[PATCH 2/2] xen/blkfront: Add BUG_ON to deal with misbehaving backends.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

Part of the ring structure is the 'id' field which is under
control of the frontend. The frontend stamps it with "some"
value (this some in this implementation being a value less
than BLK_RING_SIZE), and when it gets a response expects
said value to be in the response structure. We have a check
for the id field when spolling new requests but not when
de-spolling responses.

We also add an extra check in add_id_to_freelist to make
sure that the 'struct request' was not NULL - as we cannot
pass a NULL to __blk_end_request_all, otherwise that crashes
(and all the operations that the response is dealing with
end up with __blk_end_request_all).

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 drivers/block/xen-blkfront.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 60eed4b..8e177ca 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -145,6 +145,7 @@ static void add_id_to_freelist(struct blkfront_info *info,
 			       unsigned long id)
 {
 	info->shadow[id].req.u.rw.id  = info->shadow_free;
+	BUG_ON(info->shadow[id].request == NULL);
 	info->shadow[id].request = NULL;
 	info->shadow_free = id;
 }
@@ -746,6 +747,12 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 
 		bret = RING_GET_RESPONSE(&info->ring, i);
 		id   = bret->id;
+		/*
+		 * The backend has messed up and given us an id that we would
+		 * never have given to it (we stamp it up to BLK_RING_SIZE -
+		 * look in get_id_from_freelist.
+		 */
+		BUG_ON(id >= BLK_RING_SIZE);
 		req  = info->shadow[id].request;
 
 		if (bret->operation != BLKIF_OP_DISCARD)
-- 
1.7.7.6