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

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Date:	Fri, 25 May 2012 17:50:03 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	jbeulich@...e.com, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, axboe@...nel.dk
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: [PATCH 2/2] xen/blkfront: Add BUG_ON to deal with misbehaving backends.

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@...cle.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

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