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SubjectRe: [Xen-devel] [PATCH 2/2] xen/blkfront: Add BUG_ON to deal with misbehaving backends.
On Fri, 25 May 2012, Konrad Rzeszutek Wilk wrote:
> 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)

While we should certainly check whether bret->id is valid before
using it, is it actually correct that the frontend BUGs in response of a
backend bug?

If the IO doesn't involve the root disk, the guest might be able to
function correctly without communicating with the backend at all.
I think we should WARN and return error. Maybe also call blkfront_remove
if we can.


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