Re: Block device direct read EIO handling broken?

Dave Chinner
2019-08-06T00:25:06Z

On Tue, Aug 06, 2019 at 12:05:51AM +0000, 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.

Didn't we fix this same use-after-free in iomap_dio_rw() in commit
4ea899ead278 ("iomap: fix a use after free in iomap_dio_rw")?

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com