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    Subject[PATCH 5.4 138/244] xen-blkback: fix compatibility bug with single page rings
    Date
    From: Paul Durrant <pdurrant@amazon.com>

    [ Upstream commit d75e7f63b7c95c527cde42efb5d410d7f961498f ]

    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@amazon.com>
    Reviewed-by: Dongli Zhang <dongli.zhang@oracle.com>
    Reviewed-by: "Roger Pau Monné" <roger.pau@citrix.com>
    Link: https://lore.kernel.org/r/20210202175659.18452-1-paul@xen.org
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 1d3002d773f7..20cb0c23a22c 100644
    --- a/drivers/block/xen-blkback/common.h
    +++ b/drivers/block/xen-blkback/common.h
    @@ -316,6 +316,7 @@ 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 040d7bb21397..dca91d641f1d 100644
    --- a/drivers/block/xen-blkback/xenbus.c
    +++ b/drivers/block/xen-blkback/xenbus.c
    @@ -949,14 +949,17 @@ 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);
    @@ -964,18 +967,6 @@ 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);
    @@ -1079,10 +1070,15 @@ static int connect_ring(struct backend_info *be)
    blkif->nr_rings, blkif->blk_protocol, protocol,
    pers_grants ? "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",
    @@ -1091,8 +1087,6 @@ 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 {
    --
    2.30.2


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