[dm-devel] dm-crypt: Fix error with too large bios

[Mikulas Patocka]

On Tue, 30 Aug 2016, Ming Lei wrote:

> On Tue, Aug 30, 2016 at 5:57 AM, Mikulas Patocka <mpatocka at redhat.com> wrote:
> >
> >
> > On Mon, 29 Aug 2016, Ming Lei wrote:
> >
> >> On Sat, Aug 27, 2016 at 11:09 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
> >> >
> >> > But this patch won't work for device mapper, blk_bio_segment_split is
> >> > called from blk_queue_split and device mapper doesn't use blk_queue_split
> >> > (it can't because it frees queue->bio_split).
> >> >
> >> > We still need these two patches:
> >> > https://www.redhat.com/archives/dm-devel/2016-May/msg00211.html
> >> > https://www.redhat.com/archives/dm-devel/2016-May/msg00210.html
> >>
> >> About the 2nd patch, it might not be good enough because in theory
> >> a small size bio still may include big bvecs, such as, each bvec points
> >> to 512byte buffer, so strictly speaking the bvec number should
> >> be checked instead of bio size.
> >>
> >> Ming Lei
> >
> > This is not a problem.
> 
> I meant the following code in your 2nd patch:
> 
> + if (unlikely(bio->bi_iter.bi_size > BIO_MAX_SIZE) &&
> +    (bio->bi_rw & (REQ_FLUSH | REQ_DISCARD | REQ_WRITE)) == REQ_WRITE)
> + dm_accept_partial_bio(bio, BIO_MAX_SIZE >> SECTOR_SHIFT);
> 
> And the check on .bi_size may not work.

kcryptd_crypt_write_convert calls:
crypt_alloc_buffer(io, io->base_bio->bi_iter.bi_size)

crypt_alloc_buffer does:
unsigned int nr_iovecs = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
clone = bio_alloc_bioset(GFP_NOIO, nr_iovecs, cc->bs);

So, if io->base_bio->bi_iter.bi_size <= BIO_MAX_SIZE, then nr_iovecs will 
be less or equal than BIO_MAX_PAGES and the function bio_alloc_bioset will 
succeed.

(BTW. BIO_MAX_SIZE was removed in the current kernel, we should use 
(BIO_MAX_PAGES << PAGE_SHIFT) instead).

Mikulas