[PATCH] brd: Ensure that bio_vecs have size <= PAGE_SIZE

[Ross Zwisler]

The functions copy_from_brd() and copy_to_brd() are written with an
assumption that the bio_vec they are given has size <= PAGE_SIZE.  This
assumption is not enforced in any way, and if the bio_vec has size
larger than PAGE_SIZE data will just be lost.

Such a situation can occur with I/Os generated from in-kernel sources,
or with coalesced bio_vecs.  This bug was originally reported against
the pmem driver, where it was found using the Enmotus tiering engine.

Instead we should have brd explicitly tell the block layer that it can
handle data segments of at most PAGE_SIZE.

Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Reported-by: Hugh Daschbach <hugh.daschbach@xxxxxxxxxxx>
Cc: Roger C. Pao (Enmotus) <rcpao.enmotus@xxxxxxxxx>
Cc: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
Cc: linux-nvdimm@xxxxxxxxxxxx
Cc: Nick Piggin <npiggin@xxxxxxxxx>
---
 drivers/block/brd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index 898b4f256782..7e4873361b64 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -490,6 +490,7 @@ static struct brd_device *brd_alloc(int i)
 	blk_queue_make_request(brd->brd_queue, brd_make_request);
 	blk_queue_max_hw_sectors(brd->brd_queue, 1024);
 	blk_queue_bounce_limit(brd->brd_queue, BLK_BOUNCE_ANY);
+	blk_queue_max_segment_size(brd->brd_queue, PAGE_SIZE);
 
 	brd->brd_queue->limits.discard_granularity = PAGE_SIZE;
 	brd->brd_queue->limits.max_discard_sectors = UINT_MAX;
-- 
1.9.3

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