On 1/29/21 12:13 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jürgen Groß <[email protected]>
>> Sent: 29 January 2021 07:35
>> To: Dongli Zhang <[email protected]>; Paul Durrant <[email protected]>; xen-
>> [email protected]; [email protected];
>> [email protected]
>> Cc: Paul Durrant <[email protected]>; Konrad Rzeszutek Wilk
>> <[email protected]>; Roger Pau
>> Monné <[email protected]>; Jens Axboe <[email protected]>
>> Subject: Re: [PATCH v2] xen-blkback: fix compatibility bug with single page
>> rings
>>
>> On 29.01.21 07:20, Dongli Zhang wrote:
>>>
>>>
>>> On 1/28/21 5:04 AM, Paul Durrant wrote:
>>>> From: Paul Durrant <[email protected]>
>>>>
>>>> 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 <[email protected]>
>>>> ---
>>>> Cc: Konrad Rzeszutek Wilk <[email protected]>
>>>> Cc: "Roger Pau Monné" <[email protected]>
>>>> Cc: Jens Axboe <[email protected]>
>>>> Cc: Dongli Zhang <[email protected]>
>>>>
>>>> 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).
>>
>
> That's how it is *supposed* to be. Windows certainly behaves that way too.
>
>> Did you look into all other OS's (Windows, OpenBSD, FreebSD, NetBSD,
>> Solaris, Netware, other proprietary systems) implementations to verify
>> that claim?
>>
>> I don't think so. So better safe than sorry.
>>
>
> Indeed. It was unfortunate that the commit to blkif.h documenting multi-page
> (829f2a9c6dfae) was not crystal clear and (possibly as a consequence) blkback
> was implemented to read ring-ref0 rather than ring-ref if ring-page-order was
> present and 0. Hence the only safe thing to do is to restore that behaviour.
>
Thank you very much for the explanation!
Reviewed-by: Dongli Zhang <[email protected]>
Dongli ZHang
