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

[Jan Beulich]

On 27.01.2021 11:30, 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
> - 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.

Isn't this only the 2nd of a pair of fixes that's needed, the
first being the drivers, upon being unloaded, to fully clean up
after itself? Any stale key left may lead to confusion upon
re-use of the containing directory.

Jan