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

[Paul Durrant]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 27 January 2021 10:57
> To: Paul Durrant <paul@xxxxxxx>
> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Roger Pau
> Monné <roger.pau@xxxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>; Dongli Zhang <dongli.zhang@xxxxxxxxxx>;
> linux-kernel@xxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH] xen-blkback: fix compatibility bug with single page rings
> 
> 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.

In a backend we shouldn't be relying on, nor really expect IMO, a frontend to clean up after itself. Any backend should know *exactly* what xenstore nodes it’s looking for from a frontend.

  Paul

> 
> Jan