Re: [PATCH v2] xen-blkback: fix compatibility bug with single page rings
From: Dongli Zhang <hidden>
Date: 2021-01-29 06:22:11
Also in:
lkml, xen-devel
On 1/28/21 5:04 AM, Paul Durrant wrote:
quoted hunk ↗ jump to hunk
From: Paul Durrant <redacted> 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 <redacted> --- Cc: Konrad Rzeszutek Wilk <redacted> Cc: "Roger Pau Monné" <roger.pau@citrix.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Dongli Zhang <redacted> 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;
Is it really necessary to introduce 'multi_ref' here or we may just re-use
'nr_ring_pages'?
According to blkfront code, 'ring-page-order' is set only when it is not zero,
that is, only when (info->nr_ring_pages > 1).
1819 if (info->nr_ring_pages > 1) {
1820 err = xenbus_printf(xbt, dev->nodename, "ring-page-order",
"%u",
1821 ring_page_order);
1822 if (err) {
1823 message = "writing ring-page-order";
1824 goto abort_transaction;
1825 }
1826 }
Therefore, can we assume 'ring-page-order' can never be 0? Once we have
'ring-page-order' set, it should be >= 1 and we should read from "ring-ref%u"?
If the specification allows 'ring-page-order' to be zero with "ring-ref%u"
available, we should introduce 'multi_ref'.
Thank you very much!
Dongli Zhang
quoted hunk ↗ jump to hunk
/* 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 {