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

[Paul Durrant]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Dongli Zhang
> Sent: 27 January 2021 19:57
> To: Paul Durrant <paul@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Roger Pau
> Monné <roger.pau@xxxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>
> 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@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.
> >
> > 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>
> > ---
> >  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 {
> >