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

[Jürgen Groß <jgross@xxxxxxxx>]

On 29.01.21 07:20, Dongli Zhang wrote:
> On 1/28/21 5:04 AM, 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;
> Is it really necessary to introduce 'multi_ref' here or we may just re-use
> 'nr_ring_pages'?
>
> According to blkfront code, 'ring-page-order' is set only when it is not zero,
> that is, only when (info->nr_ring_pages > 1).
Did you look into all other OS's (Windows, OpenBSD, FreebSD, NetBSD,
Solaris, Netware, other proprietary systems) implementations to verify
that claim?

I don't think so. So better safe than sorry.


Juergen

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