Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Dec 18, 2018 at 11:29:16PM +0800, Dongli Zhang wrote:
> 
> 
> On 12/18/2018 11:13 PM, Roger Pau Monné wrote:
> > On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
> >> Hi Roger,
> >>
> >> On 12/18/2018 05:33 PM, Roger Pau Monné wrote:
> >>> On Tue, Dec 18, 2018 at 08:55:38AM +0800, Dongli Zhang wrote:
> >>>> The xenstore 'ring-page-order' is used globally for each blkback queue 
> >>>> and
> >>>> therefore should be read from xenstore only once. However, it is obtained
> >>>> in read_per_ring_refs() which might be called multiple times during the
> >>>> initialization of each blkback queue.
> >>>>
> >>>> If the blkfront is malicious and the 'ring-page-order' is set in 
> >>>> different
> >>>> value by blkfront every time before blkback reads it, this may end up at
> >>>> the "WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));" in
> >>>> xen_blkif_disconnect() when frontend is destroyed.
> >>>>
> >>>> This patch reworks connect_ring() to read xenstore 'ring-page-order' only
> >>>> once.
> >>>>
> >>>> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> >>>> ---
> >>>> Changed since v1:
> >>>>   * change the order of xenstore read in read_per_ring_refs(suggested by 
> >>>> Roger Pau Monne)
> >>>>   * use xenbus_read_unsigned() in connect_ring() (suggested by Roger Pau 
> >>>> Monne)
> >>>>
> >>>>  drivers/block/xen-blkback/xenbus.c | 70 
> >>>> ++++++++++++++++++++++----------------
> >>>>  1 file changed, 40 insertions(+), 30 deletions(-)
> >>>>
> >>>> diff --git a/drivers/block/xen-blkback/xenbus.c 
> >>>> b/drivers/block/xen-blkback/xenbus.c
> >>>> index a4bc74e..7178f0f 100644
> >>>> --- a/drivers/block/xen-blkback/xenbus.c
> >>>> +++ b/drivers/block/xen-blkback/xenbus.c
> >>>> @@ -926,7 +926,7 @@ static int read_per_ring_refs(struct xen_blkif_ring 
> >>>> *ring, const char *dir)
> >>>>          int err, i, j;
> >>>>          struct xen_blkif *blkif = ring->blkif;
> >>>>          struct xenbus_device *dev = blkif->be->dev;
> >>>> -        unsigned int ring_page_order, nr_grefs, evtchn;
> >>>> +        unsigned int nr_grefs, evtchn;
> >>>>  
> >>>>          err = xenbus_scanf(XBT_NIL, dir, "event-channel", "%u",
> >>>>                            &evtchn);
> >>>> @@ -936,43 +936,38 @@ static int read_per_ring_refs(struct 
> >>>> xen_blkif_ring *ring, const char *dir)
> >>>>                  return err;
> >>>>          }
> >>>>  
> >>>> -        err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", 
> >>>> "%u",
> >>>> -                          &ring_page_order);
> >>>> -        if (err != 1) {
> >>>> -                err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", 
> >>>> &ring_ref[0]);
> >>>> -                if (err != 1) {
> >>>> +        nr_grefs = blkif->nr_ring_pages;
> >>>> +        WARN_ON(!nr_grefs);
> >>>> +
> >>>> +        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);
> >>>> +                err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
> >>>> +                                   "%u", &ring_ref[i]);
> >>>> +
> >>>> +                if (err != 1 && (i || (!i && nr_grefs > 1))) {
> >>>
> >>> AFAICT the above condition can be simplified as "err != 1 &&
> >>> nr_grefs".
> >>>
> >>>>                          err = -EINVAL;
> >>>
> >>> There's no point in setting err here...
> >>>
> >>>> -                        xenbus_dev_fatal(dev, err, "reading 
> >>>> %s/ring-ref", dir);
> >>>> +                        xenbus_dev_fatal(dev, err, "reading %s/%s",
> >>>> +                                         dir, ring_ref_name);
> >>>>                          return err;
> >>>
> >>> ...since you can just return -EINVAL (same applies to the other
> >>> instance below).
> >>
> >> I would like to confirm if I would keep the err = -EINVAL in below because 
> >> most
> >> of the below code is copied from original implementation without 
> >> modification.
> >>
> >> There is no err set by xenbus_read_unsigned().
> > 
> > Right, but instead of doing:
> > 
> > err = -EINVAL;
> > return err;
> > 
> > You can just do:
> > 
> > return -EINVAL;
> > 
> > Which is one line shorter :).
> 
> However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is
> not set as xenbus_read_unsigned() does not return any err?
> 
> For "ring-page-order", I would still need to set err = -EINVAL with extra one
> line of code?

Given this, I don't have a strong opinion, so do as you please.

Thanks, Roger.

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