Re: [PATCH v2] xen-blkback: fix compatibility bug with single page rings

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jan 28, 2021 at 01:04:41PM +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@xxxxxxxxxx>
> 
> Prior to commit 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid
> inconsistent xenstore 'ring-page-order' set by malicious blkfront"), the
> behaviour of xen-blkback when connecting to a frontend was:
> 
> - read 'ring-page-order'
> - if not present then expect a single page ring specified by 'ring-ref'
> - else expect a ring specified by 'ring-refX' where X is between 0 and
>   1 << ring-page-order
> 
> This was correct behaviour, but was broken by the afforementioned commit to
> become:
> 
> - read 'ring-page-order'
> - if not present then expect a single page ring (i.e. ring-page-order = 0)
> - expect a ring specified by 'ring-refX' where X is between 0 and
>   1 << ring-page-order
> - if that didn't work then see if there's a single page ring specified by
>   'ring-ref'
> 
> This incorrect behaviour works most of the time but fails when a frontend
> that sets 'ring-page-order' is unloaded and replaced by one that does not
> because, instead of reading 'ring-ref', xen-blkback will read the stale
> 'ring-ref0' left around by the previous frontend will try to map the wrong
> grant reference.
> 
> This patch restores the original behaviour.
> 
> Fixes: 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid 
> inconsistent xenstore 'ring-page-order' set by malicious blkfront")
> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> 
> v2:
>  - Remove now-spurious error path special-case when nr_grefs == 1
> ---
>  drivers/block/xen-blkback/common.h |  1 +
>  drivers/block/xen-blkback/xenbus.c | 38 +++++++++++++-----------------
>  2 files changed, 17 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/common.h 
> b/drivers/block/xen-blkback/common.h
> index b0c71d3a81a0..524a79f10de6 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -313,6 +313,7 @@ struct xen_blkif {
>  
>       struct work_struct      free_work;
>       unsigned int            nr_ring_pages;
> +     bool                    multi_ref;

You seem to have used spaces between the type and the variable name
here, while neighbors also use hard tabs.

The rest LGTM:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

We should have forbidden the usage of ring-page-order = 0 and we could
have avoided having to add the multi_ref variable, but that's too late
now.

Thanks, Roger.