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

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

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.