Re: [Xen-devel] [PATCH v4 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

["Roger Pau Monné" <royger@freebsd.org>]

On Tue, Jan 8, 2019 at 10:53 AM Dongli Zhang <dongli.zhang@oracle.com> wrote:
>
> Hi Roger,
>
> On 01/07/2019 11:27 PM, Roger Pau Monné wrote:
> > On Mon, Jan 07, 2019 at 10:07:34PM +0800, Dongli Zhang wrote:
> >>
> >>
> >> On 01/07/2019 10:05 PM, Dongli Zhang wrote:
> >>>
> >>>
> >>> On 01/07/2019 08:01 PM, Roger Pau Monné wrote:
> >>>> On Mon, Jan 07, 2019 at 01:35:59PM +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@oracle.com>
> >>>>> ---
> >>>>> Changed since v1:
> >>>>>   * change the order of xenstore read in read_per_ring_refs
> >>>>>   * use xenbus_read_unsigned() in connect_ring()
> >>>>>
> >>>>> Changed since v2:
> >>>>>   * simplify the condition check as "(err != 1 && nr_grefs > 1)"
> >>>>>   * avoid setting err as -EINVAL to remove extra one line of code
> >>>>>
> >>>>> Changed since v3:
> >>>>>   * exit at the beginning if !nr_grefs
> >>>>>   * change the if statements to avoid test (err != 1) twice
> >>>>>   * initialize a 'blkif' stack variable (refer to PATCH 1/2)
> >>>>>
> >>>>>  drivers/block/xen-blkback/xenbus.c | 76 +++++++++++++++++++++-----------------
> >>>>>  1 file changed, 43 insertions(+), 33 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> >>>>> index a4aadac..a2acbc9 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]);
> >>>>> + nr_grefs = blkif->nr_ring_pages;
> >>>>> +
> >>>>> + if (unlikely(!nr_grefs))
> >>>>> +         return -EINVAL;
> >>>>
> >>>> Is this even possible? AFAICT read_per_ring_refs will always be called
> >>>> with blkif->nr_ring_pages != 0?
> >>>>
> >>>> If so, I would consider turning this into a BUG_ON/WARN_ON.
> >>>
> >>> It used to be "WARN_ON(!nr_grefs);" in the v3 of the patch.
> >>>
> >>> I would turn it into WARN_ON if it is fine with both Paul and you.
> >>
> >> To clarify, I would use WARN_ON() before exit with -EINVAL (when
> >> blkif->nr_ring_pages is 0).
> >
> > Given that this function will never be called with nr_ring_pages == 0
> > I would be fine with just using a BUG_ON, getting here with
> > nr_ring_pages == 0 would imply memory corruption or some other severe
> > issue has happened, and there's no possible recovery.
> >
> > If you want to instead keep the return, please use plain WARN instead
> > of WARN_ON.
> >
> > Thanks, Roger.
> >
>
> Is there any reason using WARN than WARN_ON? Because of the message printed by
> WARN? something like below?

Oh, so WARN also takes a condition, I was expecting WARN to not take
any parameters. Just use WARN_ON(true); then, there's no need to
re-evaluate !nr_grefs.