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Date: Thu, 28 Jan 2021 08:30:48 -0000
From: Paul Durrant <xadimgnik@...il.com>
To: "'Dongli Zhang'" <dongli.zhang@...cle.com>,
<xen-devel@...ts.xenproject.org>, <linux-block@...r.kernel.org>,
<linux-kernel@...r.kernel.org>
Cc: "'Paul Durrant'" <pdurrant@...zon.com>,
"'Konrad Rzeszutek Wilk'" <konrad.wilk@...cle.com>,
'Roger Pau Monné' <roger.pau@...rix.com>,
"'Jens Axboe'" <axboe@...nel.dk>
Subject: RE: [PATCH] xen-blkback: fix compatibility bug with single page rings
> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@...ts.xenproject.org> On Behalf Of Dongli Zhang
> Sent: 27 January 2021 19:57
> To: Paul Durrant <paul@....org>; xen-devel@...ts.xenproject.org; linux-block@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Cc: Paul Durrant <pdurrant@...zon.com>; Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>; Roger Pau
> Monné <roger.pau@...rix.com>; Jens Axboe <axboe@...nel.dk>
> Subject: Re: [PATCH] xen-blkback: fix compatibility bug with single page rings
>
>
>
> On 1/27/21 2:30 AM, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@...zon.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.
> >
> > Fixes: 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-
> order' set by malicious blkfront")
> > Signed-off-by: Paul Durrant <pdurrant@...zon.com>
> > ---
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > Cc: "Roger Pau Monné" <roger.pau@...rix.com>
> > Cc: Jens Axboe <axboe@...nel.dk>
> > Cc: Dongli Zhang <dongli.zhang@...cle.com>
> > ---
> > drivers/block/xen-blkback/common.h | 1 +
> > drivers/block/xen-blkback/xenbus.c | 36 +++++++++++++-----------------
> > 2 files changed, 17 insertions(+), 20 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;
> > /* All rings for this device. */
> > struct xen_blkif_ring *rings;
> > unsigned int nr_rings;
> > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> > index 9860d4842f36..4c1541cde68c 100644
> > --- a/drivers/block/xen-blkback/xenbus.c
> > +++ b/drivers/block/xen-blkback/xenbus.c
> > @@ -998,10 +998,15 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
> > for (i = 0; i < nr_grefs; i++) {
> > char ring_ref_name[RINGREF_NAME_LEN];
> >
> > - snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> > + if (blkif->multi_ref)
> > + snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> > + else {
> > + WARN_ON(i != 0);
> > + snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref");
> > + }
> > +
> > err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
> > "%u", &ring_ref[i]);
> > -
> > if (err != 1) {
> > if (nr_grefs == 1)
> > break;
>
> I think we should not simply break here because the failure can be due to when
> (nr_grefs == 1) and reading from legacy "ring-ref".
>
Yes, you're quite right. This special case is no longer correct.
> Should we do something as below?
>
> err = -EINVAL;
> xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
> return err;
>
I think simply removing the 'if (nr_grefs == 1)' will be sufficient.
Paul
> Dongli Zhang
>
>
> > @@ -1013,18 +1018,6 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
> > }
> > }
> >
> > - if (err != 1) {
> > - WARN_ON(nr_grefs != 1);
> > -
> > - err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
> > - &ring_ref[0]);
> > - if (err != 1) {
> > - err = -EINVAL;
> > - xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
> > - return err;
> > - }
> > - }
> > -
> > err = -ENOMEM;
> > for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
> > req = kzalloc(sizeof(*req), GFP_KERNEL);
> > @@ -1129,10 +1122,15 @@ static int connect_ring(struct backend_info *be)
> > blkif->nr_rings, blkif->blk_protocol, protocol,
> > blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");
> >
> > - ring_page_order = xenbus_read_unsigned(dev->otherend,
> > - "ring-page-order", 0);
> > -
> > - if (ring_page_order > xen_blkif_max_ring_order) {
> > + err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
> > + &ring_page_order);
> > + if (err != 1) {
> > + blkif->nr_ring_pages = 1;
> > + blkif->multi_ref = false;
> > + } else if (ring_page_order <= xen_blkif_max_ring_order) {
> > + blkif->nr_ring_pages = 1 << ring_page_order;
> > + blkif->multi_ref = true;
> > + } else {
> > err = -EINVAL;
> > xenbus_dev_fatal(dev, err,
> > "requested ring page order %d exceed max:%d",
> > @@ -1141,8 +1139,6 @@ static int connect_ring(struct backend_info *be)
> > return err;
> > }
> >
> > - blkif->nr_ring_pages = 1 << ring_page_order;
> > -
> > if (blkif->nr_rings == 1)
> > return read_per_ring_refs(&blkif->rings[0], dev->otherend);
> > else {
> >
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