>> On 8/5/19 2:27 PM, Damien Le Moal wrote:
>>> On 2019/08/06 6:26, Jens Axboe wrote:
>>>>> In any case, looking again at this code, it looks like there is a
>>>>> problem with dio->size being incremented early, even for fragments
>>>>> that get BLK_QC_T_EAGAIN, because dio->size is being used in
>>>>> blkdev_bio_end_io(). So an incorrect size can be reported to user
>>>>> space in that case on completion (e.g. large asynchronous no-wait dio
>>>>> that cannot be issued in one go).
>>>>>
>>>>> So maybe something like this ? (completely untested)
>>>>
>>>> I think that looks pretty good, I like not double accounting with
>>>> this_size and dio->size, and we retain the old style ordering for the
>>>> ret value.
>>>
>>> Do you want a proper patch with real testing backup ? I can send that
>>> later today.
>>
>> Yeah that'd be great, I like your approach better.
>>
>
> Looking again, I think this is not it yet: dio->size is being referenced after
> submit_bio(), so blkdev_bio_end_io() may see the old value if the bio completes
> before dio->size increment. So the use-after-free is still there. And since
> blkdev_bio_end_io() processes completion to user space only when dio->ref
> becomes 0, adding an atomic_inc/dec(&dio->ref) over the loop would not help and
> does not cover the single BIO case. Any idea how to address this one ?
>

May be add a bio_get/put() over the 2 places that do submit_bio() would work,
for all cases (single/multi BIO, sync & async). E.g.:

+ bio_get(bio);
qc = submit_bio(bio);
if (qc == BLK_QC_T_EAGAIN) {
if (!dio->size)
ret = -EAGAIN;
+ bio_put(bio);
goto error;
}
dio->size += bio_size;
+ bio_put(bio);

Thoughts ?