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From:   Paul Durrant <xadimgnik@gmail.com>
Reply-To: <paul@xen.org>
To:     "'Jan Beulich'" <jbeulich@suse.com>
Cc:     "'Paul Durrant'" <pdurrant@amazon.com>,
        "'Konrad Rzeszutek Wilk'" <konrad.wilk@oracle.com>,
        =?utf-8?Q?'Roger_Pau_Monn=C3=A9'?= <roger.pau@citrix.com>,
        "'Jens Axboe'" <axboe@kernel.dk>,
        "'Dongli Zhang'" <dongli.zhang@oracle.com>,
        <linux-kernel@vger.kernel.org>, <linux-block@vger.kernel.org>,
        <xen-devel@lists.xenproject.org>
References: <20210127103034.2559-1-paul@xen.org> <cd70ae5e-a389-7521-8caf-15650a276152@suse.com>
In-Reply-To: <cd70ae5e-a389-7521-8caf-15650a276152@suse.com>
Subject: RE: [PATCH] xen-blkback: fix compatibility bug with single page rings
Date:   Wed, 27 Jan 2021 11:09:43 -0000
Message-ID: <026001d6f49c$eab982b0$c02c8810$@xen.org>
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> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: 27 January 2021 10:57
> To: Paul Durrant <paul@xen.org>
> Cc: Paul Durrant <pdurrant@amazon.com>; Konrad Rzeszutek Wilk =
<konrad.wilk@oracle.com>; Roger Pau
> Monn=C3=A9 <roger.pau@citrix.com>; Jens Axboe <axboe@kernel.dk>; =
Dongli Zhang <dongli.zhang@oracle.com>;
> linux-kernel@vger.kernel.org; linux-block@vger.kernel.org; =
xen-devel@lists.xenproject.org
> Subject: Re: [PATCH] xen-blkback: fix compatibility bug with single =
page rings
>=20
> On 27.01.2021 11:30, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> >
> > 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.
>=20
> 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=E2=80=99s looking for from a frontend.

  Paul

>=20
> Jan