On 1/27/21 2:30 AM, Paul Durrant wrote:
> From: Paul Durrant <pdurr...@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.
>
> Fixes: 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid
> inconsistent xenstore 'ring-page-order' set by malicious blkfront")
> Signed-off-by: Paul Durrant <pdurr...@amazon.com>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com>
> Cc: "Roger Pau Monné" <roger....@citrix.com>
> Cc: Jens Axboe <ax...@kernel.dk>
> Cc: Dongli Zhang <dongli.zh...@oracle.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".
Should we do something as below?
err = -EINVAL;
xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
return err;
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 {
>
