Re: Block device direct read EIO handling broken?

Damien Le Moal
2019-08-06T07:05:06Z

On 2019/08/06 13:09, Jens Axboe wrote:
> On 8/5/19 5:05 PM, Damien Le Moal wrote:
>> On 2019/08/06 7:05, Damien Le Moal wrote:
>>> 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 ?
>>>
>>
>> That does not work since the reference to dio->size in
>> blkdev_bio_end_io() depends on atomic_dec_and_test(&dio->ref) which
>> counts the BIO fragments for the dio (+1 for async multi-bio case). So
>> completion of the last bio can still reference the old value of
>> dio->size.
>>
>> Adding a bio_get/put() on dio->bio ensures that dio stays around, but
>> does not prevent the use of the wrong dio->size. Adding an additional
>> atomic_inc/dec(&dio->ref) would prevent that, but we would need to
>> handle dio completion at the end of __blkdev_direct_IO() if all BIO
>> fragments already completed at that point. That is a lot more plumbing
>> needed, relying completely on dio->ref for all cases, thus removing
>> the dio->multi_bio management.
>>
>> Something like this:
>
> Don't like this, as it adds unnecessary atomics for the sync case.
> What's wrong with just adjusting dio->size if we get BLK_QC_T_EAGAIN?
> It's safe to do so, since we're doing the final put later. We just can't
> do it for the normal case of submit_bio() succeeding. Kill the new 'ret'
> usage and return to what we had as well, it's more readable too imho.
>
> Totally untested...
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index a6f7c892cb4a..131e2e0582a6 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -349,7 +349,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
> loff_t pos = iocb->ki_pos;
> blk_qc_t qc = BLK_QC_T_NONE;
> gfp_t gfp;
> - ssize_t ret;
> + int ret;
>
> if ((pos | iov_iter_alignment(iter)) &
> (bdev_logical_block_size(bdev) - 1))
> @@ -386,8 +386,6 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>
> ret = 0;
> for (;;) {
> - int err;
> -
> bio_set_dev(bio, bdev);
> bio->bi_iter.bi_sector = pos >> 9;
> bio->bi_write_hint = iocb->ki_hint;
> @@ -395,10 +393,8 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
> bio->bi_end_io = blkdev_bio_end_io;
> bio->bi_ioprio = iocb->ki_ioprio;
>
> - err = bio_iov_iter_get_pages(bio, iter);
> - if (unlikely(err)) {
> - if (!ret)
> - ret = err;
> + ret = bio_iov_iter_get_pages(bio, iter);
> + if (unlikely(ret)) {
> bio->bi_status = BLK_STS_IOERR;
> bio_endio(bio);
> break;
> @@ -421,7 +417,6 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
> if (nowait)
> bio->bi_opf |= (REQ_NOWAIT | REQ_NOWAIT_INLINE);
>
> - dio->size += bio->bi_iter.bi_size;
> pos += bio->bi_iter.bi_size;
>
> nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
> @@ -433,13 +428,13 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
> polled = true;
> }
>
> + dio->size += bio->bi_iter.bi_size;
> qc = submit_bio(bio);
> if (qc == BLK_QC_T_EAGAIN) {
> - if (!ret)
> - ret = -EAGAIN;
> + dio->size -= bio->bi_iter.bi_size;

ref after free of bio here. Easy to fix though. Also, with this, the bio_endio()
call within submit_bio() for the EAGAIN failure will see a dio->size too large,
including this failed bio. So this does not work.

One thing I do not 100% sure is the nowait case with a fragmented dio: if the
processing stops on a BLK_QC_T_EAGAIN, the code in blkdev_bio_end_io() will
complete the iocb with -EAGAIN, while this code (submission path) will return
the amount of submitted bytes for the dio (short read or write). So for nowait
&& is_sync case, this means that preadv2/pwritev2 would always get -EAGAIN
instead of the partial read/write achieved. For the nowait && !is_sync case,
similarly, aio_return() would always return -EAGAIN too even if a partial
read/write was done. Or am I missing something ? Checking again the man pages,
this does not look like the described behavior.

If this analysis is correct, I think the proper fix cannot be done only in
__blkdev_direct_IO(). blkdev_bio_end_io() needs to change too. Basically, I
think the "dio->size -= bio->bi_iter.bi_size" needs to go into
blkdev_bio_end_io() and a test added to see if dio->size is 0 (then complete
with -EAGAIN) or not (then partial completion).
Starting testing something now with these changes in blkdev_bio_end_io().