Re: dm-crypt: Fix error with too large bios

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Aug 30 2016 at  8:19P -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Tue, 30 Aug 2016, Ming Lei wrote:
> 
> > On Tue, Aug 30, 2016 at 5:57 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
> > >
> > >
> > > On Mon, 29 Aug 2016, Ming Lei wrote:
> > >
> > >> On Sat, Aug 27, 2016 at 11:09 PM, Mikulas Patocka <mpatocka@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).

Is this revised patch OK with you?

From: Mikulas Patocka <mpatocka@redhat.com>
Date: Tue, 30 Aug 2016 16:38:42 -0400
Subject: [PATCH] dm crypt: fix error with too large bcache bios

When dm-crypt processes writes, it allocates a new bio in
crypt_alloc_buffer().  The bio is allocated from a bio set and it can
have at most BIO_MAX_PAGES vector entries, however the incoming bio can be
larger if it was allocated by bcache.  If the incoming bio is larger,
bio_alloc_bioset() fails and an error is returned.

To avoid the error, we test for a too large bio in the function
crypt_map() and use dm_accept_partial_bio() to split the bio.
dm_accept_partial_bio() trims the current bio to the desired size and
asks DM core to send another bio with the rest of the data.

This fix is wrapped with a check for CONFIG_BCACHE because there
currently isn't any other code that generates too large bios.  So unless
bcache is configured there is no point wasting time making this check.

Signed-off-by: Mikulas Patocka <mpatocka redhat com>
Cc: stable@vger.kernel.org # v3.16+
---
 drivers/md/dm-crypt.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index eedba67..743f548 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -1924,6 +1924,12 @@ static int crypt_map(struct dm_target *ti, struct bio *bio)
 		return DM_MAPIO_REMAPPED;
 	}
 
+#ifdef CONFIG_BCACHE
+	if (unlikely(bio->bi_iter.bi_size > (BIO_MAX_PAGES << PAGE_SHIFT)) &&
+	    bio_data_dir(bio) == WRITE)
+		dm_accept_partial_bio(bio, ((BIO_MAX_PAGES << PAGE_SHIFT) >> SECTOR_SHIFT));
+#endif
+
 	io = dm_per_bio_data(bio, cc->per_bio_data_size);
 	crypt_io_init(io, cc, bio, dm_target_offset(ti, bio->bi_iter.bi_sector));
 	io->ctx.req = (struct skcipher_request *)(io + 1);
-- 
2.7.4 (Apple Git-66)