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Date: Thu, 28 Jan 2021 13:04:24 -0000
From: Paul Durrant <xadimgnik@...il.com>
To: <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>,
"'Dongli Zhang'" <dongli.zhang@...cle.com>
Subject: RE: [PATCH] xen-blkback: fix compatibility bug with single page rings
Apologies; I missed the v2 from the subject line. I'll re-send.
Paul
> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@...ts.xenproject.org> On Behalf Of Paul Durrant
> Sent: 28 January 2021 12:55
> To: 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>; Dongli Zhang <dongli.zhang@...cle.com>
> Subject: [PATCH] xen-blkback: fix compatibility bug with single page rings
>
> 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 (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@...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>
>
> 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;
> /* 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..6c5e9373e91c 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -998,14 +998,17 @@ 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;
> -
> err = -EINVAL;
> xenbus_dev_fatal(dev, err, "reading %s/%s",
> dir, ring_ref_name);
> @@ -1013,18 +1016,6 @@ 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 +1120,15 @@ 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 +1137,6 @@ 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 {
> --
> 2.17.1
>
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